Helen Barbara Mcfarlane

Peter Betts

Prof Mark Jessell

Dr Jérôme Ganne

Dr Lenka

The geodynamic and tectonic evolution of the Palaeoproterozoic Sefwi Greenstone Belt, West African Craton

This thesis investigates the Palaeoproterozoic crust of the West African Craton in southwest Ghana, providing insight into its controversial geodynamic and tectonic evolution. Rocks of the study area comprise greenschist-to amphibolite facies, mafic to felsic volcanic and volcaniclastic rocks, high-grade paragneisses and low-grade volcano-sedimentary packages, all of which are extensively intruded by multiple generations of granitoids. New lithological, metamorphic and structural maps are constructed using integrated field mapping and interpretation of regional airborne geophysical datasets. This approach is used to constrain the deformation history of the sparsely exposed rocks of the NE-to NNE-striking Sefwi Greenstone Belt and the adjacent volcano-sedimentary domains deformed during the Eburnean Orogeny (2150-2070 Ma).

Combined geochemical and geochronological analysis of the magmatic suites of the Sefwi Greenstone Belt reveal calc-alkaline, volcanic arc affinities, as well as a striking similarity to Neoarchean TTGs that require diverse magma sources and petrogenetic processes. Rare inherited zircon cores from the Palaeoproterozoic magmatic suite yield ages of ca. 2250 to 2270 Ma, with granitoid emplacement ages ranging between ca. 2189 and 2081 Ma. Zircon Lu-Hf analysis reveals consistently positive εHf(t) values and two-stage crustal model ages between 2650 and 2250 Ma, indicative of a radiogenic proto-crust and short crustal residence times. The magmatic evolution reveals the coeval generation of sodic, high-silica TTGs derived from partial melting of low-K mafic sources and dioritic magmas generated in a metasomatised, LILE-enriched mantle wedge at ca. 2155 Ma. Subsequent emplacement of high-K quartz monzonites at ca. 2136 Ma supports the interaction of mantle-derived magmas and remelting of existing TTGs. The final stage of magmatism is characterised by the emplacement of two-mica-granites and leucogranites along the NW margin of the Sefwi Greenstone Belt between ca. 2092 and 2081 Ma, interpreted as a terminal collisional event during the Eburnean Orogeny. Eburnean metamorphism and deformation is characterised in the study area by initial high-pressure amphibolite facies metamorphism corresponding with low apparent geothermal gradients (HP-MT, ~15-17°C/km). D1 NNW-SSE shortening generated a ubiquitous beddingparallel foliation (S1) and ~E-W striking thrust faults, resulting in the burial of supracrustal rocks and crustal thickening. In the high-grade terrane, subsequent amphibolite-granulite facies viii metamorphism is associated with anatexis. In-situ SHRIMP U-Pb monazite ages at ca. 2073 Ma, hosted within, D2 mineral assemblages, are interpreted as the initial timing of cooling and exhumation, significantly later than paroxysmal metamorphism in NW Ghana and central Ivory Coast (2150-2130 Ma). NNE-striking normal detachments and constrictional deformation structures formed during sinistral ENW-WSW transtension (D2), during which segments of the middle-and lower crust were juxtaposed with low-grade domains. Subsequent E-W directed shortening (D3) caused the dextral re-activation of NE-SW striking shear zones, associated with a localised greenschist facies metamorphic overprint. We propose that the juvenile crust of southwest Ghana was generated in an intraoceanic arc setting, associated with diverse and intense subduction-related magmatism until subsequent terrane accretion and collision. The north-western margin of the Sefwi Greenstone Belt in interpreted as a suture between the separate arc terranes, diachronously accreted during the Eburnean Orogeny. The Palaeoproterozoic crust of the southern portion of the West African Craton represents a juvenile crustal growth event, recording the unique geodynamic and orogenic processes associated with nascent subduction-related plate tectonics in the early Earth. ix Résumé Cette thèse s'intéresse à un segment de croûte d'âge Paléoprotérozoïque du craton ouestafricain. Les roches de la zone d'étude comprennent des roches volcaniques et volcanoclastiques mafiques à felsiques, des paragneiss de haut grade métamorphique et des ensembles volcano-sédimentaires faiblement métamorphisés. De nouvelles cartes lithologiques, métamorphiques et structurales sont construites à l'aide d'une approche intégrée, couplant cartographie de terrain et interprétation des données géophysiques aéroportées à l'échelle régionale. L'analyse des données géochimiques et géochronologiques des suites magmatiques de la ceinture de roches vertes de Sefwi révèle une affinité marquée avec le magmatisme calcoalcalin, produit des arcs volcaniques modernes et avec les TTGs d'âge Néoarchéen, impliquant une certaine diversité des sources et des processus pétrogénétiques. Des coeurs de zircons hérités, présents au sein de la suite magmatique livrent des âges autour de ca. 2250 à 2270 Ma. Leurs couronnes révèlent des âges de mise en place compris entre ca. 2189 et 2081 Ma. L'analyses Lu-Hf sur zircon livre des valeurs εHf positives et des âges modèles pour la croûtes situés entre 2650 et 2250 Ma. Ces valeurs indiquent l'existence d'une proto-croûte à tendance radiogènique et des temps de séjour limités pour ces magmas évoluant au sein de cette protocroûte. L'évolution des magmas montre qu'ils ont été générés de façon concomitante, vers 2155 Ma, certains dérivants de la fusion d'une source mafique faiblement enrichit en potassium et formant des magmas sodiques, riches en silice, de type TTGs, d'autres de composition plus dioritiques, générés à partir de la fusion du manteau métasomatisé et enrichit en LILE. La mise en place plus tard vers ca. 2136 Ma de monzonites, présentant de teneurs élevées en potassium, soutient l'hypothèse d'une interaction avec des magmas de refusion de TTG existants au sein de la croûte. Le dernier stade du magmatisme est caractérisé par la mise en place de granites à deux micas et de leucogranites, le long de la marge nord-ouest de la ceinture de roches vertes vers ca. 2092 et 2081 Ma, marquant le stade de la collision au sein de l'orogène Eburnéenne. L'évènement tectono-métamorphique d'âge Eburnéen est caractérisé par un métamorphisme initialement de faciès amphibolite de haute pression, associé à un gradient géothermique assez froid (HP-MT, ~ 15-17 ° C / km). Le raccourcissement D1, orienté NNO-SSE, a généré une foliation pénétrative (S1), parallèle au litage des roches et des plans de chevauchement à tendance décrochant, orienté E-O. Cette tectonique précoce a provoquée l'enfouissement de roches supra-crustales (sédiments, roches volcaniques) et un épaississement x de la croûte. Cet évènement métamorphique précoce évolue dans le temps et l'espace vers le facies amphibolite-granulite et l'anatexie. Les données SHRIMP U-Pb in-situ sur monazite livré des âges autour de ca. 2073 Ma. Ces monazites sont présentes au sein de paragénèses métamorphiques (D2) soulignant la foliation S2. Ces âges sont interprétés comme marquant le début de l'exhumation et du refroidissement de la croûte inférieure. Des détachements normaux, orientés NNE-SSO et des structures constrictives se sont formés conjointement au sein d'un régime de déformation D2 globalement transtensif, à jeux sénestre. Un régime compressif plus tardif (D3) a ensuite causé une réactivation en mouvement dextre de ces structures cisaillantes orientées NE-SO avec une rétrogression en schistes verts. Nous proposons que les segments de croute juvénile ont été générés en contexte d'arc intra-océanique, associé à un magmatisme intense et varié, issus des processus de subduction qui prendront fin lors des stades d'accrétion et de collision de ces segments d'autres terranes birimiens. La marge nord-ouest de la ceinture de roches vertes de Sefwi est interprétée comme une zone de suture entre des segments d'arc originellement séparés.

xii xiii

Résumé analytique (Executive summary) Objectifs de la thèse

Les roches d'âge Paléoprotérozoïques du craton ouest-africain, datés autour de 2300-2070 Ma, sont actuellement sous-explorées et peu représentées dans la littérature mondiale qui s'intéresse à la géodynamique protérozoïque. Ce craton expose pourtant un ensemble unique des roches crustales formées après la transition archéen-protérozoïque et qui n'ont subi que très peu de modifications après la stabilisation de ce dernier. Ce craton offre ainsi une fenêtre exceptionnelle sur le cadre géodynamique et les processus orogéniques responsables de la formation et la conservation de la croûte juvénile et des processus orogéniques au cours d'une période énigmatique de l'histoire de la Terre. Dans le cadre de l'Initiative pour l' Exploration en Afrique de l'Ouest (WAXI), les chercheurs d'universités d'Afrique de l'Ouest , d'Australie, de France et du Canada se sont associés avec les acteurs industriels, des sponsors et les bureaux nationaux de recherche en géologie, au sein d'un programme intégré qui vise à améliorer nos connaissances sur la géologie et le potentiel d'exploration du craton ouest-africain. Ce projet représente l'un des derniers projets de doctorat à l'échelle régionale du programme WAXI au Ghana, ce qui représente une opportunité pour relier la technique de recherche récente et les résultats du sud et du nord-ouest du Ghana. La ceinture de roches vertes de Sefwi, d'âge Paléoprotérozoïque, située au sud-ouest du Ghana, constitue la zone ciblée par cette thèse. Les principales questions que cette thèse vise à aborder peuvent être listées comme suit:

1. Quelle a été l'évolution structurale et métamorphique de la ceinture de roches vertes de Sefwi? 2. Quelles ont été les caractéristiques de la déformation et du métamorphisme au Paléoprotérozoïque ? Quelles ont été les forces motrices à l'origine de cet évènement tectono-métamorphique ? En outre, qu'est-ce que cela nous apprend sur le style tectonique de l'orogenèse éburnéenne et quelles en sont les implications pour la compréhension de cette orogenèse Paléoprotérozoïque? 3. Quels ont été les paramètres géodynamiques qui ont permis la formation et la préservation de grands volumes de croûte juvénile au sein du craton ouest-africain, suite à une période hypothétique de quiescence tectonique globale? Est-ce que les processus géodynamiques responsables de la formation du craton ouest-africain ont des similitudes avec les processus archéens ou ont-ils été formés dans un cadre tectonique xiv différent, de type Tectonique des Plaques? Et en quoi ces processus de Tectoniques de Plaques diffèrent-ils de ceux d'aujourd'hui? L'objectif principal de cette thèse est d'explorer le cadre géodynamique de la formation de la croûte juvénile ainsi que les processus orogéniques préservés au sein du craton ouest-africain pour avoir un aperçu de l'évolution du régime de la Tectonique des Plaques au début de l'histoire de la Terre.

Compte tenu de ces objectifs ambitieux et de la taille de l'objet d'étude, une approche multi-échelles et multi-disciplinaires a été privilégié, combinant données de terrain, cartographie régionale, analyse structurale et modélisation métamorphique, couplé à des datations et des analyses géochimies et isotopiques. Les grandes lignes de la thèse adressent différents aspects au sein de trois principaux objectifs, incluant le contexte au sein de la littérature existante et les méthodes appliquées afin d'explorer différents aspects de chaque question.

Les directeurs de thèse pour ce projet étaient le Dr. Laurent Aillères (School of Earth,

Chapitre IV Résumé

Ce chapitre est complémentaire du cadre et de la carte tectono-métamorphiques présentés au chapitre 3, fournissant une analyse pétrographique détaillée, des estimations quantitatives des conditions du métamorphisme régional dominant et des contraintes géochronologiques sur la déformation et le métamorphisme durant l'orogenèse éburnéenne. Ces nouvelles contraintes géochronologiques sont comparées aux données géochronologiques existantes afin de comprendre la chronologie des évènements métamorphiques au sein du craton, mais aussi la xvii chronologie du métamorphisme de haut grade dans la zone étudiée et son rapport aux autres terrains de haut et bas grades au sein du craton. Les assemblages métamorphiques et les âges sur monazites documentés dans ce chapitre élucident une histoire métamorphique plus complexe pour le sud-ouest du Ghana (en particulier) et l'orogenèse éburnéenne (en générale), contribuant ainsi à une meilleure compréhension de l'amalgamation du craton ouest-africain à cette époque. Cette évolution métamorphique apparait comme un régime d'exception au Paléoprotérozoïque. Ce chapitre a été soumis au Journal of Metamorphic Geology. Une version révisée sera renvoyée avant mai 2018. Les principales révisions proposées ont été prises en compte, certaines révisions étant toujours en cours.

De nouvelles données pétrographiques et géochronologiques sont présentées pour les paragneiss migmatitiques et les roches méta-volcaniques de hauts grades exposées au sud-ouest du Ghana. L'étude porte sur les domaines tectono-métamorphiques présents au coeur de la ceinture de roches vertes de Sefwi et dans les domaines adjacents à cette ceinture. Ces domaines sont bornés ou recoupés par des zones de cisaillement ductiles, fortement réactivées, d'orientation NE à ENE et présentes à l'échelle régionale. Au nord de la ceinture de Sefwi, un terrane de haut grade a été nouvellement identifié. Il comprend des paragneiss migmatitiques hétérogènes de haut grade, injecté par de volumineuses intrusions de granite à deux micas. Il est interprété comme faisant partie de la ceinture de Sefwi en raison de ses similitudes métamorphiques et structurales. Paragneiss et amphibolites à grenat révèlent de façon sporadique un métamorphisme précoce de haute pression et moyenne température (HP-MT) (10,0 à 11,5 kbar, 580-650 ° C), s'inscrivant au sein d'un faciès amphibolite de haute pression et marquant un gradient géothermique apparent assez froid, de l'ordre de 15-17 °C par km. Cet évènement métamorphique précoce évolue vers le facies amphibolite-granulite (7.5 à 9.5 kbars, 650-700 °C) atteignant le début de la fusion partielle (anatexie). Cette évolution se produit le long d'un gradient géothermique apparent moyen de l'ordre de 20-25 °C / km. La variété des chemins métamorphiques progrades enregistrés par les migmatites révèle un réchauffement durant la décompression, ou inversement un réchauffement durant l'enfouissement. L'exhumation de ces roches partiellement fondues se produit conjointement, comme l'indique le parallélisme entre la forme horaire de tous ces chemins P-T-t, montrant une baisse simultanée en pression et température, et la présence d'un métamorphisme régional dans le facies amphibolite venant se superposer aux assemblages métamorphiques de plus haut grade.

xviii Les données d'âge (U-Pb) obtenues in-situ sur monazite (SHRIMP) permettent de situer le début de cette exhumation et de ce refroidissement autour de ca. 2073 Ma. A l'inverse des migmatites, un échantillon d'amphibolite à grenat enregistre son refroidissement durant l'exhumation, sans passer par un stade de plus haute température. Nous interprétons les chemins P-T -t de forme horaire enregistré par deux des trois échantillons, comme la preuve d'un épaississement de la croûte birimienne dans ce secteur de Sefwi, en lien avec la convergence et la collision de ce segment de croûte le long de sa marge nord -ouest.

L'exhumation de la croûte inférieure le long des zones de cisaillement d'orientation NE et de plans de détachement à pendage ouest est finalisée aux environs de ca. 2092 -2081 Ma, ce qui correspond à l'âge de mise en place des leucogranites venant recouper les écailles de croûte inférieure et supérieure.

Les assemblages métamorphiques de haute pression produits le long de faibles gradients géothermiques apparents distinguent le sud-ouest du Ghana des provinces granitiques-gneissiques précambriennes. De plus, les diverses histoires thermiques et la juxtaposition tectonique de différentes écailles de la croûte issues de différentes profondeurs ne sont pas conformes aux prédictions métamorphiques et au type homogène des déformations prédits par le modèle «d'orogène chaud» proposé pour le cas des orogènes précambriens. De plus, des âges métamorphiques aussi jeunes n'ont pas été documentés auparavant dans la partie sud-est du craton ouest-africain. Ces résultats suggèrent plutôt différents épisodes de collision survenant au cours de l'assemblage du craton, produisant une histoire métamorphique diachrone et complexe. L'enregistrement métamorphique du craton Ouest africain soutient l'idée d'un régime tectonique de transition au Paléoprotérozoïque, partagé entre une Tectonique des Plaques moderne qui s'annonce et une tectonique Archaïque qui disparait. Ces gradients métamorphiques froids préfigurent en quelque sorte l'apparition des premières subductions modernes du Néoprotérozoïque et leur cortège métamorphique de ultra-haute pression (UHP).

Chapitre V Résumé

Les chapitres de recherche précédents ont exploré le style tectonique et les mécanismes d'exhumation accompagnant l'orogène éburnéenne, comme il est possible de la documenter dans le SW du Ghana. Ce chapitre se concentre sur l'évolution magmatique de la ceinture de roches vertes de Sefwi afin de mieux comprendre le contexte géodynamique de la croissance de la croûte à cette époque de la Terre. Il présente de nouvelles analyses pétrologiques et xix géochimiques des suites magmatiques représentatives de la ceinture de roches vertes de Sefwi et du domaine adjacent de Sunyani-Comoé. L'analyse des données en éléments majeurs et traces permet de remonter aux sources potentielles des magmas et des processus pétrogénétiques responsables de la formation de la croûte birimienne. Les analyses U-Pb et Lu-Hf sur zircons contraignent la chronologie du magmatisme, tout en nous fournissant des informations sur les processus et la chronologie d''extraction de ces magmas à partir du manteau paléoprotérozoïque, mais aussi sur les temps de séjour de ces magmas dans la croûte birimienne et sur la nature de cette proto-croûte en Afrique de l'Ouest. La chronologie du magmatisme et les valeurs εHf(t) sont comparés à d'autres régions du craton ouestafricain.

Cette approche permet de révéler différents sous-domaines à signature isotopique bien distincte, nous amenant à une meilleure compréhension de l'architecture du craton. Une considération d'ensemble est enfin apportée à cette géodynamique paléoprotérozoïque, responsable de la formation et de la préservation du craton ouest-africain, au regard des modèles géodynamiques proposé pour l'Archéen et ceux connus et opérant au Phanérozoïque.

De nouvelles données géochimiques et isotopiques sont présentées pour les suites magmatiques qui affleurent dans la partie sud-ouest du Ghana (ca. 2300-2070 Ma). Cette étude se concentre sur l'évolution magmatique de la ceinture de roches vertes de Sefwi, redéfinissant les caractéristiques géochimiques et isotopiques des principales suites magmatiques mises en place entre ca. 2155 et 2136 Ma. La croûte paléoprotérozoïque juvénile dérive d'un matériel crustal extrait d'un manteau appauvri entre ca.2650 et 2260 Ma, indiquant des temps de séjour assez courts dans la croûte et impliquant un remaniement très limité de la croûte continentale archéenne présente à cette époque paléoprotérozoïque.

De nouvelles données géochimiques roche totale mettent en lumière la grande variété pétrologique, géochimique et isotopique des suites magmatiques d'âge Paléoprotérozoïque présentes au sud du Ghana. Ces nouvelles données indiquent une mise en place simultanée de ces magmas issus de différentes sources présentent au sein d'arcs volcaniques. Cinq suites magmatiques ont été identifiées: 1) granitoïdes de type tonalites-trondhjémite-granodiorite (TTG), riches en sodium, dominés par la biotite et la hornblende, associés localement à des systèmes effusifs de type dacite; 2) diorites calco-alcalines, enrichies en LILE et laves andésitiques associées; 3) pyroxénites, gabbros et roches intrusives de composition mafique ; 4) monzonites riches en quarz, de composition très potassique; et 5) granites à deux micas et leucogranites à muscovite, enrichis en éléments incompatibles. Les spectres en éléments traces sont tous caractérisés par des valeurs en Nb -Ta, P, Ti très négatives et des anomalies positives en Pb et Ba dans les suites magmatiques plus évolués, ce qui indique une influence de xx la croûte au cours de la genèse du magma au sein de l'arc volcanique. Les diorites et les TTGs à biotite présentant de faibles valeurs d'HREE, se mettent en place simultanément vers ca. 2155 Ma, révélant des sources contrastées. Les magmas de type dioritique sont produits plus en profondeur, issus de la fusion partielle d'un coin de manteau métasomatisé et enrichi en éléments incompatibles, tandis que les TTGs à biotite ont probablement été produits par la fusion partielle de panneaux de croûte océanique subductés sous l'arc, au sein d'un manteau assez chaud. La genèse des TTGs à hornblende est attribuée à la fusion partielle à plus faible profondeur d'une croûte mafique. Cette hypothèse se fonde sur l'observation des faibles teneurs en Sr de ces magmas, leur forte teneur en terres rares lourdes (HREE), le faible fractionnement en terres rares légères (REE) et des rapports en éléments assez élevés par rapport aux données issues de l'expérimentale. La mise en place de magma monzonitiques vers ca. 2136 Ma est révélateur d'un processus de mélange au sein de la croûte birimienne, entre des magmas issus de la fusion du manteau et d'autres dérivant d'une refonte partielle de TTGs existants. La mise en place ultérieure de granites à deux micas et des leucogranites, entre ca. 2092-2081 Ma, est interprétée comme le produit d'une anatexie crustale lors d'un évènement de type collision d'arc. La zone de suture envisagée se situerait parallèlement à la marge nord -ouest de la ceinture Sefwi. Les analyses in-situ Lu-Hf sur zircons livrent toujours des valeurs ε Hf(t) positives, situées entre +4,9 et +8,0 pour les TTG, +2,2 et +5,5 pour les diorites et +2,7 à +5,8 pour les laves dacitiques, mettant en évidence une source nettement radiogénique durant la croissance de la croûte et représentant potentiellement une limite isotopique juvénile au sein de ce craton. Les monzonites mises en place postérieurement révèlent une plus large gamme de valeurs en Hf (t), situées entre +1,2 et +6,5, ce qui suggère une hétérogénéité croissante dans la source de ces magmas. Nous suggérons que la formation et l'accrétion du craton ouest-africain reflètent l'accumulation rapide et l'amalgamation de terranes d'arcs générés en domaine intra-océanique et associant de la subduction. Ce type de géodynamique géochimique est très semblable à celle communément proposée pour les cycles de Wilson et représentait, à ce titre, une de ses toutes premières expression dans l'histoire de la Terre.

Conclusions

Sommaire des publications et congrès

Le chapitre 4 a éte soumis á Journal of Metamorphic Geology (May 2017).

McFarlane, H. B., Block, S., Ganne, J., Baratoux, L., Ailleres, L., Betts, P. Jessell, M. W., Armstrong, R. (submitted) Petrological and geochronological evidence of collisional orogenesis and subsequent lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny, SW Ghana. Journal of Metamorphic Geology.

Les chapitres 3 et 5 seront préparé pour etre soumis á Precambrian Research apres que la thèse a été présentée.

Well, it's been a journey…

My deepest thanks must go to my main supervisor, Laurent, as well as my co-supervisors, Pete, Mark, Jérôme and Lenka. When Laurent and Pete first pitched the idea of doing a PhD with field work in West Africa, a year in France and a home base in Melbourne, it sounded like a dream! In its infant stages, I'm sure none of us had imagined just how big (and how expensive) the project would turn out to be.

Laurent, I don't think I'll ever be able to express how gratefully I am for all the experiences I've had over the last four and a half years. Thank you for proposing the project and for financing it in all its excesses; thank you for our discussions, even when they ran in circles; thank you for your support and friendship throughout the years and for having confidence in me when I had none. You provided the light, the hope and the push in spite of my stubbornness and perfectionism. I must also thank Pete for setting me on new trajectories, inspiring new debates and for always believing in subduction. To Jérôme, Lenka and Mark, many thanks to each of you for your wisdom and insight. Your expertise and patience have refined my research in a multitude of ways despite the slow-drip feed of my finished works, and each of you have aided my navigation of the French administration system. A special thanks must go to Caroline Venn for being an inspiring mentor, a wonderful friend and a brilliant second mum.

I hope I can one day make amends for texting you at 4am for months on end after finishing field work on the other side of the world. You kept me safe! Thanks must also go to the administration at the School of Earth, Atmosphere and Environment, especially to Silvana, Katie, Christine and Emily, for getting me organised when I wasn't. I couldn't have managed without you.

I gratefully acknowledge AMIRA International and the industry sponsors, including AusAid and the ARC Linkage Project LP110100667, for their support of the WAXI project (P934A). A significant amount of the research was funded by the School of Earth Atmosphere and Environment, Monash Research Initiative Fund -Dr Laurent Ailleres, with addition contributions from the School of Earth Atmosphere and Environment, Monash Research Initiative Fund -Dr Robin Armit and Prof.

Peter Betts, Institute pour le Recherche et Developpement (IRD) through Dr Jérôme Ganne. I would like to thank all those who helped make my field seasons happen, including the generous support of the Geological Survey Department (GSD) of Ghana, including Kwame Boamah, and for the supply of maps, data and logistical support. A special thank you must also go to the GSD drivers, Mr Kwasi Duah and Mr Brown. Kwasi, thank you for your help translating, navigating, organising and wielding the sledge hammer. It was an absolute pleasure to do field work with you. A number of mining companies also kindly allowed me to visit mine sites and utilise their outcrop databases, including the teams at Chirano (Kinross), Bibiani (Resolute) and Kenyase (Newmont) mines. Thanks to Clay Postlethwaite for his xxviii continued interest and support of the researchour discussions were always so valuable. I would also like to thank Philippe de Parseval and Sophie Gouy for assistance in microprobe data acquisition and Thierry Aigouy for his assistance in SEM imaging at the Géosciences Environnement Toulouse (GET) laboratory. I gratefully acknowledge the facilities and technical staff at the Monash Centre for Electron Microscopy. Thanks also to Richard Armstrong at ANU for his help extracting monazite ages and for being such a generous host for my time in Canberra. I am indebted to Christopher Spencer for conducting the Lu-Hf analyses, for answering my questions and for setting my aspirations higher upon our first meeting at AGU.

There have also been a number of researchers who have helped with the design of the project, as well as teaching me a huge number of new analytical techniques. Much of the project conception and design must be credited to Sylvain Block, who challenging me and guided me through the earlier years of this PhD. To Luis Parra-Avila and Nicolas Thébaud, thank you for initiating me into the world of SHRIMP dating. The geochronology may well have been a total disaster without your guidance.

Massive thanks to Robin Armit for your guidance, unwavering support and for your contribution in piecing together such an amazing story. The editorial contribution of Roberto Weinberg and the passionate discussions had with Oliver Nebel were also greatly appreciated.

I have done an extensive amount of travel during this PhD and as a result, I have made lifelong friends both at home and abroad. To my friends in France, thanks for helping me build a second home and for your patience when listening to my butchering of the French language in the beginning. My love of cheese now extends to such depths that I think I will never be able to live without it. To my Perth crew, thanks for adopting me in the blink of an eye. Can't wait to see more of you in the future. To my friends at home, especially Emily and Elspeth, thank you for bringing some normalcy back into my life, for drink and dancing with me, for piecing me back together when I fell apart and for sticking with me despite my frequent absence. To my friends at Monash, especially Lauren, Jackson, Dave and Geez, I could not have survived without you. You've been with me for the whole rollercoaster ride of this thesis, as well as field trips, coffees and many, many beers. Thanks for dipping your toes into insanity with me.

There is one person in particular who remained by my side, bringing me hope and making me smile through the stress, sweat, blood and tears, despite being on the other side of the world. Thank you

Chapter I 1 Introduction

General Introduction

The application of the plate tectonic paradigm and a uniformitarian approach to interpret the Precambrian Earth remains a contentious issue. Similarities between ancient and modern rock types have prompted many authors to invoke modern plate tectonics by 3000 Ma (Cawood, et al., 2006;Condie & Kröner, 2008;Smithies, et al., 2005;van Hunen & Moyen, 2012), or even earlier [START_REF] Furnes | Precambrian greenstone sequences represent different ophiolite types[END_REF], with its operation becoming widespread after 2700 Ma (Condie, 2008). However, a number of the petro-tectonic assemblages and plate tectonic indicators that we use to track the role of plate tectonics, including ophiolite sequences, blueschist facies and ultra-high-pressure metamorphism, did not appear until ca. 1000 Ma (Hamilton, 1998;Stern, 2005;Stern, 2007). Based on this, Stern (2007) suggests that there is a progression from an episodic plate tectonic regime capable of lithosphere recycling and generation of arc-like magmas at ~ 1900 Ma, with sustainable, deep subduction of negatively buoyant oceanic lithosphere beginning in the Neoproterozoic. Alternatively, the global adoption of a plate tectonic regime is suggested to be episodic and diachronous during the Archaean (O'Neill, et al., 2007;van Hunen & Moyen, 2012) or a gradual transitional from a plume-lid regime to a plate tectonic regime from Archaean to the Phanerozoic (Fischer & Gerya, 2016a;Fischer & Gerya, 2016b).

Magmatism and tectonic activity of the Earth are, in essence, the surficial manifestation of the global thermal budget balancing the loss of heat through mantle convection with heat production, primarily generated through radioactive decay (Jaupart, et al., 2007, and references therein). It is widely accepted that the mantle was hotter in the early Earth and has since undergone secular cooling (Korenaga, 2008a;Korenaga, 2008b;Labrosse & Jaupart, 2007;[START_REF] Schubert | Whole planet cooling and the radiogenic heat source contents of the Earth and Moon[END_REF], with ambient mantle temperatures at ca. 3000-2500 Ma between 150 and 250 °C higher than present day [START_REF] Ganne | Primary magmas and mantle temperatures through time[END_REF]Herzberg, et al., 2010). Elevated mantle temperatures likely affected the composition and thickness of oceanic lithosphere, as well as the rheology, composition and tectonic processes of the continental lithosphere (Burov & Yamato, 2008;Burov, 2011;[START_REF] Davies | On the emergence of plate tectonics[END_REF]Sizova, et al., 2010;van Hunen & van den Berg, 2008;[START_REF] Yamato | HP-UHP exhumation during slow continental subduction: Self-consistent thermodynamically and thermomechanically coupled model with application to the Western Alps[END_REF].

In order to understand the evolution of plate tectonics and the continental crust, we turn to the crustal record. This in itself presents geographical and statistical biases, representing a system continually affected by recycling, surface erosion, subduction-zone recycling and overprinting during crustal reworking [START_REF] Clift | Controls on tectonic accretion versus erosion in subduction zones: Implications for the origin and recycling of the continental crust[END_REF]Hawkesworth, et al., 2009;Hawkesworth & Kemp, 2006a).

Crustal growth models indicate that a significant proportion (~65%) of the continental crust had been extracted from the mantle by the end of the Archaean, with growth rates decreasing after ca. 3000 Ma, coinciding with elevated crustal reworking as indicated by zircon Hf isotopic record (e.g. Belousova, et al., 2010;Dhuime, et al., 2012). Archaean crust, however, represents a relatively low proportion of the continental crust exposed today, reflecting a low preservation rate and intense crustal recycling and reworking in the early Earth, limiting both surface exposure and, consequently, the record of early Earth processes. The igneous and detrital zircon record displays successive peaks, which are interpreted as episodes of discrete juvenile crust production and preservation (Cawood, et al., 2013;Condie, et al., 2011b;Hawkesworth & Kemp, 2006b). The Precambrian geological record shows peak activity at 3500 Ma, 2700 Ma, 2100-1900 Ma and 1100 Ma (Condie, 1998;Condie, et al., 2005;Condie, et al., 2011a;[START_REF] Kemp | Episodic growth of the Gondwana supercontinent from hafnium and oxygen isotopes in zircon[END_REF][START_REF] Mcculloch | Progressive growth of the Earth's continental crust and depleted mantle: geochemical constraints[END_REF]) associated with crust production, supercraton/ continent formation and orogenesis. These peaks have multiple interpretations including mantle avalanche events (Condie, 1998) and spikes in super-plume activity (Albarède, 1998;[START_REF] Stein | Mantle plumes and episodic crustal growth[END_REF], or are alternatively correlated with the timing of supercontinent assembly and magmatism [START_REF] Campbell | Formation of supercontinents linked to increases in atmospheric oxygen[END_REF][START_REF] Taylor | The geochemical evolution of the continental crust[END_REF]. Hawkesworth, et al. (2009), however, note that these peaks may simply represent ideal preservation conditions. Given the ambiguous nature of these crystallisation peaks, exploration of regions that contain significant additions of juvenile crust between these events, such as the ca. 2300 -2100 Ma crust of the West African Craton (Abouchami, et al., 1990;Boher, et al., 1992), may provide insight into global crustal and tectonic transitions of the early Earth.

There are many changes documented in the magmatic and the metamorphic record that coincide with the Archaean-to-Proterozoic transition, and the hypothesised advent of plate tectonics. Keller and Schoene (2012) describe a significant disruption in the secular evolution of the geochemical characteristics of mafic and felsic lithologies occurring at the Archaean-Proterozoic boundary, which are associated with changes in the degrees of mantle melting, crustal thickness and the rise of atmospheric oxygen and potentially reflect a major change in global geodynamics. MgO-rich komatiites are produced during high degrees of partial melting of a hot mantle (Condie, 1994;Condie & Benn, 2006;De Wit & Ashwal, 1997), and are found in far greater abundance in the Archaean than in either the Proterozoic or the Phanerozoic. Similarly, felsic rocks display strong compositional variation at the Archaean-Palaeoproterozoic boundary, with Archaean crust dominated by the grey gneisses including the tonalite-trondhjemite-granodiorite (TTG) suite, minor amphibolites and paragneisses. The TTG suite is characterised by high Na 2 O and SiO 2 concentrations, high light rare element (LREE) over heavy rare earth element (HREE) ratios and the absence of positive or negative Sr and Eu anomalies [START_REF] Jahn | REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution[END_REF]Moyen, 2011;Moyen & Martin, 2012), and are derived from the partial melting of hydrous metabasalts at depth. There are two contrasting schools of thought for their genesis; the first invokes subduction of oceanic crust, often involving young hot lithosphere to transport hydrated basalts to depth (Drummond & Defant, 1990;Halla, et al., 2009;Laurent, et al., 2014;Moyen, 2011), and the second suggests that TTGs are generated during the delamination of an over-thickening mafic crust (Atherton & Petford, 1993;Bédard, 2006;Smithies, 2000). Magmatism in the Neoarchaean reveals greater petrological and geochemical diversity with the emplacement of TTGs followed by high-K granitoids, two mica-granites and late sanukitoids (Laurent, et al., 2014;Martin, et al., 2009;Zeh, et al., 2009). The sanukitoids series have potassic calc-alkaline to alkalicalcic, metaluminous compositions and are enriched in both incompatible elements, and Ni and Cr, suggesting both crustal and mantle-derived contributions, or contamination during melting from basic to ultrabasic rocks (Laurent, et al., 2014;Martin, et al., 2009). As such, they are interpreted as the product of the interaction of melts derived from enriched crustal material and mantle peridotite, and are therefore a proposed proxy for the development of subduction-related processes at the Archaean-Proterozoic boundary [START_REF] Heilimo | Discrimination and origin of the sanukitoid series: geochemical constraints from the Neoarchean western Karelian Province (Finland)[END_REF][START_REF] Laurent | Geochemistry and petrogenesis of high-K "sanukitoids" from the Bulai pluton, Central Limpopo Belt, South Africa: Implications for geodynamic changes at the Archaean-Proterozoic boundary[END_REF][START_REF] Moyen | Late Archaean granites: a typology based on the Dharwar Craton (India)[END_REF].

Phanerozoic versus Precambrian orogenesis

In order to understand the development or onset of the plate tectonic regime, many geologists have long relied upon uniformitarianism, interpreting features of ancient terranes as analogues to modern geological formation (Cawood, et al., 2009;Condie & Benn, 2006;Condie & Kröner, 2008;Ernst, 2009;Stern, 2005;[START_REF] Windley | Precambrian rocks in the light of the plate-tectonic concept[END_REF]. Alternatively, based on archetypal features of Archaean provinces, authors have invoked unique geodynamic settings for crustal evolution during the early Earth (Bédard, et al., 2003;[START_REF] Bédard | The hunting of the snArc[END_REF][START_REF] Chardon | Sinking of the Dharwar basin (South India): implications for Archaean tectonics[END_REF]Hamilton, 1998;[START_REF] Smithies | Formation of Earth's early Archaean continental crust[END_REF][START_REF] Smithies | The Mesoarchean emergence of modern-style subduction[END_REF]. Here we explore the characteristics and tectonic differences of Phanerozoic, Proterozoic and Archaean orogens, in order to discuss their geodynamic significance and the contrasting schools of thought regarding Precambrian tectonics.

The modern plate tectonic paradigm is largely based on our understanding and documentation of Phanerozoic orogens that form along convergent plate margins in association with the subduction of oceanic lithosphere. Convergence and lateral shortening are responsible for the accretion of mobile orogenic belts or terranes with contrasting tectono-metamorphic histories. The horizontal forces generated by subduction lead to the centripetal growth of continental crust, evidenced by the amalgamation of younger terranes on continental margins and lateral age gradients [START_REF] Moresi | Dynamics of continental accretion[END_REF][START_REF] Percival | Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies[END_REF]. In general, Phanerozoic accretionary orogens comprise accretionary wedges, exotic crustal fragments, volcanic arcs and associated back arcs, ophiolites, clastic sedimentary basins and distinct metamorphic terranes (Cawood, et al., 2009;[START_REF] Condie | Earth as an evolving planetary system[END_REF][START_REF] Hyndman | Subduction zone backarcs, mobile belts, and orogenic heat[END_REF]. The forearc is characterised by low heat flow decreasing in a trenchward direction with the downgoing cold lithosphere recording a low geothermal gradient of 5-10 °C/km, characteristic of a "cold" orogen. In contrast, the magmatic arc and the backarc domains are characterised by high heat flow and high geothermal gradients between 20 and 40 °C/km [START_REF] Hyndman | Subduction zone backarcs, mobile belts, and orogenic heat[END_REF]. The concurrent burial and metamorphism of the accretionary prism and rocks of the arc and back-arc domains under blueschist-to-eclogite facies metamorphic conditions and amphibolite-to-granulitefacies metamorphic conditions, respectively, and their subsequent juxtaposition, creates a thermal asymmetry within the accretionary orogenic systems. The duality of thermal regimes represents a hallmark indicator of modern accretionary orogens generated in a plate tectonic regime (Brown, 2006).

Continued convergence results in the closure of oceanic basins, the onset of collision and the tectonic assembly of domains within the accretionary orogen or exisiting continental lithosphere, exemplified by the Alpine-Himalayan Orogen. In this setting, lateral shortening is accommodated by the development of major thrust faults, nappe stacking and heterogeneous crustal thickening within the accretionary wedge and foreland belts and basins [START_REF] Dahlen | Critical taper model of fold-and-thrust belts and accretionary wedges[END_REF]. The resulting elevated topographic profiles are continually subjected to erosional and tectonic processes that ultimately remove the overburden, exposing the high-pressure rocks from deep within the orogen that yield key information regarding the tectonic processes of Phanerozoic collisional orogens. First documented in the Phanerozoic Western Alps continental collisional belt by [START_REF] Chopin | Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences[END_REF] and in the ca. 400 Ma Caledonides of Norway [START_REF] Smith | Coesite in clinopyroxene in the Caledonides and its implications for geodynamics[END_REF], coesite inclusions in garnet and clinopyroxene porphyroblasts within high-grade blueschist domain and eclogite pods revealed the burial of continental upper crustal rocks within the subduction zone to depths of approximately 100 km (~ 30 kbar) and their subsequent return to the surface [START_REF] Chopin | Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences[END_REF]Chopin, 2003;[START_REF] Smith | Coesite in clinopyroxene in the Caledonides and its implications for geodynamics[END_REF]. The interpreted mechanism for the burial involves the entrainment of tracts of the leading edge of a continental plate, attributed to body forces during incipient collision (Chopin, 2003, and references therein). Greater debate, however, surrounds the mechanism for the exhumation of ultra-high pressure (UHP) rocks. Petrological and isotopic data reveal simultaneous high cooling rates and high decompressions rates, representative of rapid exhumation rates of 1-2 cm/yr [START_REF] Burov | A thermomechanical model of exhumation of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks in Alpine-type collision belts[END_REF][START_REF] Doin | Subduction initiation and continental crust recycling: the roles of rheology and eclogitization[END_REF][START_REF] Hacker | Exhumation of ultrahigh-pressure continental crust in east central China: Late Triassic-Early Jurassic tectonic unroofing[END_REF], with observed concurrent cooling during exhumation [START_REF] Carswell | Petrographic characteristics and metamorphic evolution of ultrahigh-pressure eclogites in plate-collision belts[END_REF][START_REF] Chopin | Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences[END_REF] precluding entrainment of UHP metamorphic rocks during the diapiric ascent of plutons as a viable exhumation mechanism. Burov, et al. (2001, and references therein) summarised multiple levels of exhumation in a continental collision zone. They propose that exhumation of low-pressure-lowtemperature rocks of the accretionary prism can be attributed to classical corner flow, whilst deeper high-pressure-high-temperature segments of the crust are exhumed through positive buoyancy and ascending shear flow. Alternatively, exhumation of the deepest material is attributed to convective instabilities generated through heating of crustal material by the overlying asthenospheric mantle [START_REF] Burov | A thermomechanical model of exhumation of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks in Alpine-type collision belts[END_REF], or to the break off subducted lithosphere and its subsequent exhumation along the subduction channel due to its relative positive buoyancy [START_REF] Gerya | Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation[END_REF]. These mechanisms are then complemented by the removal of overburden and thinning of the crust during extensional tectonics. Extension is attributed to extensional collapse [START_REF] Andersen | Extensional tectonics in the Caledonides of southern Norway, an overview[END_REF], lateral, ductile flow of a weak, partially molten lower crust (Vanderhaeghe & Teyssier, 2001a;Vanderhaeghe & Teyssier, 2001b) or, alternatively, to slab rollback [START_REF] Brun | Exhumation of high-pressure rocks driven by slab rollback[END_REF], ultimately exposing the complex melange of metamorphic terranes observed in orogenic belts today.

Numerical experiments and thermomechanical modelling [START_REF] Burov | Rheological and geodynamic controls on the mechanisms of subduction and HP/ UHP exhumation of crustal rocks during continental collision: Insights from numerical models[END_REF]Burov & Yamato, 2008;[START_REF] Faccenda | Styles of post-subduction collisional orogeny: influence of convergence velocity, crustal rheology and radiogenic heat production[END_REF][START_REF] Yamato | Burial and exhumation in a subduction wedge: Mutual constraints from thermomechanical modeling and natural PTt data (Schistes Lustres, western Alps)[END_REF] examining the role and interaction of these processes are complemented and constrained by petrological evidence and the determination of P-T-t-d paths of metamorphic terranes (e.g. England & Thompson, 1984), which allow us to calculate transient geotherms within the crust during orogenesis. This knowledge often guides interpretation of early Earth tectonics or alternatively highlights the differences in architecture, processes and geotherms that characterise Archaean and Palaeoproterozoic provinces. The following section explores the unique characteristics of Precambrian orogens and the range of proposed geodynamic frameworks, including ambiguity of evidence between different end-member models.

Archaean provinces, and and some Palaeoproterozoic terranes, frequently display a unique crustal architecture characterised by narrow greenstone belts, vast granite-gneiss terranes dominated by TTG magmatism and craton-scale shear zones. The interpreted geodynamic significance of such terranes follows two schools of thought, the first of which regards granite-greenstone provinces as ancient analogues of the modern, horizontal, subduction-driven plate tectonic regime (Cawood, et al., 2006;Condie & Benn, 2013;Condie & Kröner, 2008). In this model, crustal-scale shear zone are interpreted as major thrust related sutures. The second school of thought invokes a unique geodynamic model for the early Earth [START_REF] Bédard | Evidence for Regional-Scale, Pluton-Driven, High-Grade Metamorphism in the Archaean Minto Block, Northern Superior Province, Canada[END_REF][START_REF] Bédard | The hunting of the snArc[END_REF][START_REF] Smithies | Formation of Earth's early Archaean continental crust[END_REF].

Archaean metamorphism predominantly preserves non-linear exposures of greenschist to mediumpressure granulite facies rocks, with no record of blueschist facies metamorphism, nor deep continental subduction (Brown, 2007, and references therein). Strain patterns in some Archaean provinces define dome and keel geometries with strain trajectory triple junctions associated with homogeneous distributed deformation. These strain patterns are interpreted as the product of both body forces exerted by diapiric structures and regional horizontal shortening reflecting boundary forces (Bouhallier, et al., 1995;Choukroune, et al., 1995). Gravitational instabilities between the thick, dense overlying (flood) basalts or greenstone cover and the underlying warm, felsic and less dense granitoid-dominant lower crust result in a density inversion [START_REF] Abbott | Archaean plate tectonics revisited 1. Heat flow, spreading rate, and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents[END_REF][START_REF] Chardon | Strain patterns, décollement and incipient sagducted greenstone terrains in the Archaean Dharwar craton (south India)[END_REF][START_REF] Chardon | Sinking of the Dharwar basin (South India): implications for Archaean tectonics[END_REF][START_REF] Hoffman | Archean oceanic flake tectonics[END_REF]Taylor & McLennan, 1985;Van Kranendonk, et al., 2004;[START_REF] West | A model for Archean tectonism. Part I. The thermal conditions[END_REF]. Synformal structures commonly observed in anastomosing greenstone belts are attributed to the downward advection or sagduction of cold dense material, and elliptical foliation patterns form during the diapiric emplacement of high-viscosity granitoids [START_REF] Chardon | Archean granite-greenstone tectonics at Kolar (South India): Interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion[END_REF]Choukroune, et al., 1995). The dome-and-keel geometries contain vertical stretching lineations at the contact between the two domains and are thus interpreted as the result of "vertical" tectonics (Bouhallier, et al., 1995;Choukroune, et al., 1995;Lompo, 2010;[START_REF] Marshak | Deformation style way back when: thoughts on the contrasts between Archean/ Paleoproterozoic and contemporary orogens[END_REF]Van Kranendonk, et al., 2004;Vidal, et al., 2009).

However, the dichotomic debate between vertical and horizontal tectonics has recently been questioned by a number of authors [START_REF] François | Burial and exhumation during Archean sagduction in the East Pilbara granite-greenstone terrane[END_REF][START_REF] Rey | Spreading continents kick-started plate tectonics[END_REF]Thébaud & Rey, 2013). [START_REF] François | Burial and exhumation during Archean sagduction in the East Pilbara granite-greenstone terrane[END_REF] integrated structural, metamorphic and geochronological studies in the Archean East Pilbara granite-greenstone terranes, which revealed the rapid burial (9-11 kbar, 450-550 °C) of metabasalts and metasediments and their subsequent exhumation during partial melting of the lower crustal and the formation of granitoid domes. Complementary thermomechanical modelling revealed a diverse range of apparent geothermal gradients (10-45°C/km) [START_REF] François | Burial and exhumation during Archean sagduction in the East Pilbara granite-greenstone terrane[END_REF].

In addition, based on numerical experiments, Thébaud and Rey (2013) argue that sagduction can generate significant lateral shortening and intraplate deformation. The rapid burial of supracrustal sequences and associated cold apparent geothermal gradients are typically attributed to subduction along a plate boundary, however, both [START_REF] François | Burial and exhumation during Archean sagduction in the East Pilbara granite-greenstone terrane[END_REF] and Thébaud and Rey (2013) highlight the potential of sagduction as a viable alternative mechanism for burial and exhumation. A similar mechamism was proposed by Ganne, et al., (2014) for high-pressure (>10 kbar) volcano-sedimentary Palaeoproterozoic rocks of the West African Craton, which were exhumated from lower crustal depths through entrainment in ascending granitoids. It was recented noted, however, by [START_REF] Rey | Spreading continents kick-started plate tectonics[END_REF] that sagduction and subduction are not neccessarily mutally exclusive when dealing with elevated amntle temperatures. Numerical modelling suggests that early continents derived from differentiation of thick oceanic plateaus underwent intraplate gravitational collapse, which generated sufficient lateral forces and buoyancy contrasts to initiate episodic subduction of adjacent oceanic lids. Using this method, [START_REF] Rey | Spreading continents kick-started plate tectonics[END_REF] were able to generate a self-consistent geodynamic framework explaining multimodal volcanism and TTG melts and formation of a deep mantle root, characteristic of Archaean cratons, as well as coexisting stagnant-lid and subduction-like processes. Given these arguments, distinguishing between vertical and horizontal tectonics and different geodynamic settings on the basis of deformation and apparent geothermal gradients is remains problematic and nuanced.

The third end-member orogenic model invoked in the Precambrian tectonic debate is the "hot-orogen" model proposed for young, hot Precambrian orogens, sourcing evidence from field observations, strain patterns and analogue and numberical models. In this model, bulk horizontal contraction is accommodated by homogeneous deformation and metamorphism and lateral crustal flow due to rheologically weakened lithosphere (Chardon, et al., 2009;Chardon, et al., 2011;Cruden, et al., 2006;Rey & Houseman, 2006). The rheologically weak lithosphere prohibits the formation of both significant topographic relief and crustal-scale detachments capable of exhuming the lower crust.

The hot-orogen mode is invoked for Precambrian accretionary and collisional orogens (e.g. Cagnard, et al., 2007;Vidal, et al., 2009) as well as wide, mature Phanerozoic orogens such as the Variscan Belt [START_REF] Schulmann | Vertical extrusion and horizontal channel flow of orogenic lower crust: key exhumation mechanisms in large hot orogens?[END_REF].

Whilst the debate surrounding early Earth tectonics and geodynamics continues to focus on finite timing and end-member models, other datasets suggest a more gradual progression towards the modern plate tectonic regime. In the metamorphic record, Archaean metamorphic rocks commonly exhibit average metamorphic conditions from sub-greenschist facies to granulite facies with no sharp lateral metamorphic breaks (e.g. Goscombe, et al., 2009;[START_REF] Komiya | Intermediate-P/T type Archean metamorphism of the Isua supracrustal belt: Implications for secular change of geothermal gradients at subduction zones and for Archean plate tectonics[END_REF][START_REF] Percival | Giant granulite terranes of northeastern Superior Province: the Ashuanipi complex and Minto block[END_REF]. The Neoarchaean was marked by the coeval appearance of ultra-high temperature (UHT) and eclogite-high-pressure granulite (E-HPG) facies metamorphism, interpreted by Brown (2006;2007) as a key marker for the onset of a transitional Proterozoic plate tectonic regime involving increasingly widespread subduction-like process transitioning to self-sustaining subduction systems around the globe. It was not until the Neoproterozoic that the classic metamorphic indicators of the modern plate tectonic regime appearance including blueschist facies metamorphic conditions and ultra-high pressure assemblages (Stern, 2005;Stern, 2007), suggesting the progressive evolution and adoption of present day subduction dynamics.

Many datasets derived from the geological record suggest the Earth was going through significant geodynamic changes at the Archaean-Palaeoproterozoic boundary, which possibly extended into the Palaeoproterozoic (e.g. Brown, 2007;[START_REF] Condie | Plate Tectonics and crustal evolution[END_REF]Keller & Schoene, 2012). Given the breadth of time for which estimates of the commencement of plate tectonics span (~700 Ma to >4000 Ma [START_REF] Harrison | Heteroge-neous Hadean hafnium: evidence of continental crust at 4.4 to 4.5[END_REF][START_REF] Sleep | Evolution of the continental lithosphere[END_REF]Stern, 2007;van Hunen & Moyen, 2012), a focus on the geological record during this transitional period is warranted. The Palaeoproterozoic provinces of the West African Craton are characterised by juvenile crust composed of volcano-plutonic belts and granite-gneiss domains similar to Archaean granite-greenstone provinces (Abouchami, et al., 1990;Lompo, 2010;Milési, et al., 1992;Taylor, et al., 1992;Vidal, et al., 2009).The craton represents a significant crustal growth event between the two global peaks in juvenile crust production and the first orogenic event during a period of global orogenesis between 2100 and 1800 Ma, heralding the amalgamation of the first Palaeoproterozoic supercontinent, Nuna (Columbia) (Rogers & Santosh, 2002;Zhao, et al., 2002;Zhao, et al., 2004). As such, the West African Craton provides a unique opportunity to explore the enigmatic tectonic regime of the Proterozoic.

West African Craton Geology

Introduction

The Palaeoproterozoic (2300-2070 Ma) West African Craton represents one of Earth's oldest Palaeoproterozoic accretionary orogens (Windley, 1992). This orogeny is associated with high production rates of juvenile crust and extensive, protracted magmatism (Abouchami, et al., 1990;Condie, 2007), forming between two global crustal growth maxima at 2700 and 1900 Ma (Cawood, et al., 2013;Condie, 1998;Condie, et al., 2005). The formation and deformation of the craton precedes the amalgamation of the first Palaeoproterozoic supercontinent, Nuna (Columbia) and a major period of global orogenesis between 2100 and 1800 Ma (Rogers & Santosh, 2004;Zhao, et al., 2002;Zhao, et al., 2004). The following section provides the geological setting of the West African Craton, highlighting controversies that persist in the literature, reminiscent of those that arise in the Neoarchaean record.

Geological Setting

The West African Craton consists of two major geological provinces: the Reguibat Rise of northwest Africa and the Leo-Man Rise of western Africa (Fig. 2.1) (Bessoles, 1977b;[START_REF] Black | Precambrian of West Africa[END_REF]Rocci, et al., 1991). The Reguibat Rise extends across southern Morocco to southern Algeria, whilst the Leo-Man Rise is exposed throughout Sierra Leone, Liberia, Guinea, Ivory Coast, southern Mali, Burkina Faso, Ghana and Niger, and includes the Kédougou-Kéniéba and Keyes inliers of Senegal and western Mali.

The craton is centrally overlain by the intracontinental Mesoproterozoic to Devonian Taoudeni Basin and the Neoproterozoic Volta Basin in the southeast. The entire craton is bounded to the east and west by Pan African-aged mobile belts [START_REF] Ennih | The boundaries of the West African craton, with special reference to the basement of the Moroccan metacratonic Anti-Atlas belt[END_REF][START_REF] Schluter | Geological Atlas of Africa -With notes on stratigraphy, tectonics, economic geology, geohazards and geosites each country[END_REF][START_REF] Villeneuve | Structure, evolution and palaeogeography of the West African craton and bordering belts during the Neoproterozoic[END_REF]. Prior to the opening of the Atlantic Ocean in the Jurassic, correlations are made between the West African Craton, the São-Luís Craton and the north-eastern extent of French Guiana (Klein, et al., 2005;Klein & Moura, 2008;Ledru, et al., 1994;[START_REF] Nomade | The Guiana and the West African shield Palaeoproterozoic grouping: new palaeomagnetic data for French Guiana and the Ivory Coast[END_REF]Vanderhaeghe, et al., 1998).

Both the Reguibat Rise and the Leo-Man Rise each comprise an Archaean domain in the west and a Palaeoproterozoic domain to the east, separated by major shear zones (Fig. 2.1) [START_REF] Black | Precambrian of West Africa[END_REF]. The Archaean nuclei of the Leo-Man Rise comprises the Kenema-Man Domain, consisting of tonalite-trondhjemite-granodiorite gneisses emplaced in three discrete pulses at 3600-3500 Ma, 3260-3050 Ma and 2960-2850 Ma, intercalated with linear belts of supracrustal rocks including basalts, BIFs, komatiites and minor sedimentary sequences (Rollinson, 2016, and references therein).

Crust formation in the Leo-Man Rise is concomitant with the magmatism of the Reguibat Rise (Potrel, et al., 1998;[START_REF] Potrel | 3.5 Ga old terranes in the West African craton, Mauritania[END_REF]. The Kénéma-Man Domain was affected by two Archaean orogenic events, including the Leonian (3500-2900 Ma) and the Liberian (2900-2600 Ma) events (Bessoles, 1977b;Rollinson, 2016;Tagini, 1971a), associated with amphibolite-to granulite facies regional metamorphism [START_REF] Kouamelan | Pétrologie et géochronologie Sm-Nd des granulites de haute pression de Kouibli (Dorsale de Man, Côte d'Ivoire)[END_REF]Kouamelan, et al., 1997;[START_REF] Thiéblemont | A 3.5 Ga granite-gneiss basement in Guinea: further evidence for early archean accretion within the West African Craton[END_REF].

The final orogenic event is attributed to the accretion of the Palaeoproterozoic domains during the Eburnean Orogeny (ca. 2100 Ma) (Abouchami, et al., 1990;Boher, et al., 1992;Liégeois, et al., 1991).

Palaeoproterozoic rocks of the West African Craton form the eastern domains of the Reguibat and Leo-Man Rises, known as the Yetti-Eglab and Baoulé-Mossi Domains, respectively.

The Palaeoproterozoic Baoulé-Mossi domain of the West African Craton (WAC) is characterised by elongate meta-volcanic greenstone belts, meta-sedimentary basins, and large granite-gneiss provinces consisting of variably deformed grey gneisses and tonalite-trondhjemite-granodiorite (TTG) suites, separated by craton-scale shear zones (e.g. Abouchami, et al., 1990;Bessoles, 1977b;Feybesse & Milési, 1994). Volcanic and volcano-sedimentary sequences for the Birimian Supergroup were first described in the Birim Valley in Ghana by Kitson (1918). Greenstone belts predominantly comprise metamorphosed Birimian volcanic sequences, which range in composition from tholeiitic and calcalkaline basalts through to rhyolites and associated minor volcaniclastic, greywacke and chemical sedimentary sequences (Baratoux, et al., 2011;Boher, et al., 1992;Doumbia, et al., 1998;Pouclet, et al., 2006). Birimian volcano-sedimentary rocks are made of low-grade argillite, shale, greywacke and rare intercalated intermediate volcanoclastic or volcanic layers (Adadey, et al., 2009;Agyei Duodu, et al., 2009;Junner, 1940). The youngest Palaeoproterozoic sequence comprises fluvio-deltaic detrital sedimentary formations of the Tarkwa Group, which are often spatially restricted to fault bounded basins overlying central portions of greenstone belts (Ledru, et al., 1994). Tarkwaian rocks consist of conglomerates, quartzites and immature sediments, defined by the sequences exposed in the Ashanti Belt in southern Ghana (Junner, 1935;Perrouty, et al., 2012). Timing of deposition was originally constrained by detrital zircons from the basal units of the formation yielding single U-Pb and Pb-Pb ages between ca. 2150 and 2143 Ma (Davis, et al., 1994;Hirdes & Nunoo, 1994). The maximum age of deposition has subsequently been revised to 2107 -2097 Ma (Perrouty, et al., 2012;Pigois, et al., 2003). Tarkwa Group sedimentary rocks are interpreted as syn-orogenic to late stage basins (Davis, et al., 1994;Perrouty, et al., 2012), or alternatively as a molasses sequence deposited at the onset of deformation in a foreland basin (Eisenlohr & Hirdes, 1992).

Craton-scale isotopic studies using whole rock Nd and Lu-Hf analysis of magmatic and detrital zircons predominantly indicated a radiogenic source for the Palaeoproterozoic crust [START_REF] Abati | Insights on the crustal evolution of the West African Craton from Hf isotopes in detrital zircons from the Anti-Atlas belt[END_REF]Abouchami, et al., 1990;Block, et al., 2016a;Boher, et al., 1992;Klein, et al., 2005;Tapsoba, et al., 2013). Hf-and Nd-isotope signatures yield model ages on average between 2500 and 2100 Ma, indicating short crustal residence after mantle extraction. A number of studies, however, reveal negative εNd (t) and εHf (t) values in far south-eastern Ghana and southern Mali, indicating an older crustal influence or reworking of Archaean crust, with model age ranging from 4000 to 2800 Ma in southern Mali (Parra-Avila, et al., 2016;Petersson, et al., 2016;Taylor, et al., 1992). The oldest detrital zircons in the Palaeoproterozoic domain yield U-Pb ages between ca. 3600 and 2700 Ma sourced from southern Mali (Parra-Avila, et al., 2016). These ages correlate well with the Kénéma-Man Domain, yet are not found elsewhere in the craton. Inherited zircon cores from throughout the domain predominantly yield ages between ca. 2300 and 2200 Ma (de Kock, et al., 2011;Gasquet, et al., 2003;Siegfried, et al., 2009;[START_REF] Thomas | Geological Map Explanation[END_REF], whilst the oldest recorded crystallisation ages are yielded by granodiorite gneisses in Burkina Faso at 2265± 17 Ma (Parra-Avila, et al., 2017) and2253 ± 9 Ma (Tshibubudze, et al., 2013).

Bimodal volcanism of the Birimian greenstone belts is dated across the craton at between 2300 and 2180 Ma (Block, et al., 2016a;Delor, et al., 1995;Feybesse, et al., 2006;Hirdes & Davis, 1998;Hirdes, et al., 1996;Hirdes, et al., 2007). Similar age ranges are yielded by the earliest phase of TTG and calc-alkaline plutonism in Ghana (de Kock, et al., 2011;Doumbia, et al., 1998;Feybesse, et al., 2006;Siegfried, et al., 2009), Burkina Faso (Parra-Avila, et al., 2016;Tshibubudze, et al., 2013;Tshibubudze, et al., 2015), Ivory Coast (Gasquet, et al., 2003;Pouclet, et al., 2006) and Senegal [START_REF] Dioh | Diversity of the Palaeoproterozoic granitoids of the Kédougou inlier (eastern Sénégal): petrographical and geochemical constraints[END_REF]Gueye, et al., 2008;Gueye, et al., 2007), with granitoid emplacement occurring near continuously until 2100 Ma. Post-2100 Ma volcanism and calc-alkaline plutonism is documented in Kedougou-Kéniéba Inlier of Senegal (Hirdes & Davis, 2002), prompting authors to propose a diachronous accretion of the Palaeoproterozoic crust; however, magmatism in the remainder of the craton at this time is limited to the emplacement of late potassic granites and leucogranites along the Palaeoproterozoic-Archaean boundary and minor occurrences in southern Ghana (Egal, et al., 2002;Gueye, et al., 2007;Hirdes, et al., 1992;Hirdes, et al., 2007).

Deformation and metamorphism of the Palaeoproterozoic domains is attributed to the Eburnean Orogeny (2150 -2070 Ma) (Bonhomme, 1962;Feybesse, et al., 2006). Eburnean deformation is characterised as an early crustal thickening event, often associated with extensive magmatism and greenschist to granulite facies metamorphism, followed by the development of craton-scale shear zones during transcurrent tectonism (Baratoux, et al., 2011;Eisenlohr & Hirdes, 1992;Feybesse, et al., 2006;Hein, 2010;Jessell, et al., 2012;Perrouty, et al., 2012;Tagini, 1971a;Vidal, et al., 2009). The early phase of deformation was responsible for juxtaposition of domains of different composition and grade and the generation of high-grade terranes, which authors attribute to a range of mechanisms, discussed in detail in Section 2.3. Major strike-faulting in the later stages of the Eburnean (ca. 2100 Ma) is commonly associated with a pervasive greenschist facies overprint and wide-spread gold mineralisation hosted in shear zones and palaeoplacer deposits (Feybesse, et al., 2006;Galipp, et al., 2003;Ledru, et al., 1991;[START_REF] Milesi | West African gold deposits in their Lower Proterozoic lithostructural setting[END_REF]Oberthür, et al., 1998;Pigois, et al., 2003;Vidal, et al., 2009). Terminal collision or docking of the Palaeoproterozoic and Archaean domains is constrained by formation and exhumation of high-pressure granulite facies rocks between 2050 and 2030 Ma [START_REF] Kouamelan | Géochronologie et Géochimie des Formations Archéennes et Protérozoïques de la Dorsale de Man en Côte d'Ivoire. Implications pour la Transition Archéen-Protéozoïque[END_REF]Kouamelan, et al., 1997;Pitra, et al., 2010). The craton records very few magmatic ages after ca. 1980 Ma, indicating a prolonged period of quiescence until ca. 1000 Ma (Ennih & Liégeois, 2008, and reference therein).

This dramatically contrasts the global crustal and orogenic record, which is dominated by the elevated crustal growth and widespread formation of linear, mobile orogenic belts during the formation and assembly of the supercontinent, Nuna (Condie, 1998;Zhao, et al., 2002).

Controversies

Stratigraphy and nomenclature

The Palaeoproterozoic West African Craton is characterised by narrow volcanic greenstone belts, wide meta-sedimentary "basins" and vast granite-gneiss terranes, bounded and separated by crustal-scale transcurrent shear zones that formed during the Eburnean Orogeny. Sporadic exposures of high metamorphic grade gneissic to migmatitic rocks occur throughout the Baoulé-Mossi domain, juxtaposed with low-grade rocks. Questions that continue to arise include: What is the tectono-stratigraphic architecture of the craton? Can we correlate stratigraphic units and deformation events at a craton-scale?

Was the Eburnean Orogeny monocyclic or polycyclic? What do the strain patterns and metamorphic record of the craton tell us about the tectonic style during the Eburnean? Previous studies have employed geochemical, geochronological and structural investigations in order to determine the stratigraphic framework, the tectonic environment and the phases and styles of tectonism recorded within the craton. Persisting controversies and questions regarding the Palaeoproterozoic evolution of the craton are mainly due to the limited geochemical, metamorphic and geochronological data, and the integrations of such datasets at a regional or craton scale, as well as complex geo-political relationships. This has produced a variety of evolutionary models for the West African Craton (Abouchami, et al., 1990;Baratoux, et al., 2011;Block, et al., 2016b;Boher, et al., 1992;de Kock, et al., 2012;Egal, et al., 2002;Feybesse, et al., 2006;Feybesse & Milési, 1994;Hein, 2010;Ledru, et al., 1994;Liégeois, et al., 1991;[START_REF] Milesi | West African gold deposits in their Lower Proterozoic lithostructural setting[END_REF][START_REF] Tshibubudze | The Markoye Shear Zone in NE Burkina Faso[END_REF][START_REF] Vidal | The palaeoproterozoic (Birimian) of Haute-Comoé in the West African craton, Ivory Coast: a transtensional back-arc basin[END_REF]Vidal, et al., 2009). Some of the earliest controversies generated in literature regarding the West African Craton, arose from conflicting interpretation of the stratigraphy of the craton between French and English geologists in the 1940 -1970s. Early literature, predominantly proposed by Anglophone geologists, divided the Birimian Supergroup into a lower series of greywackes, phyllites, shales and minor lavas, and an upper series of mafic to intermediate lavas and subordinate volcaniclastic rocks, greywacke and phyllite (Junner, 1935;Junner, 1940;[START_REF] Kemp | Episodic growth of the Gondwana supercontinent from hafnium and oxygen isotopes in zircon[END_REF][START_REF] Ledru | Tectonique transcurrente et évolution polycyclique dans le Birrimien, Protérozoïque inférieur, du Sénégal-Mali (Afrique de l'Ouest)[END_REF][START_REF] Lemoine | The Burkinian, an orogenic cycle, precursor of the Eburnean of West Africa[END_REF][START_REF] Milési | Caractérisation Lithostructurale de Deux Ensembles Successifs dans les séries Birimiennes de la Boutonniére de Kédougou (Mali-Sénégal) et du Niandan (Guinée): Implications Gîtologiques[END_REF]. Conversely, francophone geologists proposed a basal Birimian volcanic and volcano-sedimentary unit, overlain by shallow basins filled with flysche-type and molasse sedimentary sequences [START_REF] Aubouin | Propos sur les géosynclinaux[END_REF]Bessoles, 1977b;Tagini, 1971b). In addition, structural, geochemical and isotopic studies by Eisenlohr and Hirdes (1992) and Leube, et al. (1990) suggest that the Birimian metavolcanic rocks and metasedimentary formations are contemporaneous distal, lateral facies equivalents.

Most recently, Adadey, et al. (2009) and Perrouty, et al. (2012) proposed a stratigraphy for the Birimian Supergroup in southern Ghana using a compilation of the growing geochronological data for the region. The stratigraphy comprises the basal Sefwi Group metavolcanics and mica schists (>2174 Ma), constrained by the syn-tectonic emplacement of the Sekondi granitoid (Oberthür, et al., 1998). The overlying Kumasi Group phyllites and volcanoclastic rocks were deposited between ca. 2154-2125 Ma, based on detrital zircon ages, syn-depositional volcanism and late-depositional granitoid intrusion (Adadey, et al., 2009;Oberthür, et al., 1998;Perrouty, et al., 2012).

Whilst the advent of precise geochronology has largely settled the stratigraphic debate, there are contrasting interpretations of deformation relationships. This includes suggestions of an early deformation event recorded by the lower meta-volcanic units that is not recognised in upper meta-sedimentary units, which suggests a polycyclic evolution for the Eburnean Orogeny (de Kock, et al., 2012;[START_REF] Feybesse | Le domaine de Toulepleu-Ity (Côte-d'Ivoire); une unité «birrimienne» charriée sur les gneiss archéens du domaine de Kénéma-Man lors des premiers stades de l' orogène éburnéen[END_REF]Ledru, et al., 1991;Perrouty, et al., 2012). This model suggests that there was an early orogenic event prior to the emplacement of the Birimian meta-sedimentary rocks. Authors suggest that high-grade terranes and some greenstone belts record elevated metamorphic conditions and polyphase deformation, interpreted as an older basement upon which meta-sedimentary units were deposited and subsequently deformed during the Eburnean Orogeny sensu stricto (s.s.) (e.g. [START_REF] Arnould | Étude géologique des migmatites et des granites précambriens du Nord-est de la Côre d'Ivoire et de la Haute-Volta méridionale: cadre géologique, classification, principaux types[END_REF]Lemoine, et al., 1990;Perrouty, et al., 2012). The age and terminology of the early deformation event differs throughout the Baoulé-Mossi Domain, including the "Burkinian" orogenic cycle in the Ivory Coast (2400 -2150 Ma) (Lemoine, et al., 1990), the "Tangaean" event of Burkina Faso (2170 -2130 Ma) (Hein, 2010;[START_REF] Tshibubudze | The Markoye Shear Zone in NE Burkina Faso[END_REF], or, more recently, "Eburnean I" (2266 -2150 Ma) (Allibone, et al., 2002) or "Eoeburnean" event in Ghana (2195 -2150 Ma) (Baratoux, et al., 2011;de Kock, et al., 2011;de Kock, et al., 2012;Perrouty, et al., 2012). Early deformation often corresponds with an initial phase of magmatic accretion and crustal thickening (e.g. Feybesse, et al., 2006;Milési, et al., 1992;Vidal, et al., 2009). Each of these events are interpreted to precede the tectonic assembly of the Paleoproterozoic and Archaean domains and transcurrent tectonism and greenschist facies metamorphism attributed to the Eburnean Orogeny s.s. (2130( -1980 Ma) Ma) (Feybesse, et al., 2006;Jessell, et al., 2012;Ledru, et al., 1991;Lompo, 2010;Oberthür, et al., 1998;Pitra, et al., 2010).

Monocyclic interpretations for the Eburnean Orogeny deem volcano-plutonic belts and metasedimentary basins to be contemporaneous, lateral facies equivalents or representative of crustal segments of varying depths. This interpretation requires greenschist facies metasedimentary provinces to be coeval, supracrustal equivalents to high grade metamorphic terranes, which were juxtaposed during later deformation (Block, et al., 2015;Block, et al., 2016b;Eisenlohr & Hirdes, 1992;Hirdes, et al., 1996;Hirdes, et al., 2007;Leube, et al., 1990;Opare-Addo, et al., 1993). These models suggest that deformation and metamorphism across the craton reflect a long-lived, progressive orogenic event (Block, et al., 2016b;Eisenlohr & Hirdes, 1992;Hirdes, et al., 2003). Given the variety of interpretations and discrepancies between the terranes, additional geochronologically-constrained structural and metamorphic studies are required in order to elucidate the spatial and temporal variations of deformation events across the craton.

Metamorphic record and tectonic style of the West African Craton

Metamorphism of the meta-volcanic and meta-sedimentary rocks within greenstone belts reveal predominantly low-grade metamorphism from greenschist to amphibolite facies (Galipp, et al., 2003;John, et al., 1999;Kříbek, et al., 2008). In contrast, metamorphism and migmatization of orthogneisses and paragneisses in high-grade terranes is associated with upper amphibolite to granulite facies conditions (Bessoles, 1977a;Block, et al., 2015;de Kock, et al., 2011;John, et al., 1999;Opare-Addo, et al., 1993), with high-pressure granulites predominantly restricted to the tectonic contact between the Archaean Kénéma-Man and Palaeoproterozoic Baoulé-Mossi domains (Pitra, et al., 2010). Whilst geochronological data is relatively limited, the timing of Eburnean metamorphism is constrained by U-Pb ages of metamorphic monazite, zircon and titanite in between 2110 and 2080 Ma (e.g. de Kock, et al., 2011;Oberthür, et al., 1998), with older paroxysmal metamorphism at ca. 2130 Ma proposed by Block, et al. (2015;2016b) based on metamorphic monazite and zircon ages. Sm-Nd garnet-whole rock isochron ages from central Ivory Coast are older still at ca. 2150 Ma (Boher, et al., 1992).

Analyses of strain patterns and metamorphic relationships have drawn a range of interpretations on the tectonic styles and geodynamic mechanisms responsible for the tectonic evolution of the West African Craton. Some authors interpret dome and basin geometries within the craton, which they suggest was associated with Archaean-like diapirism-related vertical tectonics, with late horizontal shortening purely accommodated by transcurrent shear systems (e.g. Lompo, 2010;Pouclet, et al., 2006;Pouclet, et al., 1996;Vidal, et al., 1996). In such studies, amphibolite facies metamorphism is attributed to contact metamorphism during granitoid emplacement (Debat, et al., 2003;Gasquet, et al., 2003;Pons, et al., 1995;[START_REF] Soumaila | Le metamorphisme des formations de la ceinture de roches vertes birrimienne (paleoproterozoique) de Diagorou-Darbani (Liptako, Niger, Afrique de l'Ouest)[END_REF]. In the case of the Sefwi Greenstone Belt, large tracts of regional amphibolite facies gneisses are peculiarly attributed to an underlying, unexposed K-rich pluton (Hirdes, et al., 1993). Alternative proposals for orogenic styles in West Africa include "modern-type" thrust-related tectonics and nappe stacking (Feybesse, et al., 2006;Feybesse & Milési, 1994;Milési, et al., 1992). Indeed, the cold (~15°C/km) apparent geothermal gradients deduced for rocks metamorphosed at the greenschist-blueschist facies transition are interpreted as evidence of modern-style subduction during the formation and assembly of the craton (Ganne, et al., 2012). Preservation of cold geothermal gradients is inconsistent with many studies which interpret deformation and metamorphism of the West African Craton as a weak, hot orogen (Caby, et al., 2000;Lompo, 2009;Pons, et al., 1995;Vidal, et al., 2009). The weak, hot orogen model consists of extensive magmatism, often diapirically emplaced, with subsequent lateral shortening associated with homogeneous thickening and late-orogenic development of large-scale shear zones. The high-pressure granulite facies metamorphism at the Archaean-Palaeoproterozoic boundary along the Sassandra Shear Zone in western Ivory Coast may be elucidated by either crustal thickening by unit stacking in a collisional setting or homogeneous thickening of a warm, weak crust (Pitra, et al., 2010). In NW Ghana, late-orogenic exhumation of amphibolite-granulite facies rocks occurs during orogenic collapse following crustal thickening, with the formation of anatectic domes resembling modern collisional orogens (Block, et al., 2016b). Given the diversity of tectonic models, deformation styles and distribution of metamorphic terranes proposed for the West African Craton, it is possible that the craton preserved spatial and temporal variations in the crust. Elucidation of such variations requires continued integration of structural, metamorphic and geochronological studies.

Geodynamic setting and crustal formation

Based on εNd (t) and εHf (t) data, the Palaeoproterozoic crust of the West African Craton displays a dominantly juvenile character (Abouchami, et al., 1990;Block, et al., 2016a;Boher, et al., 1992;Taylor, et al., 1992), with short crustal residence times. Recent studies, however, show a greater Archaean influence in southern Mali and far southeast Ghana (Parra-Avila, et al., 2016;Petersson, et al., 2017;Petersson, et al., 2016). The magmatic processes and geodynamic setting of juvenile crust formation are attributed to a number of scenarios including: an oceanic plateau setting (Abouchami, et al., 1990;Boher, et al., 1992); the accretion of multiple volcanic island arcs (Baratoux, et al., 2011;Dampare, et al., 2008;Pawlig, et al., 2006;Senyah, et al., 2016;Sylvester & Attoh, 1992); the development of a volcanic island arc system within an oceanic plateau setting (Boher, et al., 1992); or intra-cratonic rifting leading to the opening of oceanic domains that were subsequently inverted (Leube, et al., 1990).

Alternatively, [START_REF] Begg | Continental versus crustal growth: Understanding the paradox[END_REF] suggest that the West African Craton represents the amalgamation of Archaean-aged lower-lithospheric domains.

Geochemical studies reveal the bimodal nature of early volcanism across the craton, with a basal sequence of tholeiitic basalts overlain by calc-alkaline volcanic rocks noted in a number of greenstone belts across the domain (Abouchami, et al., 1990;Baratoux, et al., 2011;Dampare, et al., 2008;Hirdes, et al., 1996;Pouclet, et al., 2006;Senyah, et al., 2016). Lompo (2009) interpret tholeiitic volcanism as a mantle plume event, similar to the differentiated oceanic flood basalt analogy proposed by Abouchami, et al. (1990). Intrusive suites in the southern portion of the West African Craton are frequently described as tonalite-trondhjemite-granodiorite (TTG) series (Doumbia, et al., 1998;Vidal, et al., 2009), based on normative feldspar discrimination plots after Barker (1979). These intrusions have historically been divided into three main groups, as defined by the primary ferro-magnesian mineral and the host domain: 1) variably foliated, amphibole-bearing granitoids hosted in volcanic belts; 2) biotite-bearing granites hosted in meta-sedimentary "basins;" and, 3) potassic, alkaline biotite-muscovite granites and muscovite leucogranites (Baratoux, et al., 2011;Block, et al., 2016a;Egal, et al., 2002;Gueye, et al., 2008;Hirdes, et al., 1992;Leube, et al., 1990;Lompo, 2009;Oberthür, et al., 1998;Tapsoba, et al., 2013;Vegas, et al., 2008). Extensive geochemical characterisation of these intrusions reveal calc-alkaline affinities, negative Nb-Ta, Ti and P anomalies coupled with positive Pb anomalies, with diverse Eu and Sr anomalies and REE fractionation patterns (Baratoux, et al., 2011;Block, et al., 2016a;Egal, et al., 2002;Eglinger, et al., 2017;Feybesse, et al., 2006;Gueye, et al., 2008;Hein, 2010;Leube, et al., 1990;Petersson, et al., 2016;Tapsoba, et al., 2013;Vegas, et al., 2008), bearing striking similarities to classical arc geochemical signatures (Arculus, et al., 1999). TTG magmas and associated lavas are interpreted as the production of partial melting of the down-going slab in a subduction setting (Baratoux, et al., 2011;Peucat, et al., 2005;Pouclet, et al., 2006;Sylvester & Attoh, 1992), with high-pressure, low-HREE TTGs and LILE-enriched diorites in NW Ghana interpreted as the product of delamination of the lower crust and lithospheric mantle (Block, et al., 2016a).

Whilst these lines of evidence suggest that crustal accretion occurred primarily in a subduction setting, a number of characteristics of the craton do not conform with those of modern accretionary orogens. The major site of juvenile crustal growth on the modern Earth is associated with extensive magmatism in magmatic arcs forming along convergent plate margins [START_REF] Arculus | Aspects of magma genesis in arcs[END_REF]. Lateral accretion of juvenile terranes or mobile belts against older craton margins results in lateral crystallisation age gradients and isotopic boundaries [START_REF] Gastil | The distribution of mineral dates in time and space[END_REF][START_REF] Hoffman | United plates of America, the birth of a craton: Early Proterozoic assembly and growth of Laurentia[END_REF]Zeh, et al., 2009). In the West African Craton, significant volumes of juvenile crust form in an oceanic domain, distal to Archaean crustal influences. Furthermore, there are no clear age gradients preserved in sub-parallel greenstone belts emplaced between 2300 and 2180 Ma. The youngest juvenile crust is located in Kedougou-Kéniéba Inlier of Senegal, suggesting a westward gradient (Hirdes & Davis, 2002). Hirdes and Davis (2002) propose a major suture in central Ivory Coast, joining an older eastern sub-province with a 50-100 M.y. younger western sub-province. A ca. 2130 Ma suture between two crustal blocks is hypothesised by Parra-Avila, et al. (2017) for a similar location, based on a comprehensive geochronological transect across Burkina Faso, northern Ghana, southern Mali and eastern Guinea; however, the studies are not in exact agreement, making the extent of the suture somewhat ambiguous. Finally, the metamorphic record and litho-structural architecture do not preserve the same asymmetries, exotic crustal fragments, accretionary prism and metamorphic conditions as modern accretionary systems, thus convoluting the application of a subduction collision model for the West African Craton.

Synthesis

The greenstone-granite-gneiss terranes of the Palaeoproterozoic West African Craton bear a number of similarities to Archaean provinces. The lithological assemblage, craton-scale shear zones and dominant low-to medium-grade metamorphism are explained by a variety of geodynamic and tectonic models, reminiscent of both Archean and Phanerozoic processes. As such a number of questions arise when attempting to elucidate the geological evolution of the region. These include: What was the geodynamic setting in which the juvenile crust was generated and does it conform to the modern plate tectonic regime? What were the orogenic processes responsible for the amalgamation and stabilisation of such large areas of crust? What are the implications of these finding in understanding the evolution of geodynamic processes and lithospheric properties in the early Earth?

2 Research aims and rationale

Research aims

The ca. 2300-2070 Ma Palaeoproterozoic rocks of the West African Craton are presently underexplored, and poorly represented in the global literature regarding Proterozoic geodynamics. It represents a unique subset of the crustal rocks forme++d after the Archean-Proterozoic transition, with no significant reworking following the craton stabilisation. It thus provides an exceptional window into the geodynamic setting and orogenic processes responsible for juvenile crust formation and preservation and orogenic processes during an enigmatic period of the Earth's history. The Palaeoproterozoic Sefwi Greenstone Belt of SW Ghana in the West African Craton provides the area of focus for this thesis. The primary questions this thesis aims to address include:

1. What was the structural and metamorphic evolution of the Sefwi Greenstone Belt? 3. What was the geodynamic settings which allowed for the formation and preservation of large juvenile crust volumes of the West African Craton, following a hypothesised period of global tectonic quiescence? Do the geodynamic processes responsible for the formation of the West African Craton those active during the Archean or were they formed in a plate tectonicsetting? How do these processes differ from today?

The overarching aim of this thesis is to explore the geodynamic setting of juvenile crust formation as well as the orogenic processes preserved in the West African Craton to gain insight into the evolution of the plate tectonic regime early in the Earth's history. Given the temporally and spatially expansive nature of this aim, this thesis employs a multi-scale and multi-discipline approach combining field evidence, including regional structural and metamorphic mapping, with quantitative P-T-t constraints, geochemical and isotopic data. The following thesis outline addresses different aspects of the three primary aims, including the context within existing literature and the methods applied in order to explore different aspects of each question.

Thesis outline

This thesis is presented as a series of three manuscripts. As each chapter/manuscript is designed as a stand-alone publication, they each require general background information, therefore some repetition and overlap may exist between chapters. Chapter IV was submitted for publication in the Journal of Metamorphic Geology, and was returned for resubmission by May 2018 pending revisions. The manuscripts represented by Chapters III and V are ready for submission at Precambrian Research upon finalisation of the thesis.

Chapter 3: Transtension-related lower crust exhumation in the late stages of the

Palaeoproterozoic Eburnean Orogeny, SW Ghana: Evidence for diachronous assembly of the São Luís-West African Craton

A number of difficulties arise when applying a uniformitarian approach to understanding Archean and Proterozoic terranes, whereby their comparison to modern accretionary and collisional orogens contains a multitude of uncertainties due to ambiguous field relationships (e.g. Condie, 1981;de Wit, 1998), the absence or sporadic formation and preservation of plate tectonic indicators (Hamilton, 1998;Stern, 2005) and the influence of the warmer upper mantle temperatures in the early Earth (Gerya, 2014;Sizova, et al., 2014;[START_REF] Van Hunen | Continental collision and slab break-off: a comparison of 3-D numerical models with observations[END_REF]. The ca. 2150-2070 Ma Eburnean-Transamazonian Orogeny of the West African-Sao Luis Craton represents the first Palaeoproterozoic orogenic event during a period of global orogenesis between ca. 2100 and 1800 Ma, heralding the first Palaeoproterozoic supercontinent, Nuna (Betts, et al., 2016;[START_REF] Feybesse | Relationships between metamorphismdeformation-plutonism in the Archean-Paleoproterozoic contact zone of West Africa[END_REF]Rogers & Santosh, 2002;Zhao, et al., 2004). A number of conflicting tectonic models are invoked for the West African Craton, with the lack of consensus primarily resulting from extensive overprinting by late transcurrent tectonism, a pervasive greenschist facies metamorphic overprint, poor exposure and limited geochronological constraints. Existing tectonic models include crustal thickening through thrusting and tectonic stacking during subduction-driven lateral shortening (e.g. Feybesse, et al., 2006;Feybesse & Milési, 1994;Ledru, et al., 1994;Liégeois, et al., 1991), archaic dome and basin formation associated with granitoid diapirism and greenstone sagduction (e.g. Lompo, 2009;Lompo, 2010;Pons, et al., 1995), or, alternatively, homogeneous, distributed deformation and metamorphism attributed to a Proterozoic hot-orogen model (e.g. Chardon, et al., 2009;Vidal, et al., 2009).

The primary aim of this chapter was to develop a more detailed regional geological map of the Sefwi Greenstone Belt and the adjacent Sunyani-Comoé and Kumasi Afema Domains. It presents a synthesis of the geophysical signatures and petrophysical properties of lithologies within the study area, a new regional structural framework and the interpretation of regional airborne geophysical datasets. The integration of field observations, a structural framework and regional geophysical datasets constrained the interpretation of new litho-structural and metamorphic maps of the study area. Metamorphic constraints mentioned in this chapter are presented in Chapter 4, with detailed metamorphic petrography, quantitative metamorphic modelling and U-Pb monazite ages. The tectonic evolution of southern Ghana is discussed in relation to the deformation and metamorphism of granite-greenstone terranes of the West African Craton during the ~2150-2100 Ma Eburnean Orogeny. Finally, structures, strain patterns and distribution of tectono-metamorphic domains are compared to those of Archean and Phanerozoic orogen in order to better understand the characteristics, tectonic setting and style of Palaeoproterozoic orogenesis.

Chapter 4: Petrological and geochronological evidence of collisional orogenesis and lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny, SW

Ghana, West African Craton

Metamorphic conditions and gradients within and between different orogenic provinces inform geologists about the tectonic style, crustal architecture and thermal regimes that characterise individual orogens and tectonic settings. The metamorphic record of Phanerozoic subduction-related orogens, including blueschist facies rocks and ultra-high pressure (UHP) metamorphism contrasts the "ordinary" greenschist-to granulite facies metamorphism that dominates most Archean granitegreenstone provinces (Brown, 2006;Brown, 2007, and references therein). Indeed, the major changes in the metamorphic record at the Archean-Proterozoic and the Proterozoic-Phanerozoic transition coupled with changes in structural styles that are interpreted as major changes in the global tectonic regime (e.g. Condie, 1994a;Stern, 2007). Palaeoproterozoic greenstone-granite-gneiss terranes of the West African Craton have often been characterised by apparent homogeneous low-grade metamorphism and transcurrent tectonism attributed to the Palaeoproterozoic Eburnean Orogeny (ca. 2150 -2070 Ma) (e.g. Junner, 1935;Leube, et al., 1990), with amphibolite to granulite facies metamorphism often interpreted as the product of contact metamorphism (e.g. Caby, et al., 2000;Pons, et al., 1995;Vidal, et al., 1996;Vidal, et al., 2009). Small exposures of orthogneisses and paragneisses are sporadically preserved throughout the craton, with varying interpretations, ranging from a discordant gneissic basements prompting polycyclic models for the Eburnean Orogeny (e.g. de Kock, et al., 2011;Lemoine, et al., 1990), to the interpretation of gneissic and migmatitic textures as syn-tectonic granitoid emplacement (Gasquet, et al., 2003), with local migmatisation (Caby, et al., 2000). Alternatively, a number of studies have documented transitional greenschist-blueschist facies rocks indicative of cold apparent geothermal gradients (~15°C/km) (Block, et al., 2015;Ganne, et al., 2012), characteristic of modern subduction-related deformation. Proponents of monocyclic orogenesis during the Eburnean Orogeny suggest that low-grade terranes are the supra-crustal equivalents of high-grade rocks, metamorphosed during the same tectonic event and subsequently juxtaposed during structurally controlled exhumation (Block, et al., 2016;Hirdes, et al., 2007).

Existing metamorphic studies of greenstone belts in southern Ghana, including the Sefwi and Ashanti belts, suggest prevailing metamorphic conditions were greenschist to amphibolite facies (Galipp, et al., 2003;John, et al., 1999;[START_REF] Klemd | Metamorphism of the country rocks hosting gold-sulfidebearing quartz veins in the Paleoproterozoic southern Kibi-Winneba belt (SE-Ghana)[END_REF]. Thus far, regional scale upper greenschist to amphibolite facies metamorphism within the Sefwi Belt is poorly elucidated, with elevated metamorphic grades attributed to "the updoming of a large, hidden, underlying potassic pluton" (Hirdes, et al., 1993). This chapter aims to explore the metamorphism and tectonic contacts between tectono-metamorphic domains within the Sefwi Greenstone Belt relative to the adjacent low-grade volcano-sedimentary Sunyani-Comoé and Kumasi-Afema domains. Quantitative assessment of the metamorphic conditions are investigated through the construction of whole-rock pseudosections to constrain P-T paths in key locations within the study area. New constraints on the timing of metamorphism are provided by new in-situ SHRIMP U-Pb dating of metamorphic monazite. These constraints are used to understand spatial and temporal variations in the metamorphic conditions and relative timing of metamorphism between discrete terranes within the West African Craton. The results of this chapter provide insight into the monocyclic versus polycyclic nature of the Eburnean orogeny, aiding our understanding of the architecture and assembly of the West African Craton. Finally we aim to discuss the significance of these results in understanding Palaeoproterozoic geodynamics.

Chapter 5: Palaeoproterozoic juvenile crust formation in southern Ghana, West

Africa: New insights from igneous geochemistry and U-Pb-Hf zircon data

The mechanisms and growth rates of continental crustal genesis have undergone temporal changes since the Hadaean as a product of the Earth's cooling and differentiation. The most important period of change occurs at the end of the Archean Eon, between 3000 and 2500 Ma, by the end of which more than 50% of the Earth's continental crust had formed (Belousova, et al., 2010;Condie, et al., 2011;Dhuime, et al., 2012;Taylor & McLennan, 1985;[START_REF] Taylor | The geochemical evolution of the continental crust[END_REF]. Archean cratons are systematically characterized by greenstone belts, voluminous juvenile tonalite-trondhjemitegranodiorite granitoids, komatiites and banded-iron formations (Condie, 1994b;Martin, et al., 2005;Moyen, 2011;[START_REF] Sylvester | Archean granite plutons[END_REF], commonly bounded by craton-scale shear zones (De Wit & Ashwal, 1997). In contrast, crustal growth in the Phanerozoic is largely limited to accretionary orogens, characterized by calc-alkaline magmatism, lateral age gradients, ophiolite sequences and suturing of juvenile crustal ribbons or fragments onto plate margins (Cawood, et al., 2009, and references therein). The Palaeoproterozoic greenstone belts and granite-gneiss domains of the West African Craton displays both lithological and architectural similarities to Archean provinces, characterised by apparent lack of lateral age gradients and the coeval injection of juvenile TTGs across hundreds of kilometres. The magmatic processes and geodynamic settings for its formation, however, continue to be debated, with authors invoking an ocean plateau setting (Abouchami, et al., 1990;Boher, et al., 1992;Lompo, 2009) or the accretion of volcanic island arcs (Baratoux, et al., 2011;Dampare, et al., 2008;Senyah, et al., 2016;Sylvester & Attoh, 1992).

In this chapter, we investigate the geochemical and isotopic signature of magmatic rocks in the archetypal Sefwi Greenstone Belt. We define the major suites present in the study and use petrological and major and trace element analysis to define possible sources and petrogenetic processes active during crust formation. Geochemical analysis is complemented by zircon U-Pb and Lu-Hf isotopy to constrain the timing of magma emplacement, as well as the timing of magma extraction from the depleted mantle. We present our results within the broader context of magmatism in the West African Craton to discuss the crustal evolution of the craton. Finally, this chapter assesses the viability of various geodynamic settings for crustal growth in the context of the Archean-Proterozoic transition, including subduction-related settings, oceanic plateaus, or tectonically or magmatically over-thickened crust (e.g. Bédard, et al., 2003;Defant & Drummond, 1990;Laurent, et al., 2014;Moyen, 2011;Moyen & Martin, 2012;Smithies & Champion, 2000).

Chapter 6: Discussion and conclusions

This chapter provides a synthesis of the research aims and the significance of the results covered in this thesis. The discussion is composed of a brief summary of literature regarding the West African Craton, including the different geodynamic models proposed for the evolution and preservation of the craton. The significance of the metamorphism and deformation attributed to the Eburnean Orogeny is then considered in the context of the evolution of metamorphism throughout geologic time. We outline the unique nature of orogenic processes recorded in the craton during the Palaeoproterozoic at the commencement of a major period of global orogenesis, heralding the supercontinent era. The subsequent discussion explores the possible geodynamic setting for the formation of juvenile crust preserved in southern Ghana relative to Archean and modern geodynamic processes. Finally we discuss the significance of our findings in the West African Craton within the Palaeoproterozoic rock record in the context of secular cooling of the Earth and the transition to from Archean geodynamics to a modern plate tectonic regime.

Introduction

This chapter focuses on the lithologies and deformation sequence preserved in the Palaeoproterozoic Sefwi Greenstone Belt and adjacent tectono-metamorphic domains of SW Ghana in the West African Craton. We present new litho-structural and metamorphic maps of the study area, which are the product of the integration of field mapping, petrophysical data for individual lithologies and regional airborne geophysical data sets. The multi-scale structural investigation and incorporation of multiple datasets provides insight into the poorly understood structural history, tectonic relationships and processes of a sparsely exposed, yet archetypal greenstone belt of the craton. We consider the deformation style and history in the context of both existing tectonic models for the West African Craton and orogenic models proposed for the early Earth relative to today.

Metamorphic petrology, quantitative metamorphic estimates and metamorphic monazite U-Pb ages referred to in this chapter are discussed in detail in Chapter 4.

Introduction

The applicability of the plate tectonic paradigm and a uniformitarian approach when investigating accretionary and collisional orogenesis in the Precambrian is a continued source of controversy and debate. Modern subduction-related convergent settings are characterised by a number of tectonic and metamorphic features including ophiolites, accretionary mélanges, fold and thrust belts, blueschist facies rocks and ultra-high-pressure metamorphism (UHPM) (Chopin, 2003;[START_REF] Ernst | Alpine and Pacific styles of Phanerozoic mountain building: subduction-zone petrogenesis of continental crust[END_REF][START_REF] Matsuda | On the Pacific-type orogeny and its model-extension of the paired belts concept and possible origin of marginal seas[END_REF][START_REF] Miyashiro | Paired and unpaired metamorphic belts[END_REF]. The similarities in datasets between the ancient and modern rock record and tectonic processes are interpreted as evidence for the operation of plate tectonics by at least ca. 3000 Ma, based on the appearance of mantle wedge-related geochemical signatures, collages of accretionary-type terranes, palaeomagnetic data indicating ocean crust consumption, metamorphic and structural data indicating collisional orogenesis and numerical modelling of oceanic subduction (Cawood, et al., 2009;Cawood, et al., 2006;Condie, 2008;Condie & Benn, 2013;[START_REF] Pease | When did plate tectonics begin? Evidence from the orogenic record[END_REF][START_REF] Smithies | The Mesoarchean emergence of modern-style subduction[END_REF]. Alternatively, the absence of key tectono-metamorphic features, or differences in their appearance, in Precambrian terranes has prompted a number of authors to propose alternative geodynamic and orogenic processes in the early Earth (e.g. Bédard, et al., 2003;[START_REF] Hamilton | Plate tectonics began in Neoproterozoic time, and plumes from deep mantle have never operated[END_REF]Stern, 2005;Stern, 2007;van Hunen & van den Berg, 2008).

The continental record of the early Earth is characterised by high crustal growth rates (e.g. Dhuime, et al., 2012), archetypal granite-greenstone terranes (e.g. Condie, 1994;de Wit, et al., 1992) and warmer mantle temperatures (Herzberg, et al., 2010;[START_REF] Korenaga | Archean Geodynamics and the Thermal Evolution of Earth[END_REF]. Recently, a significant amount of research using numerical, analogue and thermo-mechanical modelling has focused on the role of elevated mantle temperatures and its influence on the mode and viability of subduction and orogenic processes (e.g. Ernst, 2009;Gerya, 2014;O'Neill, et al., 2007;Sizova, et al., 2014;Sizova, et al., 2010). The secular cooling of the mantle corresponds with changes in subduction behaviour, including episodicity and slab break off, as well as changes in lithospheric rheology [START_REF] Burov | Plume-like upper mantle instabilities drive subduction initiation[END_REF]Burov, 2011;Rey & Coltice, 2008;Rey & Houseman, 2006). This, in turn, is reflected in the metamorphic record and the strain patterns of ancient orogens (e.g. Bouhallier, et al., 1995;Brown, 2006;Brown, 2007;Choukroune, et al., 1995).

The Palaeoproterozoic rock record represents a unique period of the Earth's history defined by nascent or episodic plate tectonics and ambient mantle temperatures approximately 100 -200 °C greater than present day (Herzberg, et al., 2010;Korenaga, 2008a;Korenaga, 2008b). Whilst Archaean-like granite-greenstone terranes persist into the Proterozoic, significant changes are observed in the geochemical record at 2500 Ma (Keller & Schoene, 2012;Moyen, 2011) corresponding with the diversification of preserved metamorphic conditions including ultra-high-temperature metamorphism, high-pressure granulite facies metamorphism and rare formation and exhumation of eclogites between 3000 and 2000 Ma (e.g. Brown, 2006;Brown, 2007;Collins, et al., 2004). Elevated magmatism and crustal growth coincides with the development of collisional orogenesis in the Neoarchaean and Palaeoproterozoic, prompting the invocation of a hot-orogen model characterised by monotonous HT-LP metamorphic conditions and three-dimensional deformation during the compression of hot lithosphere (Cagnard, et al., 2006;Chardon, et al., 2009;Vidal, et al., 2009).

The Palaeoproterozoic (2300-2070 Ma) granite-greenstone terranes of the West African Craton (Abouchami, et al., 1990;Boher, et al., 1992) represents an enigmatic juvenile cratonic domain and a unique opportunity to explore orogenic processes in the early phases of these global transitions.

This study focuses on the tectonic evolution of the Palaeoproterozoic crust of southwest Ghana in the West African Craton, comprising the Sefwi Greenstone Belt and the adjacent Sunyani-Comoé and Kumasi-Afema meta-sedimentary domains. Newly interpreted lithological and structural maps of the region are presented, yielded by the integration of field observations and regional airborne geophysical data. Structural analysis of the region defines a new deformation sequence during the ca. 2100 Ma Eburnean Orogeny, with timing constraints interpreted from existing geochronological data. Field observations of metamorphic assemblages and foliations are presented in a new metamorphic map, which highlights tectonically juxtaposed high-grade domains with low-grade domains. The multiscale, multi-discipline approach provides insight into the tectonic styles and exhumation mechanisms preserved within the craton. Evidence presented in this study for the tectono-metamorphic history of the Palaeoproterozoic Transamazonian-Eburnean Orogeny of the São Luís-West African Craton reveals episodic collisional orogenesis during craton assembly. Finally, we discuss the implications of our findings relative to the Precambrian orogenic record and the secular evolution of metamorphism and thermodynamic properties of the lithosphere in the early Earth.

Geological Setting

The southern portion of the West African Craton (WAC) comprises the Archaean Kénéma-Man province (3600-2700 Ma) and the juvenile granite-greenstone terranes of the Palaeoproterozoic (2300-1980 Ma) Baoulé-Mossi Domain to the east and north (Bessoles, 1977) (Fig. 3.1a). The Baoulé-Mossi Domain is characterised by elongate volcanic-plutonic greenstone belts and volcanosedimentary basins of the Birimian Supergroup (Fig. 3.1), which are bounded by regional-to cratonscale shear zones (Junner, 1940;Leube, et al., 1990) and voluminous granite-gneiss terranes. Birimian volcanic and volcanoclastic sequences range in composition from basalt to rhyolite, with tholeiitic to calc-alkaline affinities (e.g. Baratoux, et al., 2011;Dampare, et al., 2008;Leube, et al., 1990;Pouclet, et al., 2006). Sedimentary rocks of the Birimian Supergroup include sequences of volcaniclastic greywacke, argillite, shale and chemical sediments, intercalated with intermediate volcanic layers (e.g. Leube, et al., 1990;Oberthür, et al., 1998). Greenstone belts are sporadically overlain by fluviodeltaic sedimentary sequences of conglomerate, quartzite and phyllite of the Tarkwa Group, which were deposited in narrow, fault bounded basins (Davis, et al., 1994;[START_REF] Eisenlohr | The structural geology of Birimian and Tarkwaian rocks of southwest Ghana. Republic of Ghana, Federal Republic of Germany[END_REF]. Multiple generations of Palaeoproterozoic granitoids are gneissic to undeformed, bearing both calc-alkaline affinities as well as geochemical similarities with the tonalite-trondhjemite-granodiorite (TTG) suite, with compositions ranging from dioritic to granitic (e.g. Doumbia, et al., 1998;Gasquet, et al., 2003;Hirdes, et al., 1996).

Prior to the advent of absolute geochronology, the stratigraphic relationship between Birimian metavolcanic and metasedimentary sequences drew a number of conflicting interpretations due to ambiguous, highly tectonised field relationships (e.g. Junner, 1935;Junner, 1940;[START_REF] Milési | West African gold deposits in their Lower Proterozoic lithostructural setting[END_REF]Pouclet, et al., 1996;Tagini, 1971;Whitelaw & Junner, 1929). U-Pb and Pb-Pb zircon crystallisation ages indicate volcanic activity occurred across the craton between ca. 2250 and 2180 Ma (Agyei Duodu, et al., 2009;Delor, et al., 1992;Hirdes & Davis, 1998;Hirdes & Davis, 2002;Hirdes, et al., 1996;Loh, et al., 1999;Oberthür, et al., 1998). Volcanism was considered to be partially contemporaneous with early granitoid emplacement (e.g. de Kock, et al., 2011;Feybesse, et al., 2006;Gueye, et al., 2008;Pawlig, et al., 2006;Tshibubudze, et al., 2013).

The Palaeoproterozoic rocks of Ghana and eastern Ivory Coast include, from south to north, N-to NE-striking volcano-plutonic greenstone belts of the Kibi-Winneba, Ashanti, Sefwi, Bui, Bole-Nangodi and Lawra belts. The intervening metasedimentary domains, often referred in to in the literature as basins, including the Kumasi domains, and the Sunyani-Comoé, or Haute-Comoé domain. A new stratigraphy for the Birimian Supergroup was recently established in southern Ghana by Adadey, et al. (2009) and Perrouty, et al. (2012), comprising Birimian metavolcanic rocks and subordinate mica-schists of the Sefwi Group, which were emplaced between ca. 2195 and 2170 Ma, and the volcano-sedimentary and sedimentary sequences of the Kumasi Group. Detrital zircon U-Pb ages from the Kumasi Group indicate deposition commenced at ca. 2150 Ma (Adadey, et al., 2009;Davis, et al., 1994;Oberthür, et al., 1998). The Kumasi Group is intercalated with andesitic layers dated at 2142 ± 24 Ma in the Kumasi Domain (Adadey, et al., 2009), with similar ages indicated for sedimentation in the Sunyani-Comoé Domain (Delor, et al., 1992;Hirdes, et al., 2007). Tarkwaian sediments comprise clastic material sourced from both the Sefwi and Kumasi Groups. The Tarkwaian sediments have a maximum age of deposition, constrained by U-Pb and Pb-Pb detrital zircons of 2132 ± 3 Ma (Davis, et al., 1994;Hirdes & Nunoo, 1994), with the youngest detrital age of 2090 ± 17 Ma (Pigois, et al., 2003). Tarkwaian units in the Ashanti Belt are intruded by granitoids at 2097 ± 2 Ma (Oberthür, et al., 1998). Voluminous amphibole and biotite-bearing, NaO-and CaO-rich TTG and calc-alkaline plutonism occurred between ~2190 and 2130 Ma (Gasquet, et al., 2003;Hirdes, et al., 1996;Tapsoba, et al., 2013). K 2 O-rich granitoids and young felsic lavas are emplaced between ~2130 and 2070 Ma in the west of the Baoulé-Mossi Domain in Senegal, along the Archaean-Palaeoproterozoic contact in the Ivory Coast, and to the southeast of the domain in southwest Ghana and southeast Ivory Coast (e.g. Egal, et al., 2002;Hirdes & Davis, 2002;Hirdes, et al., 1992;Petersson, et al., 2016).

Deformation of Palaeoproterozoic terranes of the Baoulé-Mossi Domain developed during the

Eburnean Orogeny (Bonhomme, 1962) between ca. 2150 and 1980 Ma (Davis, et al., 1994;Oberthür, et al., 1998). However, there is conjecture about the number and timing of deformation events, and the deformation mechanisms of different terranes (Feybesse, et al., 2006;Lemoine, et al., 1990;[START_REF] Milési | West African gold deposits in their Lower Proterozoic lithostructural setting[END_REF]Vidal, et al., 2009). Throughout the craton, Palaeoproterozoic metamorphism, attributed to the Eburnean Orogeny, is largely restricted to lower-greenschist to amphibolite facies (e.g. Béziat, et al., 2000;Feybesse, et al., 2006;Kříbek, et al., 2008). It has been proposed by some authors that peak metamorphism reached upper amphibolite facies within the contact aureole of plutons (e.g. Pons, et al., 1995). Regional medium-pressure, medium temperature (MP-MT) amphibolite facies metamorphism; however, prevailed in the Sefwi and Ashanti belts (Galipp, et al., 2003;John, et al., 1999), whilst rare, relic high-pressure, low-temperature (HP-LT) assemblages have been documented in NW Ghana (Block, et al., 2015) and eastern Burkina Faso (Ganne, et al., 2012), suggesting a greater diversity in metamorphic conditions than previously assumed. Feybesse et al. (2006) propose that an initial phase of the Eburnean Orogeny corresponds to magmatic accretion followed by partly contemporaneous deposition of flysch-type sequences of the Kumasi Group between 2135 and 2100 Ma. Synchronous Eburnean crustal thickening and medium-to high-grade metamorphism are attributed to thrust tectonism and lateral contractional deformation (Block, et al., 2015;Block, et al., 2016;Feybesse, et al., 2006;Feybesse & Milési, 1994;Ganne, et al., 2012), with subsequent transcurrent tectonism and greenschist facies metamorphism occurring across the craton between 2100 and 2070 Ma (Delor, et al., 1995;Feybesse, et al., 2006;Jessell, et al., 2012;Ledru, et al., 1991;Lompo, 2010).

Juxtaposed terranes with contrasting peak high-grade and low-grade conditions were interpreted to have undergone a concomitant evolution during monocyclic, progressive deformation (Block, et al., 2015;Block, et al., 2016;Galipp, et al., 2003;Hirdes, et al., 2007;Leube, et al., 1990;Loh, et al., 1999), and therefore represented different crustal depths.

Alternatively, some authors propose a two-phase orogenic evolution and consequently divide the orogenic event into the Eburnean I (2200-2150 Ma) and Eburnean II (2110-2088 Ma) deformation events, which were separated by a period of extension and sedimentation (Allibone, et al., 2002a).

These events are also referred to as the Eoeburnean and Eburnean, respectively (de Kock, et al., 2012;Perrouty, et al., 2012). They suggest deformation and metamorphism of high-and low-grade terranes is diachronous, with >2150 Ma gneissic crust interpreted as an older basement, indicative of a polycyclic orogenic history (de Kock, et al., 2012;Feybesse, et al., 2006;Hein, 2010;Lemoine, et al., 1990;Tshibubudze, et al., 2013;Vidal, et al., 2009).

The tectonic processes that generate and exhume high-grade terranes are an additional source of discussion. Some authors argue for unit stacking and nappe folding of rheologically strong continental crust during tectonic accretion, reminiscent of modern-style tectonic regimes (e.g. [START_REF] Billa | Banded ferruginous quartzite formations of the Nimba and Simandou ranges: Tectonically stacked units on an Archean plutonic 'basement' (Kenema-Man craton), during the Eburnean orogeny. Les formations a quartzites rubanes ferrugineux des Monts Nimba et du Simandou : Des unites empilees tectoniquement, sur un 'soubassement' plutonique Archeen (craton de Kenema-Man)[END_REF][START_REF] Fabre | Le Protérozoïque inférieur (Birimien) du centre de la Côted'Ivoire: évolution tectonique et corrélations[END_REF]Feybesse, et al., 2006;[START_REF] Milési | West African gold deposits in their Lower Proterozoic lithostructural setting[END_REF]. Others have argued that increased metamorphic grade is localised to pluton aureoles and found in association with dome and basin strain patterns (Pons, et al., 1995;Vidal, et al., 2009). Alternatively, exhumation of migmatitic rocks is interpreted as the product of buoyancy-driven ascent in a rheologically weak, hot orogenic crust, following homogeneous and distributed crustal thickening and extensive granitoid emplacement (e.g. Caby, et al., 2000;Ganne, et al., 2014;Pouclet, et al., 1996;Vidal, et al., 2009).

Study area

Litho-structural domains

The geology of southwest Ghana and southeast Ivory Coast comprises the NE-striking Sefwi-Greenstone Belt, which contains a diverse range of metamorphic rocks from greenschist facies to migmatitic packages, flanked to the NW and SE by two vast, low-grade meta-sedimentary rocks of the Sunyani-Comoé and Kumasi-Afema Domains (Fig. 3.1b) (Agyei Duodu, et al., 2009;Feybesse, et al., 2006;Hirdes, et al., 2007). These domains are often referred to as basins in the literature; however, given the pervasive deformation and metamorphism of the meta-sedimentary units and the unknown nature of both the basement and their tectonic contacts, we hereon in refer to them as tectono-metamorphic domains. Discrete domains are bounded and intersected by ENE-, NE-or ~N-striking shear zones.

High-grade domains of the Sefwi Belt

The Chiraa Domain (CD) is a triangular area in the northeast of the study area, reaching ~30 km in width. It is included as a subdomain of the Sefwi Belt due to metamorphic and structural similarities.

The Chiraa Domain comprises highly heterogeneous lithological packages of paragneisses, rare granite orthogneisses and hornblendites that are extensively intruded by foliated two-mica granites and pegmatites. Paragneisses are often observed as elongate rafts and are sometimes migmatitic. They frequently preserve stratigraphic layering with metamorphic assemblages and textures indicating greywackes, pelitic or volcaniclastic protoliths. Metamorphic petrology and quantitative estimates of metamorphic conditions recorded by the domains are detailed in Chapter 4. This contrasts with previous mapping, which classifies the Chiraa Domain as a homogenous granite batholith (Agyei Duodu, et al., 2009;Hirdes, et al., 1993). A biotite-muscovite granite and a muscovite pegmatite in the Chiraa domain yielded zircon U-Pb crystallisation ages of 2093 ± 2 and 2093 ± 2 Ma (Petersson, et al., 2016), respectively, within error of a previous zircon U-Pb age of 2092 ± 2 Ma (Agyei Duodu, et al., 2009) in the north of the domain. The Chiraa Domain is bounded to the west by a poorly exposed, west-dipping NNE-striking shear zone that is rotated into an east-west orientation along its south-dipping southern margin. The eastern margin of the domain is unconformably overlain by Neoproterozoic sediments of the Volta Basin.

The Kukuom-Juaboso Domain (KJD) is a medium-to high-grade domain that forms the core of the Sefwi Belt. This domain is rhomboidal in shape and is 150km in length and 45km at its widest point. The core of the domain is dominated by poly-deformed upper-greenschist to amphibolite facies mafic to intermediate amphibolites and quartz-rich meta-greywackes and metamorphosed volcanosedimentary rocks. It is extensively intruded by multiple generations of granodiorite, granite and maficultramafic plutons. However, very few geochronological dates have been published. Granodiorite intrusions in the north-west of the domain were emplaced at ca. 2135 Ma (Amponsah, 2012). The ENE-striking Hwidiem Shear Zone separates the Kukuom-Juaboso Domain from garnet amphibolites and weakly foliated to gneissic volcaniclastic packages, quartzites and meta-cherts of the Hwidiem Domain (HD). The Hwidiem Shear Zone contains migmatitic mafic orthogneisses characterised by fluid-present melting (detailed in Ch. 4). The Bechem Granitoid Domain (BGD) is dominated by weakly deformed granitoids with compositions varying significantly from quartz diorites to garnet-biotite granites and late potassic biotite granites (Hirdes, et al., 1993). Pyroxenite, gabbro and norite intrusions also form much of the domain displaying a strong spatial associated with high strain shear amphibolite gneiss. Whilst little dating has been undertaken within the domain, a greenschist facies meta-rhyolite flow on the southern margin of the domain yield a U-Pb zircon age of 2189 ± 1 Ma (Hirdes & Davis, 1998). Southeast of the rhyolite, strongly sheared Tarkwa-like conglomerates with abundant rounded quartzo-feldspathic lithics are observed along the tectonic contact between the Sefwi Belt and the Kumasi-Afema Domain.

Low-grade domains of the Sefwi Belt

The NE-SW striking Sefwi-Wiawso volcano-plutonic domain (SWD) represents the southern portion of the Sefwi Belt and extends for ~200 km from Bibiani in southwest Ghana into south-eastern Ivory Coast (Hirdes, et al., 2007;Hirdes, et al., 1993). The domain comprises greenschist facies tholeiitic to intermediate volcanic rocks, including pillow basalts, with minor volcaniclastic and volcanosedimentary sequences. It is extensively intruded by granites that range in compositions from quartz diorite, monzodiorite, granodiorite and tonalite to biotite granite. These granites were emplaced between 2178 and 2136 Ma (Agyei Duodu, et al., 2009;Hirdes, et al., 1992;Hirdes, et al., 2007), pene-contemporaneous with the emplacement of felsic volcanic and volcaniclastic rocks that have ages between ca. 2180 and 2166 Ma (Hirdes, et al., 2007). The domain is bound to the north by the Ketesso high strain zone, to the east by the NNE-SSW-striking Bibiani and Chirano shear zones. The Sefwi-Wiawso Domain is juxtaposed with the Kumasi metasediments at its southern boundary along the Afema high strain zone (Hirdes, et al., 2007). Immature, polymictic conglomerates occur at the easternmost boundary of the domain bound by the Bibiani shear zone (Agyei Duodu, et al., 2009).

The detrital material is sourced from both the volcano-plutonic rocks and the adjacent low-grade metasedimentary domains (Hirdes, et al., 1993).

Low-grade meta-sedimentary domains/basins

The Sunyani-Comoé and Kumasi-Afema meta-sedimentary domains coincide with the poorest outcrop conditions in the study area. These domains comprise isoclinally folded volcaniclastic rocks, wackes, and argillites, intercalated with felsic to intermediate volcanic and volcaniclastic rocks (Hirdes, et al., 1993). The Sunyani-Comoé Domain has a greater abundance of turbiditic sequences, whilst the Kumasi-Afema Domain is suggested to comprise a more significant volcaniclastic rock component (Hirdes, et al., 1993). Initiation of sedimentation in the domains is constrained by a 2142 ± 24 Ma U-Pb zircon crystallisation age of an andesitic layer that is intercalated with metasedimentary rocks in the Kumasi Basin (Adadey, et al., 2009). Metasedimentary rocks of the Kumasi-Afema Domain are intruded by numerous granodiorite and granite plutons between 2136 and 2090 Ma (Adadey, et al., 2009;Hirdes, et al., 1992). Within the Sunyani Basin, elongate, rounded leucogranite intrusions flank the Sefwi Belt to the northwest. Granites and leucogranites emplaced in the Sunyani-Comoé domain parallel to the NW margin of the Sefwi Belt decrease in age from the northeast to southwest. U-Pb zircon crystallization ages of 2088 ± 1 Ma and 2081 ± 1 were yielded by the muscovite-bearing Kawtiago granite and the Apouasso granite in southeast Ivory Coast (Delor, et al., 1992;Hirdes, et al., 1992;Hirdes, et al., 2007), with corresponding granite emplacement ages of ca. 2093 Ma in the neighbouring Chiraa Domain to the northeast (Agyei Duodu, et al., 2009;Petersson, et al., 2016).

Major Shear Zones

The crustal architecture of the study area is defined by the NE-striking Sefwi shear system (Jessell, et al., 2012), which extends >300 km from central Ghana to southeast Ivory Coast. The southernmost Afema shear zone marks the tectonic contact between the Sefwi-Wiawso volcano-plutonic domain and the Kumasi-Afema meta-sedimentary domain. Delor, et al. (1992) document NWdirected thrust movement along SE-dipping portions of the Afema Shear Zone in southeast Ivory Coast. The Ketesso high-strain zone transects the length of the Sefwi Belt, from central Ghana to south-east Ivory Coast, covered by Neoproterozoic sediments to the northeast and Miocene-Pliocene sediments to the southwest (Hirdes, et al., 2007). It separates the Sefwi-Wiawso and Kukuom-Juaboso domains, forming the southern extent of the Bechem granitoid domain (Hirdes, et al., 2003;Jessell, et al., 2012). Domains of the Sefwi Belt are bounded to the north by the Kenyase-Yamfo shear zone, comprising reactivated sub-vertical and south-dipping shear zones. Within the Kukuom-Juaboso domain, early shear zones are folded around ENE-WSW striking structures, with the Hwidiem shear zone truncating on the Kenyase-Yamfo shear zone. Both the Kenyase-Yamfo and the Ketesso shear zones are transected by discrete, late NNE-striking sinistral faults, the latter being intersected by the extension of the Bibiani shear zone, forming the contact between the Sefwi-Wiawso domain and the Kumasi-Afema meta-sedimentary rocks.

Methodology and data

Methodology

The new interpreted geological map of the study area integrates traditional mapping techniques with regional geophysical data sets. Outcropping geology was exceedingly sparse due to the presence of an extensive, thick laterite and saprolite sequence (5 to >40m thick) and significant areas of densely vegetated forest reserves covering the study area, precluding continuous mapping. Interpretation of potential field, radiometric and remote sensing data was constrained by lithological, structural and petrophysical information acquired at 484 outcrop localities. This data was complemented by outcrop maps and observations from Hirdes, et al. (1993), as well as outcrop databases from mining companies operating in the field area (Kinross and Newmont).

Processing and interpretation of geophysical data

Datasets used included regional airborne magnetic and radiometric data covering SW Ghana were lines that were spaced 500m apart and collected from an elevation of 152 m (Fig. A1SI). Raw data was processed using Geosoft Oasis Montaj software and used for regional interpretations. Processed images are shown in supplementary files (Supplementary Material A; Fig. 2SI-4SI).

Aeromagnetic data

Airborne magnetic surveys provide us with a map of magnetic anomalies from which we can derive lithological, structural and kinematic information, as constrained by field observations [START_REF] Aitken | Multi-scale integrated structural and aeromagnetic analysis to guide tectonic models: An example from the eastern Musgrave Province, Central Australia[END_REF][START_REF] Betts | Kinematic analysis of aeromagnetic data: Looking at geophysical data in a structural context[END_REF][START_REF] Lindsay | Categorising features of geological terranes with geodiversity metrics: Enhancing exploration of multiple geological models[END_REF][START_REF] Stewart | Multi-scale analysis of Proterozoic shear zones: An integrated structural and geophysical study[END_REF]). The regional airborne magnetic data covering SW Ghana was gridded as a composite residual magnetic intensity (RMI) grid with a resolution of 100 m. The residual magnetic intensity data covering the Ivory Coast was gridded to a cell size of 125 m. Conventional reduction of data to the magnetic pole (RTP) becomes unstable near the equator due to the low inclination of the Earth's magnetic field (Li, 2008;MacLeod, et al., 1993), simultaneously generating north-south trending artefacts and inaccurately representing true north-south structures or anomalies [START_REF] Beard | Detection and identification of north-south trending magnetic structures near the magnetic equator[END_REF] (Fig. A1SI). Both residual magnetic intensity datasets covering SW Ghana and the Ivory Coast and the A1 total magnetic intensity (TMI) grid were reduced to the equator (RTE), with a 180-degree phase reversal or inversion (negative RTE) (Fig. ASI3) according to the method of [START_REF] Luo | reduction to the pole at the geomagnetic equator[END_REF]. Interpretation of lithology distribution relied on both the negative RTE and analytical signal grids (MacLeod, et al., 1993). Filters included the first vertical derivative, tilt derivative and amplitude normalisation (Automatic Gain Control) aided extraction of structural information and lithological boundaries by highlighting magnetic features and steep gradients in the magnetic dataset [START_REF] Gunn | Interpreting aeromagnetic data in areas of limited outcrop: an example from the Arunta Block, Northern Territory[END_REF][START_REF] Milligan | Enhancement and presentation of airbourne geophysical data[END_REF][START_REF] Pilkington | The utility of potential field enhancements for remote predictive mapping[END_REF][START_REF] Verduzco | New insights into magnetic derivatives for structural mapping[END_REF].

Radiometric (gamma-ray) data

Gamma-ray spectrometry data shows the content and distribution of radioactive elements in the rocks and regolith in the uppermost 30cm of the Earth's crust [START_REF] Minty | Calibration and data processing for airborne gamma-ray spectrometry[END_REF][START_REF] Wilford | Apllication of airborne gamma-ray spectometry in soil/ regolith mapping[END_REF].

Combined use of topographic information and geochemical data highlights lithology distribution and laterite and regolith cover [START_REF] Dickson | Interpretation of aerial gamma-ray surveys-adding the geochemical factors[END_REF][START_REF] Wilford | Apllication of airborne gamma-ray spectometry in soil/ regolith mapping[END_REF], applied during similar mapping in Burkina Faso (Metelka, et al., 2011) and NW Ghana (Block, et al., 2016) and SW Ghana (Perrouty, et al., 2012). U, Th, and K bands were gridded individually at 100 m resolution (Fig. A4SI).

The bands are combined in a ternary RBG (Red-Green-Blue) diagram and draped over a shaded image of the digital elevation model (90m resolution; Shuttle Radar Topographic Mission, SRTM, 2000). Band ratio (U/K and Th/K) and Principal Component Analysis (PCA) images were derived from the original bands. Relative abundance ratios were used to distinguish changes in rock type and alteration.

Gravimetric data

Gravity data is frequently utilised in 2D and 3D geophysical modelling for constraining the geometry of lithological bodies and structures at depth (e.g. [START_REF] Attoh | Structure, gravity models and stratigraphy of an early Proterozoic volcanic-sedimentary belt in northeastern Ghana[END_REF][START_REF] Direen | Architecture of Proterozoic shear zones in the Christie Domain, western Gawler Craton, Australia: Geophysical appraisal of a poorly exposed orogenic terrane[END_REF] Gravimetric data was sourced from the International Gravimetric Bureau (http://bgi.omp.obs-mip.fr/) as EGM08 Free Air and Bouguer anomaly grids. The Bouguer anomaly map (Fig. 2SI) shows long wavelength anomalies is most effective at differentiating regional rock domains, large lithological packages and the extent of major structures along strike.

ALOS PALSAR data

Advanced Land Observing Satellite; Phase Array type L-Band Synthetic Aperture Radar highresolution data (ALOS PALSAR data) collected in 2009 (downloaded from http://www.eorc.jaxa. jp/ALOS/en/) was used as a complementary topographic information dataset to the SRTM. Two polarisation channels (HH + HV) were gridded with a resolution of 25m with a third band ((HH +HV)/2) used in a ternary RGB diagram. Unlike its application in the neighbouring Ashanti belt (Perrouty, et al., 2012), limited structural and detail could be gleaned from this dataset.

Lithological associations and geological map

The geophysical datasets were integrated within a GIS software for interpretation, according to the method detailed by Metelka, et al. (2011). Field observations, including lithological and petrophysical data, were used to constrain lithology distribution, complemented by radiometric and topological maps. Field structural relationships were used to constrain the interpretation of gradients and signals in the processed magnetic data images. Excellent correlation was observed between the structure and topology of lithologies and magnetic fabrics in airborne magnetic data. Integration of field data, petrophysical data and geophysical signatures were used to characterise the principal lithological associations (Table 1) in the new litho-structural map, as outlined below. Measured magnetic susceptibility values for each unit are presented in Appendix B. Gravimetric data facilitated the demarcation of major domains with high anomalies following the greenstone belt and adjacent metasedimentary domains corresponding with moderate to low gravity response, with no visible gradients at the domain contacts. Further interpretation of the gravity data was precluded by the low resolution.

Values for magnetic anomalies are derived from the negative RTE image colour bar legend, and, as a product of data processing, are given as negative integers.

Lithologies

Basalt/metabasalt/metadolerite

Metabasalts and metadolerites in the field area have been metamorphosed under greenschist facies to amphibolite facies with foliations indicating varying intensity of deformation, often depending on proximity to larger structures such as faults and regional shear zones. Garnet-bearing amphibolites are exposed in the Kukuom-Juaboso domain. Greenschist facies pillow basalts are observed in the Sefwi-Wiawso domain are weakly deformed and pervasively altered, sub-greenschist facies metabasalts. In the airborne magnetic surveys, mafic volcanic rocks correspond with high-amplitude, high-frequency magnetic anomalies, The anomalies are elongate and curvilinear, ranging between 10 and 40 km in length, with bimodal amplitudes of -618 to -509 nT (red hues corresponding to high magnetic anomalies) and -816 to -745 nT(blue hues corresponding to low amplitude), becoming more heterogeneous in proximity to major structures indicating both magnetite growth and destruction during fault movement. They are characterised by a strong magnetic fabric, displaying excellent correlation with S 1 foliation measurements suggesting the magnetic data is imaging the earliest foliation. They have a multi-modal magnetic susceptibility range from 0.05 to 13.08-84 x 10 -3 SI.

The dark radiometric signal of basalts indicated depletion in incompatible elements; however, units located along shear zones on domain boundaries show evidence of potassic alteration.

Meta-andesite/dacite

Significant areas of andesitic and dacitic rocks occur both within both the Kukuom-Juaboso and Sefwi Wiawso domains of the Sefwi Belt as well as rare intercalated sills in the adjacent metasedimentary domains. Units in the Sefwi-Wiawso domain are variably deformed and metamorphosed under greenschist facies conditions and are often associated with juvenile metasediments. Intermediate lavas are associated with a homogeneous to slightly stippled, low to moderate amplitude magnetic anomalies.

They are consistently lower amplitude (-710 to -620 nT) and often longer wavelength in their magnetic response relative to the metabasalts. Late andesitic dykes in the Kukuom-Juaboso domain are observed cross-cutting earlier structures. The meta-andesitic layers have a bimodal magnetic susceptibility of 0.03-0.40 x 10 -3 SI and 7.41-39.50 x 10 -3 SI. The radiometric response ranges from low to high intensity, often featuring a magnetic fabric. The units are often associated with distinct U enrichment relative to Th and K in the radiometric data.

Rhyolite/felsic volcaniclastic rocks/ meta-rhyolite

Rhyolites and felsic volcanoclastic and pyroclastic units are found in the Sunyani-Comoé and Kumasi-Afema domains as narrow layers, parallel to stratigraphic layering in schists derived from volcanosedimentary rocks. These felsic lava flows and volcaniclastic layers correspond with discrete, elongate bimodal, high-and low-amplitude magnetic anomalies (-783 to -750 nT; -618 to -585 nT), interpreted as parallel to stratigraphic layers and volcanic flows.

Magnetic susceptibilities tend to be low (< 0.60 x 10 -3 SI); however, more magnetite-rich felsic volcanoclastic layers have high magnetic susceptibility (6.25-29.14 x 10 -3 SI). Their radiometric signal reveals high K and Th content relative to U.

Volcaniclastic/pyroclastic rocks

Immature, volcaniclastic and pyroclastic rocks with subordinate epiclastic meta-greywacke units and rare iron-rich quartzites cover significant proportions of the Kukuom-Juaboso and Hwidiem domains. This unit is metamorphosed at upper greenschist to amphibolite facies conditions. Foliations vary from biotite defined schistose to gneissic, with alternating quartz and biotite/amphibolite bands.

Garnet is a common mineral; however, aluminosilicates were not observed. Textural and petrological evidence indicates water-present melting occurred along early shear zones. The magnetic response of the volcaniclastic metasedimentary rocks indicates a heterogeneous distribution of magnetite in alternating highly magnetic and non-magnetic layers, with amplitudes of -585 to -509 nT and -816 to -760 nT, respectively, with short, high-frequency wavelengths. The strong magnetic fabric shows excellent correlation with measured S 0 and S 1 measurements and is therefore used to extrapolate S 1 trajectories. Magnetite-rich strata well define structures within this unit, and excellent correlation was observed between the earliest foliation defined by biotite and/or amphibole and the magnetic fabric displayed in the processed aeromagnetic data. The magnetic susceptibility of this rock was bimodal, with values ranges from low (< 1 x 10 -3 SI) to extremely high (20.01-80.30 x 10 -3 SI). The radiometric signal indicates Th enrichment relative to U and K, commonly masked by regolith cover, indicating extensive weathering.

Volcano-sedimentary schist

The volcano-sedimentary schist unit comprises greywackes, argillitic schists, graphitic shale and

Composite gneiss

Large proportions of the Chiraa domain are composed of paragneisses, rare granitic orthogneisses, varying proportions granitic and trondhjemitic melts and pegmatite dykes. The gneissosity in the paragneiss units is defined by alternating mica-rich and quartz-plagioclase-rich bands. Varying modal proportions garnet and micas between layers suggests stratigraphic variability, or possibly bedding, with aluminosilicate minerals limited to rare Al-rich layers. The observed gneissosity in the paragneiss is parallel to the stratigraphic variations, as is the weak gneissosity or biotite foliation in some granite bodies. The magnetic response of the composite gneiss is associated with low-amplitude (-816 to -765 nT), long wavelength anomalies and a weak magnetic fabric that is often truncated by linear responses associated with structural or lithological contacts. Minor proportions of the domain are characterised by a short wavelength, higher magnetic signal (-618 to -550 nT) defining a stronger magnetic fabric, possibly indicating heterogeneous magnetite concentration in stratigraphic layers. Measured magnetic susceptibility values of paragneisses and granites are predominantly low (0.04-1.30 x 10 -3 SI). The radiometric signal indicates moderate U, Th and K enrichment. A similarly heterogeneous unit is mapped in NW Ghana (Block, et al., 2016), thus the term "composite gneiss" has been adopted for this study.

Mafic orthogneiss

Mafic orthogneisses are high strain units exposed along shear zones marking domain boundaries including along the Hwidiem shear zone and the southern margin of the Chiraa domain. The migmatitic mafic orthogneiss in the Chiraa domain has a metamorphic mineral assemblage of garnet and clinopyroxene with plagioclase-rich leucosomes indicating metamorphism at high-pressure amphibolite-granulite transition. Orthogneisses along the Hwidiem shear zone contain hydrous peritectic minerals, including hornblende, suggesting congruent, fluid-present melting (Weinberg & Hasalová, 2015, and references therein). The magnetic response of the mafic orthogneiss is generally characterised by high-amplitude (-585 to -509 nT), elongated anomalies, often seen subparallel to D 1 and D 2 shear zones; however, amplitudes below -750 nT are associated with migmatitic mafic orthogneisses containing high melting proportions in the western section of the Hwidiem Shear Zone. This reflects magnetite destruction during high-grade metamorphism with associated fluid flow. The measured magnetic susceptibility of mafic orthogneisses is bimodal with values ranging from 0.12 to 9.58 x 10 -3 SI.

Gabbro-pyroxenite-norite

This unit contains a variety of lithologies, including variably deformed hornblende-bearing gabbros, norite and clinopyroxene-rich pyroxenite displaying extensive replacement by hornblende, also associated with layered, hornblende-bearing mafic and dioritic intrusive sequences. Pyroxenites located along Hwidiem shear zones display interconnected plagioclase-quartz melt veins. These lithologies are limited to the Kukuom-Juaboso, Bechem and Hwidiem domains in the central Sefwi Belt. These units have a highly irregular magnetic texture with low frequency, low amplitude (-784 to -683 nT) anomalies and a poorly developed magnetic fabric, increasing in strength along faulted margins. They are characterised by low to moderate magnetic susceptibility (0.27 ̶ 2.48 x 10 -3 SI) and gamma ray emissions indicate a strong depletion in incompatible elements. Where more strongly deformed, the radiometric signal indicates high U and Th relative to K, indicating extensive lateritic or regolith cover [START_REF] Dickson | Interpretation of aerial gamma-ray surveys-adding the geochemical factors[END_REF]. This intrusive complex has not been directly dated; however, their chronology relative to the granitoid series is described below.

Granitoids

(1) G1. Trondhjemite, tonalite and rare granodiorite plutons contain biotite as the main ferromagnesian phases, with minor amphibole. They are foliated and locally migmatitic. K-feldspar is present in (micro) granodiorite plutons. G1 granitoids have a low amplitude (-810 to -706 nT), moderate frequency response in the aeromagnetic data and a stippled magnetic texture. Magnetic susceptibility is multi-modal, ranging from 0.15-25.30 x 10 -3 SI, and are associated with a welldefined magnetic fabric. Their gamma-ray emissions indicate a heterogeneous depletion in radioactive elements.

(2) G2. These granitoids comprise biotite-bearing, foliated to gneissic trondhjemite and tonalite plutons, with subordinate to rare amphibole. G2 granitoids are rounded to elongate and have a smooth, low to moderate (-783 to -684 nT) magnetic response. Magnetic susceptibility values ranging from 0.12 -1.03 x 10 -3 SI. The relative chronology of G1 and G2 granitoids is deduced from overprinting relationships observed in airborne magnetic surveys, where the magnetic fabric visible within G1 granitoids is truncated along the margins of the smooth, long wavelength, low amplitude responses of G2 granitoids, which suggests relative geochronology of G2 granitoids post-dating G1 granitoids. G2 granitoids in the Bechem Granitoid Domain displayed pervasive silica alteration and magnetite growth proximal to shear zones. Gamma-ray emissions indicate an overall depletion in radioactive elements and relative enrichment in Th. The 50km long Tano River trondhjemitic batholith, located immediately north of Wiawso, is associated with a large low response in the gravity data.

(3) G3a. Small, irregular elliptical bodies of calk-alkaline, coarse-grained, equigranular amphibolebearing granodiorites and quartz diorites, with subordinate biotite, titanite and K-feldspar, often observed as interlocking bodies. The magnetic signal is sub-elliptical in shape with a heterogeneous, high frequency, low to high amplitude (-717 to -651 nT) magnetic response. This unit has a strong spatial association with G3b granitoids; however, they have distinctly different radiometric signals. Their radiometric signal indicates heterogeneous Th enrichment relative to U and K, with other areas indicating depletion in all incompatible elements. The magnetic susceptibility of these granitoids is bimodal, with values ranging from low (< 1 x 10 -3 SI), to locally highly magnetic (4.25 -24.04 x 10 -3 SI).

(4) G3b. These granitoids are porphyritic potassic granites with abundant biotite and K-feldspar. They contain minor amphibole and accessory titanite. They are characterised as elliptical, sometimes, slightly elongate, with a distinct radiometric signal indicating enrichment in K relative to Th and U.

The magnetic response of G3b granitoids is distinguished from G3a granitoids due to its moderate amplitude (-637 to -600 nT) and a more stippled texture, suggesting a heterogeneous distribution of magnetite. G3b granitoid magnetic susceptibility values are low to moderate (0.47-3.88 x 10 -3 SI) and display a weak to well-defined magnetic fabric in airborne magnetic data.

(5) G4. These leucogranites contain muscovite, +/-biotite, and are often associated with coarsegrained, muscovite-rich pegmatitic bodies. Airborne magnetic data reveals elongate (~ 30 -40 km), elliptical bodies parallel to the NW-striking tectonic contact between the Sefwi Belt and the Sunyani-Comoé Domain. The magnetic response of G4 granitoids is characterised by smooth, low amplitude (-816 to -750 nT), and long wavelength (>10 km) magnetic responses. Rounded batholiths intruding the Kumasi-Afema Domain are structurally ambiguous in the potential field data. Their magnetic susceptibility values are homogeneously low (< 0.15 x 10 -3 SI). The radiometric signal for G4 indicates enrichment in K and Th, relative to U. Gamma-ray emissions of large pegmatite bodies indicate enrichment in all incompatible elements. These bodies share similar petrological and petrophysical characteristics to ME3 of Baratoux, et al. (2011) and Metelka, et al. (2011) and G5 of Block, et al. (2016).

Chemical sediment (Chert)

Chemical sediment in the field area is largely composed of massive or banded chert, located along the north-western and far western margins of the Kukuom-Juaboso and Hwidiem domains, within the Sefwi Belt. The magnetic response of the chert is associated with asymmetrical, elongate (~5km), low amplitude (-783 to -771 nT), irregular anomalies. Magnetic susceptibility values are generally low (0.35-1.59 x 10 -3 SI) and the radiometric signal indicates a depletion in all incompatible elements.

Polymictic conglomerate and sandstone

Two types of polymictic conglomerate and sandstone units are observed in the field area, confined to the Ketesso Shear Zone and the Bibiani Shear Zone. Where exposed along the Ketesso Shear Zone, this lithology comprises strongly sheared sandstone, siltstone and matrix supported conglomeratic beds with intensely sheared, rounded quartzo-feldspathic clasts. Conglomerates located along the Bibiani Shear Zone contain metasedimentary schist, meta-volcanic and granite-derived lithics, indicating local sources. A steeply dipping S-C fabric with a horizontal mineral stretching lineation is defined by the metamorphic assemblage of either quartz-chlorite-white mica or quartz-chloriteactinolite, often pervasively altered to sericite and albite. This lithology is associated with narrow, elongate heterogeneous magnetic anomalies, largely controlled by fluid flow along shear zones and has a radiometric signal that indicates high K and Th.

Dolerite dyke

Dolerite dykes in the field area are characterised by high-amplitude (-509 nT), linear magnetic anomalies, less than a kilometre in width and 40 to 200 km in length. The dykes crosscut all Palaeoproterozoic formations with three primary orientations at N 340°, N 020° and N 070°. The dipole response characterising some dykes indicates strong magnetic susceptibility and remanence; however, this was not further investigated within this study. In the northeast of the field area, some dyke generations are overlain by Neoproterozoic Volta basin sediments, whilst others intrude the Neoproterozoic sedimentary pile, indicating multiple emplacement events.

Volta basin sediments

Covering the north-eastern extent of the field area, the Neoproterozoic Volta Basin sediments unconformably overlie the Palaeoproterozoic units. The sediments comprise horizontal beds of thick cross-bedded sandstone, fine-grained arkoses and rare conglomerates [START_REF] Junner | Geology and hydrology of the Volta basin[END_REF]Kalsbeek, et al., 2008). Magnetic anomalies of Palaeoproterozoic unit continue beneath the basin sediments; however, their magnetic signal of is smoothed and slightly reduced in amplitude. The basin margin is most clearly defined in the combined DEM and radiometric image, associated with an elevated topography and extensive K depletion relative to U and Th.

Geophysical response of tectono-metamorphic domains

The study area is divided into a number of tectono-metamorphic domains based on their geology, metamorphic grade and geophysical response. Given the sparsity of outcrop and the rarity of exposure of major shear zones, domain boundaries are interpreted from magnetic lineaments, truncation of magnetic fabrics or contrasting magnetic, radiometric or gravity responses. The tectono-metamorphic domains of the Sefwi Greenstone Belt, including the high-grade Chiraa Domain, correspond with a NE-striking, high amplitude gravity anomaly (Fig. A2SI). Moderate to low gravity responses flank the belt to the northwest and southeast, corresponding with meta-sedimentary units of Sunyani-Comoé and Kumasi-Afema Domains, respectively. High amplitude gravity anomalies also correspond to the parallel-striking Ashanti Greenstone Belt and the northern Bui Greenstone Belt (Fig. A2SI).

The aeromagnetic data provides significantly more detail on the lithology distribution and domain boundaries within the Sefwi Belt based on contrasting magnetic characteristics (Fig. In the far northeast of the study area, the Chiraa Domain is characterised by predominantly by a low magnetic signal containing discrete, high magnetic anomalies, consistent with the heterogeneous high-grade paragneiss and granite exposures within the domain. The Bechem Granitoid domain is characterised by a low-to moderate magnetic response, with a weak, heterogeneous magnetic fabric, with high amplitude, linear to curvilinear magnetic anomalies demarcating its southern boundary represented by the ~300 km-long Ketesso Shear Zone. The southern portion of the Sefwi Belts comprises the Sefwi-Wiawso Domain, which is characterised by highly magnetic mafic to intermediate volcanic rocks, featuring a NE-to ENE-striking magnetic fabric and low amplitude, low-frequency magnetic anomalies associated with multiple generations of pluton emplacement. Contrasting all domains within the Sefwi Belt, the Sunyani-Comoé Domain is characterised by alternating high and low NE-striking magnetic domains. Smooth, low amplitude, long wavelength magnetic domains are associated with meta-sedimentary units bounded by highly magnetic lineaments, which are interpreted as shear zones activating during different phases of deformation. Within the high magnetic domains, ENE-striking magnetic fabrics are observed in the metamorphosed volcanosedimentary rocks, surrounding and along strike of elongate, elliptical magnetic lows associated with leucogranitic plutons. Whilst the Kumasi-Afema Domain bearing some lithological similarity to the Sunyani-Comoé Domain, it is characterised by and low-to moderate magnetic response with poorly developed NE-striking magnetic lineaments. The entire study area is cross-cut by late NW-, NNWand N-striking, dolerite dykes associated with narrow, elongate dipole magnetic anomalies.

Tectono-metamorphic history

Deformation sequence

We distinguish five discrete deformation events in the Sefwi Greenstone Belt and adjacent metasedimentary domains attributed to the Palaeoproterozoic Eburnean Orogeny. The structural framework for these five events is deduced from field observations across the study area, kinematic analysis of processed potential field data and the macroscale geometries of discrete tectonometamorphic domains.

D1 NNW-SSE directed shortening

D 1 is defined by a ubiquitous, penetrative metamorphic foliation present in both high-grade rocks and low-grade rocks throughout the study area. In the Sunyani-Comoé and Kumasi-Afema domains, S 1 trajectories define the axial surface of kilometric NE-SW striking isoclinal folds of thick stratigraphic units, parallel to the crustal-scale shear zones that dominate the architecture of the study area.

Conversely, S 1 trajectories of domains within the Sefwi Belt, as well within the Chiraa Domain, are routinely oblique to NE-SW-striking shear zones. In the Kukuom-Juaboso Domain, S 1 trajectories and D 1 shear zones vary in strike between E-W and ENE-WSW, with evidence of overprinting during subsequent deformation. In the high-grade Chiraa Domain, gneissic S 1 trajectories follow an arcuate geometry, elongate toward ~N250, overprinted by subsequent deformation and vertically transposed along domain boundaries.

In the Kukuom-Juaboso Domain (Fig. 3.3a, b, d), S 1 is defined by a shallowly-to steeply-dipping, schistose to gneissic metamorphic foliation of aligned biotite and amphibole in metavolcanic rocks, para-amphibolites and orthogneisses, which are well imaged in aeromagnetic data. S 1 is parallel to bedding (S 0 ) in metamorphosed volcaniclastic units. The limited number of isoclinal, centimetric F 1 folds observed in this domain are intrafolial, with fold axes parallel to bedding in metasedimentary units.

In the core of the domain, locally migmatitic orthogneisses, interpreted as intermediate metavolcanic In the high-grade Chiraa Domain, S 1 is defined by a biotite-rich gneissosity parallel to stratigraphic layering in migmatitic garnet-bearing paragneisses (Fig. 3.3c), also observed in subordinate granitic orthogneisses. No reliable L 1 stretching lineations were observed in this domain. S 1 is moderate-to steeply-dipping with a highly variable orientation due to overprinting and transposition (Fig. 3.3d).

Migmatitic paragneisses of the domain display stromatic leucosomes, parallel to the S 1 gneissic banding, as well as discordant leucocratic veins and dykes, cross-cutting S 1 foliations. Similarly, migmatitic metabasalts exposed in the Hwidiem Shear Zone display an S 1 gneissosity, defined by aligned hornblende alternative with plagioclase and quartz, also featuring stromatic leucosomes, which suggest partial melting occurred in both domains during, or in the latter stages of, D 1 (Fig. 3.3a). In contrast, in the Sunyani-Comoé and Kumasi-Afema Domains, S 1 is characterised by a chlorite-white mica foliation, parallel to bedding (S 0 ) in the low-grade metasediments, defining the axial surface of isoclinal F 1 folds. In mafic and intermediate meta-volcanic rocks of the Sefwi-Wiawso Domain, S 1 is predominately characterised by a steeply dipping E-W to NE-SW, chlorite-actinolite-white mica schistosity (Fig. 3.3d) and mineral alignment in biotite and hornblende bearing granitoids. Based on field observations, D 1 is likely characterised as an NNW-SSE directed shortening event associated with isoclinal folding and thrusting, high-grade metamorphism, and partial melting in its latter stages. D 1 structures in the airborne magnetic data are primarily preserved within the Kukuom-Juaboso Domain and the Hwidiem Domain, corresponding with high magnetic E-W to ENE-WSW striking lineaments interpreted as D 1 shear zones, often coinciding with a sharp change in the amplitude and texture of the magnetic response. They are parallel to the ENE-striking magnetic fabric of the Kukuom-Juaboso Domain, which corresponds with the bedding-parallel S 1 foliation observed in the field. D 1 shear zones are often refolded around tight ENE-striking folds developed during D 2 , or truncated by later faults. The combined evidence of a bedding-parallel S 1 , rare isoclinal intra-folial F 1 folds, coupled with the documented thrust movement along D 1 structures is consistent with lateral contractional deformation and crustal thickening during D 1 NNW-SSE directed shortening. During this event, the Kenyase-Yamfo Shear Zone likely represented an oblique reverse fault; however, due to extensive reactivation, this was not confirmed by field evidence.

D2 ENE-WSW directed transtension

The structural expression of D 2 deformation within the study area is highly varied, depending on both the orientation of pre-existing structures and proximity to major D 1 shear zones. In the amphibolite to granulite facies paragneisses of the Chiraa Domain, S 2 is defined by a spaced biotite schistosity, forming the axial surface of tight, gently-inclined F 2 folds (Fig. 3.4a), with fold axes observed approximately parallel to L 2 mineral stretching lineations, defined by elongate kyanite or biotite in favourable lithologies. S 2 progressively rotates in orientation from ~N040 to N270 in an anticlockwise direction, parallel to the arcuate geometries of the S 1 trajectories (Fig. 3.5), dipping steeply to the west in the north of the domain, and shallowing slightly to the south (60-80°S). In the far north of the domain, L 2 and F 2 plunging moderately (18-40°) towards the ~N230, rotating with the metamorphic fabric to an E-W orientation along the southern margin of the domain, plunging shallowly to moderately (~7-44°) towards both the east and west due to subsequent deformation. In a raft of migmatitic mafic orthogneisses (SB177) in the south of the domain, the S 1 gneissosity and plagioclase-rich leucosomes are refolded by centimetric, rootless F 2 folds (Fig. 3.4b), with minor melt accumulation in fold hinges.

Megacrystic garnet and clinopyroxene porphyroblasts display S 1/2 foliation parallel stretching (Fig.

3.4c

). Whilst the tectonic contact between migmatitic rocks of the Chiraa Domain and greenschist facies metasedimentary rocks of the Sunyani-Comoé Domain to the west and south is poorly exposed, it is interpreted as significant crustal detachment, juxtaposing domains of contrasting metamorphic grade. Along the western boundary of the Chiraa Domain, the dip direction of the S 2 foliation in both the high and low grade rock progressively rotates counter-clockwise from NW to W and then to the south, along the southern boundary of the domain. The corresponding L2 stretching lineation pitches shallowly to the SW, progressively rotating to the east. The combined structural and architectural features of this tectonic contact is interpreted as a normal, left lateral detachment fault, with foliation rotation attributed to a regional scale drag fold along the larger shear system along strike.

Within the Kukuom-Juaboso and Hwidiem domains, S 1 trajectories and D 1 shear zones are refolded along tight F 2 folds elongate towards N230-N270. S 2 is a moderate-to steeply-(40-82°) NNW to SSE-dipping, discontinuous biotite and/or amphibole foliation (Fig. 3.5), depending on the lithology. The ENE-WSW oriented Hwidiem Shear Zone marks the tectonic contact between the Kukuom-Juaboso and Hwidiem domains. Within the shear zone, metamorphic banding (S 1 ) and parallel leucosomes in migmatitic mafic orthogneisses are refolded around open, upright F 2 folds, displaying prominent elongation and mineral stretching parallel to F 2 fold axes (Fig. 3.4d). F 2 fold hinges plunge 17-30° towards ~N080, whilst L 2 amphibole stretching lineations plunge near coaxially to N065-N109. This geometry indicates greater hinge parallel elongation relative to hinge-perpendicular shortening, which is characteristic of a transtensional regime [START_REF] Dewey | Transpression and transtension zones[END_REF]. A similar relationship is observed in a garnet-hornblende paragneiss (SB464) in the Kukuom-Juaboso Domain with isoclinal, near recumbent F 2 folds displaying near coaxial L 2 amphibole stretching lineations. Unlike the Hwidiem Domain, L 2 mineral stretching lineations observed in the core of the Kukuom-Juaboso Domain consistently plunge shallowly (7-34°) towards the west, with more varied orientation proximal to domain boundaries due to later transposition. The coaxial relationship between L 2 and F 2 , their obliquity relative to the major Kenyase-Yamfo Shear Zone attest to constrictional deformation and folding under transtension (Fossen, et al., 2013). The role of constrictional deformation is further demonstrated by the presence of L>>S tectonites in garnetamphibole-bearing dioritic amphibolites (L 2 : 4-11° → N230) in the Ketesso high strain zone (Fig. 3.4e).

Along strike, metre-scale boudin flow structures are observed in a strongly sheared, highly altered granite (L 2 : 11° → N225). According to [START_REF] Sullivan | L tectonites[END_REF] (Fig. 3.4f), which overprint the S 1 foliation. We, therefore, interpret the shear zone as a D 1 fault reactivated as a left-lateral normal fault under an ENE-WSW tensional regime. In the greenschist facies volcano-sedimentary rocks of the Sunyani-Comoé Domain, bedding and S 1 are refolded by isoclinal, SW-plunging F 2 folds evident in tight, millimetric crenulations of S 1 by an upright S 2 foliation in F 2 fold hinges (Fig. 3.4 g, h), or locally as a NE-SW oriented, differential S 1/2 cleavage parallel to the regional scale crustal architecture. In the southern volcano-plutonic Sefwi-Wiawso Domain, D 2 strcutres are poorly defined, and are variably expressed as E-W to NE-SW striking, tight folds associated with spaced dissolution crenulation cleavage or a weak chlorite schistosity in favourable lithologies. On the eastern margin of the Sefwi-Wiawso Domain, the NNE-striking brittle-ductile Bibiani Shear Zone separates folded metasediments of the Kumasi-Afema domain in the footwall from polymictic conglomerates in the hanging wall. The Bibiani Shear Zone and the parallel Chirano Shear Zone are interpreted as extensional detachments, bounding a syn-extension graben hosting conglomeratic units containing lithics sourced from both the Sefwi-Wiawso and Kumasi-Afema domains.

The D 2 Hwidiem Shear Zone is characterised by a curvilinear, elongate anomalies with highly varied magnetic amplitudes (-816 to -585 nT) and short wavelengths (500 m -2 km). The western extent of the shear zone is low magnetic, likely reflecting magnetic destruction during fluid-fluxed partial melting of mafic rocks within the shear zone. Distal to the boundaries of the Kukuom Juaboso Domain, D 1 magnetic fabrics and D 1 shear zones are refolded by regional-scale ENE-striking, tight In the low-grade metavolcanic and metasedimentary domains, D 3 deformation generates a N-S to NE-SW oriented dissolution cleavage, most poorly developed in the Sefwi-Wiawso Domain.

D 3 E-W shortening transposes tectonic fabrics proximal to domain boundaries to sub-vertical orientations, reactivating pre-existing shear zone with favourable orientations. In the high-strain Ketesso-Shear Zone and the Kenyase-Yamfo Shear Zone chlorite-muscovite NE-SW S-C 3 fabrics, sigmoidal asymmetries and sub-horizontal stretching lineations indicate dextral shearing under greenschist facies conditions. Conglomerate units within the Ketesso Shear Zone display prominent shearing of clasts coupled with the development of chlorite-white-mica S-C 3 fabrics (Fig. 3.6d). In contrast, the NNE-striking Bibiani on the eastern boundary of the Sefwi Wiawso Domain is at high angles to D 3 shortening and is, therefore, less favourable for strike-slip reactivation. Instead, Tarkwatype conglomerates deposited during D 2 transtension were thrust over low-grade metasediments of the Kumasi-Afema Domain during D 3 .We correlate D 3 deformation with brittle-ductile shearing and west-side-up movement associated with greenschist facies metamorphism documented in the Chirano lode horizon, parallel to the Bibiani shear zone [START_REF] Allibone | Paleoproterozoic gold deposits hosted by Albite-and carbonate-altered tonalite in the Chirano District, Ghana, West Africa[END_REF]. D 3 deformation in the airborne magnetic data sets is associated with high amplitude, elongate NE-striking anomalies between 250 and 300 km in length, associated with the linear Ketesso Shear Zone and the curvilinear Kenyase-Yamfo and Afema Shear Zones. These anomalies truncate those associated with the Hwidiem Shear Zone and the southern margin of the Chiraa Domain. Gentle N-S F 3 folds reform early magnetic fabrics within the Kukuom-Juaboso, Hwidiem and Chiraa Domains. (Delor, et al., 1992;Hirdes, et al., 2007;[START_REF] Lüdtke | Géologie de la région Haute Comoé Sud -fueilles Dabakala (2b, d et 4b, d)[END_REF][START_REF] Lüdtke | Géologie de la Haute Comoé Sud -feuilles Kong (4b et 4d) et Téhini-Bouna (3a à 3d)[END_REF]. The magnetic expression of D 5 faults is interpreted from the truncation and lateral displacement of pre-existing magnetic lineaments and fabrics. Faults are often localised, with a maximum length of 10 km.

Metamorphic history

We present a new metamorphic map for the study area (Fig. 3.7) based on field observations of petrological assemblages and metamorphic textures. Here we describe the key assemblages and metamorphic relationships for each domain (mineral abbreviations after Whitney & Evans, 2010).

More detailed petrological analysis and quantitative P-T estimates and geochronological constraints from in situ monazite SHRIMP U-Pb ages are presented in Chapter 4.

Rocks from the Chiraa and Kukuom-Juaboso domains preserve high-grade metamorphic assemblages. The peak pressure assemblages of paragneiss sample SB398 comprises Bt + Grt + Ky + Pl + Ph + Rt + Qz, whilst garnet amphibolites comprise Hbl + Grt + Pl + Ilm + Qz indicative of high-pressure amphibolite facies metamorphic conditions. Initial conditions of 10.0-11.5 kbar, 580-650 °C, deduced from unzoned garnet porphyroblast compositions are associated with low apparent geothermal gradients (15-17 °C/km) (detailed in Chapter 4). Migmatitic paragneisses of the Chiraa Domain contain leucosomes parallel to the S 1 foliation, often refolded by F2 folds or feeding into dykes that cross -cut both D1 and D2 structures. This suggests that partial melting was late-D 1 to syn-D 2 . Anatexis in the Chiraa Domain matches a higher apparent geothermal gradient of 20-25 °C/ km, consistent with the peak metamorphic assemblage observed in a migmatitic metabasic rock in the same domain of Pl + Grt + Cpx + Hbl + Ilm + L, which suggests anatexis occured at >10 kbar and >700 °C (Pattison, 2003). Conversely, garnet amphibolites from the Kukuom-Juaboso Domain do not undergo partial melting. In contrast, metamorphosed volcano-sedimentary rocks of the Sunyani-Comoé Domain preserve a peak assemblage of fine-grained chlorite-white mica ± actinolite.

Field observations revealed a localised greenschist facies metamorphic overprint along the boundaries of the amphibolite facies and migmatitic domains and along major shear zones, whilst little to no retrogression of high-grade assemblages was observed away from domain boundaries. This suggests exhumation and juxtaposition of domains of contrasting metamorphic grade occurred prior to the greenschist facies metamorphic overprint. In the Kukuom-Juaboso Domain, retrogression the garnet amphibolite sample (SB344) is marked by the localised chlorite growth along the foliation planes and on the rim of garnets and the overgrowth of titanite around foliation-parallel ilmenite (Chapter 4). The incomplete replacement of hornblende and garnet by chlorite and syn-kinematic growth of chlorite on foliation plane indicates late deformation is associated with a clockwise P-T path from upper amphibolite facies conditions to upper greenschist facies conditions (Spear, 1993). This is consistent with the clockwise P-T paths displaying simultaneous cooling and decompression and an overprinting transcurrent-related, retrograde mineral assemblage of chlorite-carbonate-epidotealbite, documented in the Kukuom-Juaboso Domain by [START_REF] Galipp | Metamorphism of the Paleoproterozoic (Birimian) Sefwi volcanic belt, Ghana[END_REF]2003).

The timing of the initiation of cooling and exhumation in the high-grade paragneisses (samples SB398 and SB389) is constrained by in situ monazite U-Pb SHRIMP ages of 2073 ± 2 Ma and 2074 ± 3 Ma (see Chapter 4). Monazite grains were located primarily in the matrix, elongate parallel to the main tectonic fabric, or in the case of SB398, parallel to the L 2 kyanite stretching lineation. The monazite age is interpreted as the timing of exhumation initiation, post-dating zircon U-Pb crystallisation ages of 2093 ± 2 Ma and 2092 ± 2 Ma yielded by granites in the Chiraa Domain (Petersson, et al., 2016). This relationship is consistent with observations from many high-grade terranes in which monazite populations are younger than zircon populations from the same area (Kelsey, et al., 2008). methodology is utilised by Block, et al. (2016) for Palaeoproterozoic rocks exposed in NW Ghana and is applied here to facilitate comparison of strain patterns in comparable terranes in the West African Craton. Fault generations are colour-coded according to their primary deformation phase or the event during which they were last reactivated. This map illustrates that:

Structural-metamorphic map

-The early S 1 and subparallel S 2 fabrics preserve an E-W to ENE-WSW strike within the higher grade subdomains of the Sefwi Belt, complemented by the major trends displayed in rose diagrams of S 1 measurements for the Kukuom-Juaboso and Hwidiem Domains in Figure 3.4.

The Kenyase-Yamfo Shear Zone along the north-western margin of the belt demarcates the transition from ENE-WSW striking foliation trajectories to the predominantly NE-striking S 1 trajectories of the Sunyani-Comoé Domain. The shear zone is likely to have been inherited from either the primary crustal architecture or D 1 and has been reactivated and transposed during subsequent deformation. Within the Sefwi Belt, smaller D 1 shear zones and isograds associated with partial melting at the upper amphibolite to granulite facies conditions commonly strike ENE and are variably refolded or truncated by younger structures. The NNE-striking western margins of the Chiraa and Kukuom-Juaboso domains coincide with the most significant lateral metamorphic breaks, which are attributed to D 2 ENE-WSW transtension (Figs. 3.7 and 3.8).

-Refolded S 1 trajectories with the Kukuom-Juaboso Domain reveal regional-scale tight F 2 folds, generating a low-angle relationship between S 1 and S 2 . D 2 structural measurements (Fig. 3.5) indicate a near coaxial relationship of shallowly E-to NE-and W-to SW-plunging L 2 and F 2 measurements, most prominent in the higher metamorphic grade domains. Figure 3.8 highlights the obliquity between the E-W to ENE-WSW striking S 1/2 measurements in the Chiraa Domain (Fig. 3.8a) and the Kukuom Juaboso Domain (Fig. 3

Discussion

Timing of Eburnean deformation in the Sefwi Belt

Palaeoproterozoic crust exposed in southwest Ghana comprises tholeiitic to calc-alkaline volcanic suites and TTG-like to calc-alkaline plutons suites was emplaced between 2190 and 2130 Ma, recognised as a phase of magmatic accretion (e.g. Amponsah, 2012;Hirdes & Davis, 1998;Hirdes, et al., 2007;Hirdes, et al., 1993;Loh, et al., 1999;Taylor, et al., 1992). Subsequent deformation and metamorphism attributed to the Eburnean Orogeny are bracketed across the craton between ca. 2150 and ca. 1980 Ma (e.g. Davis, et al., 1994;Feybesse, et al., 2006). Deposition of metasediments in the Kumasi-Afema Domain, considered to be partially contemporaneous with sedimentation in the Sunyani-Comoé Domain, is constrained by the youngest detrital zircon age of 2154 ± 2 Ma (Oberthür, et al., 1998), with intercalated andesitic flows dated at 2142 ± 2 Ma (Adadey, et al., 2009), and latesedimentary emplacement of granodiorite plutons at 2136 ± 19Ma (Adadey, et al., 2009). Perrouty, et al. (2012) bracket Kumasi Group deposition between 2154-2125 Ma, in agreement with a period of tectonic extension and emplacement of alkaline granites and quartz monzonites in the Sefwi-Wiawso domain between 2150 and 2130 Ma proposed by Hirdes, et al. (2007). Figure 3.9 illustrates the interpreted tectonic setting and relative distribution of key terranes in southwest Ghana prior to the commencement of D 1 deformation at ca. 2100 Ma. The tectonic contacts between domains of the Sefwi Belt and the adjacent metasedimentary domains have been significantly reworked during subsequent deformation, and therefore their nature remains unclear. Gravity data; however, defines major litho-tectonic domains, with high gravity anomalies matching the boundaries of the volcanoplutonic and high-grade domains of both the Sefwi and Ashanti belts, and low to moderate strength anomalies associated with the metasedimentary domains, likely reflecting the architecture generated during early crust formation.

In the study area, D 1 deformation is characterised by a ubiquitous bedding parallel foliation, development of E-W oriented regional scale thrusts and kilometric folds in association with HP-MT temperature metamorphism (Fig. 3.9b), consistent with crustal thickening (England & Thompson, 1984). These observations are consistent with field observations of a subparallel relationship bedding and S 1 in the Sunyani-Comoé Domain exposed in southeast Ivory Coast (Delor, et al., 1992;Delor, et al., 1992b). The timing of D 1 crustal thickening remains unconstrained in this study; however, P-T-t paths indicate rapid burial of supracrustal rocks precedes anatexis in the Chiraa Domain. We, therefore, suggest that the minimum timing of D 1 is at ca. 2093 Ma indicated by the oldest granite crystallisation age from the Chiraa domain (Petersson, et al., 2016). D 1 field observations potentially correspond with the shear-related, bedding-parallel foliation documented in the neighbouring Ashanti belt by (Allibone, et al., 2002a;Allibone, et al., 2002b). The timing of early Eburnean deformation is poorly constrained in southern Ghana, with authors suggesting it extended from 2187 to 2158 Ma (D1; Perrouty, et al., 2012) or 2116 to 2088 Ma (Eburnean II; Allibone, et al., 2002a). It must be noted that Allibone, et al. (2002a) suggest that metamorphism of Birimian mafic volcanic rocks was attributed to extensive magmatism between ca. 2200 and 2150 Ma, referred to as Eburnean I. Alternatively, thrustrelated crustal thickening in southern Ghana is hypothesised by Feybesse, et al. (2006) between 2130 and 2100 Ma. This likely corresponds with monazite and vein-hosted zircon growth at 2102 ± 1 Ma and 2104 ± 3 Ma, respectively, in migmatitic orthogneisses in the southern extent of the Ashanti belt (Loh, et al., 1999). Lobjoit (1964) suggested that the deflection of the strike of meta-sedimentary rocks around a foliated tonalite pluton east of Bibiani was the product of "forceful emplacement" of the pluton. We instead suggest that the structural evidence may indicate syn-to late-kinematic emplacement of the pluton during D 1 deformation, providing a minimum age for the commencement of D 1 deformation of ca. 2116 Ma (Hirdes, et al., 1992). Coast (Vidal, et al., 2009). Furthermore, we propose that the conglomerate and sandstone sequences west of the Bibiani Shear Zone were deposited in NNE-striking half-graben structures that developed during D 2 transtension.

Younger D 3 deformation was associated greenschist facies metamorphism and dextral reactivation of the Sefwi shear system during E-W shortening (Grey shear zones: Fig. 3.9d). The timing of D 3 remains unconstrained; however, it is potentially correlated with retrograde greenschist facies metamorphism and hydrothermal xenotime crystallisation at 2063 ± 9 Ma post-dating sinistral shearing in the Ashanti Belt (Pigois, et al., 2003).

Rheology, tectonic style and exhumation

Strain patterns and lateral changes in metamorphic grade are widely investigated in Precambrian provinces due to the insight they provide into the rheological structure of the lithosphere and its influence on tectonic style and exhumation processes during orogenesis. In Phanerozoic orogenic belts, lateral tectonic forces are accommodated through thrust-related, heterogeneous crustal thickening and unit stacking. Conversely, Archaean and some Palaeoproterozoic terranes are associated with warmer lithospheres and elevated continental geotherms, which results in weaker and more buoyant lithosphere [START_REF] Griffin | Secular variation in the composition of subcontinental lithospheric mantle: geophysical and geodynamic implications[END_REF]. Under such conditions, convergence is accommodated through homogeneous crustal thickening, limited topography and lateral ductile flow (Cagnard, et al., 2006;Cruden, et al., 2006;Rey & Houseman, 2006). Alternatively, crustal thickening is attributed to the diapiric emplacement of voluminous buoyant granitoids and the sagduction of negatively buoyant greenstone terranes, resulting in the characteristic dome and basin strain pattern, with subsequent lateral shortening accommodated by craton-scale strike-slip shear zones [START_REF] Bouhallier | Diapirism, bulk homogeneous shortening and transcurrent shearing in the Archaean Dharwar craton: the Holenarsipur area, southern India[END_REF]Choukroune, et al., 1995). In the Sefwi Greenstone Belt, we argue that the combined evidence of a bedding-parallel foliation (S 1 ), early D 1 thrust faulting and initial HP-MT metamorphism indicates burial of supracrustal rocks reflects the early stages of modern collisional orogens (England & Thompson, 1984). During D 1 deformation, we propose that large-scale thrusting leads to heterogeneous crustal thickening and burial of supracrustal rocks during horizontal convergence, early in the tectonic history of SW Ghana. This is consistent with hypothesised accommodation of crustal thickening by nappe stacking (Allibone, et al., 2002b;Feybesse, et al., 2006;Milési, et al., 1992). Such evidence does not conform to dome and basin geometries, homogeneous crustal thickening and diapirism-related deformation as proposed for areas of the West African Craton (e.g. Delor, et al., 1995;Lompo, 2009;Vidal, et al., 1996;Vidal, et al., 2009).

HP-LT and HP-MT metamorphic conditions and low geothermal gradients are rare in ancient orogenic domains (Brown, 2007).Whilst few in number, tectono-metamorphic studies of Palaeoproterozoic rocks in the West African Craton sporadically preserve peak pressures of 10 -14 kbars associated with relatively cool apparent geothermal gradients, indicating a minimum crustal thickening of 33 -50 km, recorded in NW Ghana (Block, et al., 2015), within the Sefwi Belt (Ch. 4, this study), eastern Burkina Faso (Ganne, et al., 2012) and along the tectonic contact between the Archaean Kénéma-Man Domain and the Palaeoproterozoic Baoulé-Mossi Domain (Pitra, et al., 2010).

Documentation of such conditions in the West African Craton suggests that localised areas of the lithosphere were cool enough and strong enough to support moderate amounts of crustal thickening, suggesting the development of thermo-mechanical properties similar to that of the Phanerozoic lithosphere (Rey & Houseman, 2006).

The metamorphic record of the study area reveals high-pressure mineral assemblages including two garnet amphibolites documented in the Kukuom-Juaboso Domain, a Grt-Ky-Rt bearing paragneiss (SB398) and an orthopyroxene-free garnet-clinopyroxene migmatitic mafic orthogneiss (SB177) hosted within the Chiraa Domain. The phase assemblage of the mafic orthogneiss is consistent with metamorphism at pressures greater than 10 kbar and 700 °C (Pattison, 2003). It was noted by Pattison (2003), however, that whilst this phase assemblage is characteristic of high-pressure granulite facies metamorphism, it was also possible to generate similar assemblages under amphibolite facies conditions. Metamorphic constraints coupled with field observations, including stromatic leucosomes folding of horizontal layers in a heterogeneous lithology as a response to constrictional strain (Fossen, et al., 2013), most clearly developed within with ENE-WSW striking Hwidiem Shear Zone and within the Kukuom-Juaboso Domain. Within the Ketesso Shear Zone, a quartz dioritic garnet amphibolite preserves an L>>S tectonite fabric (SB294; Fig. 3.4g) consistent with transtension in the lower crust [START_REF] Dewey | Transtension in arcs and orogens[END_REF]. It is juxtaposed with a weakly deformed, greenschist facies metabasalt (SB347) of the Sefwi-Wiawso Domain 13 km to the east-southeast. Differential exhumation juxtaposing highgrade tectono-metamorphic domains is attributed the normal movement along NNE-SSW-striking detachments faults and normal, left-lateral reactivation of major NE-striking shear zones, providing evidence against homogeneous uplift proposed for Palaeoproterozoic orogenic domains involving weak, juvenile crust (Cagnard, et al., 2006;Cagnard, et al., 2007;Chardon, et al., 2009;Cruden, et al., 2006).

There are two possible interpretations of the driving force of transtension and constrictional strain. Within the hot orogen model, lateral constrictional flow develops during orogen-normal shortening in response to gravitational forces acting on thickened, hot lithosphere (Chardon, et al., 2009;Chardon, et al., 2011;Cruden, et al., 2006). Significant topographic relief and heterogeneous crustal thickening are prohibited by rheologically weak lithosphere and thermally homogeneous lower crust, with lateral shortening resulting in orogen-parallel flow, noted in the later stages of Neoarchaean convergence in the Gawler and Terre Adélie cratons [START_REF] Duclaux | Orogen-parallel flow during continental convergence: Numerical experiments and Archean field examples[END_REF]. A similar concept is applied to wide, mature collision orogens that have undergone extensive partial melting in the low crust, where accumulated gravitational stress results in extensional collapse (e.g. Brown, 2010;Royden, et al., 2008;Vanderhaeghe & Teyssier, 2001) and the formation of gneiss domes (Whitney, et al., 2004). It is likely that partial melting of middle and lower crustal rocks documented in the northeast of the study area contributed to the localised rheological weakening of the crust. However, the observed metamorphic textures and calculated metamorphic P-T paths do not display isothermal decompression, with steep retrograde paths instead indicating concurrent cooling and exhumation, therefore we do not consider gravitational collapse to be the primary driver of transtension-related exhumation. For SW Ghana, we hypothesise that constrictional deformation and transtension-driven late-orogenic exhumation indicates a change in boundary conditions to ~ENE-WSW divergence. This thermo-mechanical behaviour is analogous to the exhumation of HP and UHP rocks of the Western Gneiss Region of western Norway, driven by sinistral oblique plate divergence of Laurentian and Baltic plates (Krabbendam & Dewey, 1998).

Orogenic model, regional correlations and Transamazonian ties

Some of the major complexities when considering an orogenic model for the West African Craton arise from the regional or craton-scale correlation of metamorphic or deformation events between discrete greenstone belts, both volcano-plutonic and meta-sedimentary, and high-grade gneissic terranes. Polycyclic orogenic models for the West African Craton are inferred from exposures of high-grade terranes, thought to represent a Palaeoproterozoic (> 2150 Ma) basement deformed and metamorphosed prior the low-grade metamorphism and transcurrent tectonics of the Eburnean

Orogeny (e.g. de Kock, et al., 2012;Hein, 2010;Pouclet, et al., 1996). In SW Ghana, D 1 deformation in both the Ashanti Belt (Perrouty, et al., 2012) and the Sefwi Belt (this study) is associated with an initial phase of N-S to NNW-SSE shortening. Whilst we were unable to directly constrain the timing of D 1 in this study, Perrouty, et al. (2012) propose an Eoeburnean (D1) event between 2187 -2158 Ma, observed in metavolcanic rocks and mica schists of the Sefwi Group exposed in the southern extent of the Ashanti Belt. This timing corresponds with extensive magmatism in the Sefwi Belt, regarded by other authors as a magmatic accretionary event (Allibone, et al., 2002b;Feybesse, et al., 2006;Milési, et al., 1992). Perrouty, et al. (2012) bracket a subsequent extensional event associated with deposition of Kumasi Group volcano-sedimentary units between 2154 and 2125 Ma, prior to the deposition of Tarkwaian sediments between 2107-2097 Ma.

Correlation of the regional scale thrust faulting and crustal thickening documented in the Sefwi Belt, also associated with N-S shortening, is convoluted by the tectonic setting and timing of the deposition of Kumasi Group interpreted by Perrouty, et al. (2012). Rocks in the Chiraa and Kukuom-Juaboso domains indicate burial or supracrustal rocks to depths of >33 km, interpreted as a narrow collisional front between the Sefwi Belt and the Sunyani-Comoé Domain. The obliquity between D1 shortening and the NE-SW strike of the Sefwi Belt potentially suggests oblique collision along pre-existing crustal discontinuities associated with juvenile crust formation. Feybesse, et al. (2006) describe a similar period of thrust tectonism and crustal thickening between 2130 and 2105 Ma in SW Ghana, which they attribute to D 1 deformation of the Eburnean Orogeny. Such deformation may also provide a mechanism for the formation of shear-related, bedding-parallel foliations documented in both the Sefwi Belt and the Ashanti Belt (this study; Allibone, et al., 2002b).

Poor correlation is found between subsequent deformation events between the Sefwi and the Ashanti belts. D 2 constrictional deformation described in this study is attributed to ENE-WSW transtension, normal left-lateral movement along major shear zones and a change in boundary conditions. Sinistral movements within the Sefwi shear systems is supported by field observations of Feybesse, et al. (2006) and in the Ivorian extent of the Sunyani-Comoé Domain (Vidal, et al., 2009).

In situ monazite ages constrain the initiation of D 2 exhumation of high-grade rocks at ca. 2073 Ma (Ch. 4; this study), post-dating granite and leucogranite emplacement in the Sunyani-Comoé domain between 2093 and 2081 Ma (Agyei Duodu, et al., 2009;Hirdes, et al., 1992;Hirdes, et al., 2007), attributed to partial melting in the lower crust after to D 1 crustal thickening. We suggest that deposition of the Tarkwa-like immature conglomerates and sandstone along the NNE-striking Bibiani Shear zone is associated with graben formation and normal fault movement during D 2 . As such, we caution the correlation of these conglomerates with those exposed in the Ashanti Belt. Whilst D 2 potentially corresponds with the D 4 NNW-SSE shortening event responsible for the sinistral reactivation of the Ashanti fault (Allibone, et al., 2002b;Perrouty, et al., 2012), we are unable to reconcile the D 3 NW-SE shortening of Perrouty, et al. (2012) with deformation recorded in the study area. In addition, late dextral reactivation of regional NE-striking shear zones is not observed in the Ashanti Belt.

In NW Ghana, high-grade metamorphism develops during N-S shortening between 2140 and 2130 Ma (Block, et al., 2015;Block, et al., 2016). Exhumation of the lower crust is attributed to N-S extension during gravitational collapse generating anatectic domes. Block, et al. (2016) hypothesise a subsequent collision and corresponding E-W shortening between independent crustal domains represented by NW Ghana (D3; Block, et al., 2016) and SW Burkina Faso (D1; Baratoux, et al., 2011) between striking Jirapa shear zone of NW Ghana (Block, et al., 2016). Sinistral shear zones in NW Ghana are subsequently overprinted by dextral NE-striking strike-slip shear zones during E-W shortening (D5; Block, et al., 2016), which we interpret as synchronous with our D 3 deformation. Such deformation may be the distal expression of the collision of the Archaean Kénéma-Man Domain and the Baoulé-Mossi domain between 2050 and 2030 Ma (Kouamelan, et al., 1997;Pitra, et al., 2010).

On a more global scale, geological, structural and geochronological studies employed in paleogeographic and geochronological reconstructions have supplied strong evidence that West

Africa and the São Luís Craton were contiguous during the Palaeoproterozoic (e.g. [START_REF] Hurley | Test of continental drift by comparison of radiometric ages[END_REF]Klein & Moura, 2008, and references therein;[START_REF] Torquato | Brazil-Africa geological links[END_REF]. Magmatism of the São Luís craton and the basement of the Gurupi belt is defined by three periods of magmatic activity (2240 ± 5 Ma; 2160 ± 10 Ma; 2080 ± 20 Ma), sharing lithological and structural characteristics with SW Ghana (Klein & Moura, 2008). Peraluminous collision-type granitoids (2086 -2090 Ma), produced by partial melting of crustal material during collision (Klein, et al., 2005;Klein & Moura, 2008), are of comparable age to the collisional orogenesis and leucogranites emplacement in the Sunyani-Comoé Domain (Agyei Duodu, et al., 2009;Hirdes, et al., 1992;Hirdes, et al., 2007). A stronger correlation may be made between the magmatic and tectono-thermal evolution of the Sefwi Greenstone Belt and the Ile de Cayenne complex, French Guiana. Both the Sefwi Belt and the Cayenne complex are characterised by early tholeiitic magmatism followed by emplacement of TTG magmas between ca.

2174 and 2144 Ma , with some bearing calc-alkaline affinities (Delor, et al., 2003;Feybesse, et al., 2006;Hirdes, et al., 2007;Vanderhaeghe, et al., 1998). Both terranes yield positive εHf (t) and εNd (t) values that attest to the juvenile character of the crust (Delor, et al., 2003;Petersson, et al., 2017;Taylor, et al., 1992), which formed distal to Archaean cratonic material with a subsequent period tectonic accretion (Gruau, et al., 1985;[START_REF] Kroonenberg | Paleoproterozoic evolution of the Guiana Shield in Suriname: A revised model[END_REF]Vanderhaeghe, et al., 1998). Crustal thickening, anatexis and emplacement of potassic granitoids and peraluminous leucogranites along major shear zones between ca. 2093 and 2083 Ma in French Guiana is attributed to oblique collision and late orogenic lateral extrusion (Vanderhaeghe, et al., 1998), bearing a striking similarity to the sequence of events recorded in southwest Ghana. The final correlation between the two terranes comes from the timing of crustal extension and formation of granulite facies metamorphism is in the Guiana Shield at 2070 to 2060 Ma (Delor, et al., 2001), which we interpret to be a lateral continuation of the D 2 deformation and metamorphism recorded on the northwest margin of the Sefwi Belt. Furthermore, if we assign sinistral movement, as documented in this study, to the 400km of displacement along the Sefwi shear system proposed by Jessell, et al. (2012), it would place modern day SW Ghana, and likely the São Luís Craton, in NE Brazil in the French Guiana Shield.

Implications for Paleoproterozoic geodynamics

Structural and metamorphic evidence yielded by rocks in the West African Craton provide an opportunity to explore orogenic processes during the Palaeoproterozoic, prior to the appearance of blueschists and ultra-high-pressure metamorphism (UHPM) and other tectonic indicators attributed to a modern plate tectonic regime in the Neoproterozoic (Stern, 2005;Stern, 2007). The Neoarchaean to Palaeoproterozoic period, however, coincides with a number of significant changes in the Earth, including changes in the tectonic style, secular evolution recorded in the metamorphic record and evolving chemistry of magmatic rocks (e.g. Brown, 2006;Brown, 2007;Dhuime, et al., 2012;Keller & Schoene, 2012;Laurent, et al., 2014). Indeed, it is widely accepted that the ambient upper-mantle temperatures were hotter in the past (e.g. Herzberg, et al., 2010;Labrosse & Jaupart, 2007), likely affecting the tectonic style and the rheology of the lithosphere (Burov & Yamato, 2008;Burov, 2011;Sizova, et al., 2010;van Hunen & van den Berg, 2008). The orogenic model we propose for SW Ghana reveals structural and metamorphic evidence of significant crustal thickening and clockwise P-T-t paths reminiscent of modern collisional orogens, forming a growing body of research suggesting collision between discrete Palaeoproterozoic terranes in the West African Craton (Block, et al., 2016;Parra-Avila, 2015;Parra-Avila, et al., 2016).

We iterate that the hot-orogen model does not sufficiently explain the strain patterns and metamorphism documented in southwest Ghana which contrast the homogeneous lithospheric deformation, monotonous metamorphism and limited exhumation capacity of hot orogens proposed for Neoarchaean and Palaeoproterozoic orogens (Cagnard, et al., 2011;Cagnard, et al., 2006;Cagnard, et al., 2007;Chardon, et al., 2009;Rey & Houseman, 2006;Rey, et al., 2003), proposed also for areas of the West African Craton (Vidal, et al., 2009). Instead, the tectonic style and orogenic model proposed for SW Ghana bears a number of similarities to collisional orogenesis documented in the Archaean Barberton terrain (Diener, et al., 2013;Moyen, et al., 2006) and the Archaean Abitibi Subprovince (Daigneault, et al., 2002;Mueller, et al., 1996), including initial HP-MT metamorphism interpreted as a collisional event followed by late orogenic extension.

The period of global orogenesis between ca. 2100 and 1800 Ma is characterised by extremely diverse metamorphic conditions and tectonic style including the pene-contemporaneous 1810 Ma hot orogen of the southern Finnish Svecofennides (Cagnard, et al., 2007) and the Himalaya-like Trans-Hudson Orogen, associated with the formation and exhumation of eclogite facies rocks (Weller & St-Onge, 2017). As such, we believe the tectono-metamorphic history of SW Ghana supports the notion of non-unique orogenic processes for the Neoarchaean-Palaeoproterozoic period, potentially reflecting the thermal divergence in the mantle between 2.500 and 2000 Ma, as proposed by [START_REF] Condie | A great thermal divergence in the mantle beginning 2.5 Ga: Geochemical constraints from greenstone basalts and komatiites[END_REF] and the progressive, and possibly localised, development of plate tectonics and subduction settings (Condie & Kröner, 2008).

Conclusions

A new interpretation of the tectonic evolution and development of the crustal architecture of SW Ghana during the Palaeoproterozoic Eburnean Orogeny has been presented, based on the integration of regional field mapping, interpretation of regional airborne geophysical datasets and incorporation of geochronological constraints. The re-interpretation of the litho-tectonic map, combined with the regional structural framework and complementary metamorphic data suggests that the domains within the Sefwi Belt record significant crustal thickening during NNW-SSE shortening, relative to adjacent low-grade domains. The tectonic contact between the Sefwi Belt and the Sunyani-Comoé Domain is interpreted as a narrow, collisional front, representing a suture zone between SW Ghana and central Ghana/Ivory Coast that formed during oblique convergence. Differential exhumation of high-grade domains occurred after thermal relaxation of the orogenic crust during D 2 transtension, contrasting the surface geology predicted for Precambrian hot orogens. Furthermore, based on previous studies of elongate granites coupled with macro-geometries of domains, we interpret major normal, left lateral movement within the Sefwi shear system. Based on both structural and metamorphic evidence transtension is interpreted as the product of orogenic extension reflecting changes in boundary conditions and oblique plate divergence rather than gravitational collapse. Localised, late dextral reactivation of NE-striking shear zones is generated by far-field stresses. We highlight the diachronous nature of high-grade metamorphism and discrepancies in deformation histories across the craton during the Eburnean Orogeny, which we interpret as the product of episodic collisional orogenesis between discrete juvenile terranes during the assembly and stabilisation of the West African Craton.

Introduction

This chapter is complementary to the tectono-metamorphic framework and map presented in Chapter 3, providing detailed petrographic analysis, quantitative estimates of prevailing metamorphic conditions and geochronological constraints on deformation and metamorphism during the Eburnean Orogeny. These new geochronological constraints are compared to existing geochronological data in order to understand the timing of metamorphism in high-and low-grade terranes, as well as the timing of high-grade metamorphism in the study area relative to other high-grade terranes within the craton. The metamorphic assemblages and monazite ages documented in this chapter elucidate a more complex metamorphic history for the SW Ghana and the Eburnean Orogeny, thus aiding our understanding of the amalgamation of the West African Craton. We consider the significance of the metamorphism documented in SW Ghana relative to the global metamorphic record, which appears to be unique the Palaeoproterozoic plate tectonic regime.

This chapter was submitted to the Journal of Metamorphic Geology and was returned for revision for resubmission in May 2018. The major suggested revisions have been addressed, with some revisions still in progress.

Introduction

Precambrian geodynamics and lithosphere tectonics continue to provide a source of controversy in scientific literature (e.g. Cawood, et al., 2006;Condie & Benn, 2006;[START_REF] Reddy | Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere[END_REF]. Of particular interest are the tectonic settings and processes associated with Precambrian accretionary and collisional orogenesis, culminating in craton assembly and stabilisation (Condie, 2007;Windley, 1992). Precambrian accretionary orogens include all Archaean cratons and several Palaeoproterozoic provinces (Windley, 1992) and are characterised by high crustal production rates, voluminous juvenile magmatism, elevated geothermal gradients and low lithospheric strength relative to the Phanerozoic (Condie, 2007;Gerya, 2014;Sizova, et al., 2010). The comparison of metamorphic conditions and tectonic settings preserved in Precambrian orogens relative to the Phanerozoic, therefore, remains uncertain. Archaean orogens commonly comprise elongate greenstone belts and vast granite-gneiss domains, featuring low-pressure granulite facies metamorphism and homogeneous strain patterns (e.g. Bouhallier, et al., 1995;Choukroune, et al., 1995). Whilst some authors interpret such features within the modern plate tectonics paradigm (e.g. Cawood, et al., 2006;de Wit, et al., 1992), alternative tectonic processes have also been proposed by a number of authors (e.g. Bouhallier, et al., 1995;Choukroune, et al., 1995;Hamilton, 1998;Rey & Houseman, 2006), emphasising the potential significance of diapirism, vertical movement and gravitational forces in granite-greenstone regions during lithospheric deformation. Additionally, the "hot orogen" model proposed for some Precambrian provinces accommodates lateral shortening in weak, hot crust through distributed deformation, with minimal topographic expression due to lateral or three-dimensional flow (Cagnard, et al., 2006;Cruden, et al., 2006;Rey & Houseman, 2006). Such orogens are commonly characterized by broad regions with isobaric metamorphism and lateral temperature gradients [START_REF] Bleeker | New structural-metamorphic constraints on Early Proterozoic oblique collision along the Thompson Nickel Belt, Manitoba, Canada[END_REF]Caby, et al., 2000;Cagnard, et al., 2007;Chardon, et al., 2009), resulting from homogeneous uplift of the entire domain (e.g. [START_REF] Gapais | Mountain building and exhumation processes through time: inferences from nature and models[END_REF]. In contrast, Phanerozoic accretionary orogens involve subduction of cold, rigid lithosphere, able to support significant crustal thickening, thus generating and exhuming high-pressure (HP) to ultra-high-pressure (UHP) metamorphic terranes, inferred as the hallmark of modern-style plate tectonics (Stern, 2005;2007).

A number of studies, however, have demonstrated the sporadic appearance in the Neoarchaean and Palaeoproterozoic of high-pressure and/or eclogite facies metamorphic conditions and diverse geothermal gradients within metamorphic domains of Precambrian orogens (e.g. [START_REF] Block | The Murchison Greenstone Belt, South Africa: accreted slivers with contrasting metamorphic conditions[END_REF]Collins, et al., 2004;Ganne, et al., 2012;Moyen, et al., 2006;[START_REF] Nicoli | Rapid evolution from sediment to anatectic granulite in an Archean continental collision zone: the example of the Bandelierkop Formation metapelites, South Marginal Zone, Limpopo Belt, South Africa[END_REF][START_REF] Volodichev | Archean eclogites in the Belomorian mobile belt[END_REF]. In a review of the secular evolution of metamorphism, Brown (2007;2009) notes the appearance of complementary eclogite-high-pressure granulite (E-HPG) and ultra-high temperature (UHT) metamorphism, which he infers to be the ancient expression of subduction-to-collision orogenesis. Brown (2006) proposes a transitional "Proterozoic plate tectonic" regime based on the diversification of recorded metamorphic conditions after the Neoarchaean. Rare high P/T and low geothermal gradients (<20 °km -1 ) and evidence of structurally controlled exhumation are documented in the MesoArchaean Barberton greenstone-granitoid terrane (Moyen, et al., 2006), the Neoarchaean to Palaeoproterozoic Jiaodong Terrain in the North China Craton [START_REF] Liu | Neoarchean to Paleoproterozoic high-pressure mafic granulite from the Jiaodong Terrain, North China Craton: Petrology, zircon age determination and geological implications[END_REF], the Salem block of southern India [START_REF] Anderson | High-pressure granulites at the dawn of the Proterozoic[END_REF], the Usagaran Orogen , Tanzania (Collins, et al., 2004), the Fada N'Gourma area of Burkina Faso (Ganne, et al., 2012) and high-grade paragneisses in NW Ghana (Block, et al., 2015), where the latter two examples are from the West African Craton (WAC).

However, debate continues to surround the tectonic setting producing HP-UHP metamorphic rocks and the mechanism(s) of exhumation in the Neoarchaean and Palaeoproterozoic (Block, et al., 2015;Block, et al., 2016b;Brown, 2007;Ganne, et al., 2014;van Hunen & van den Berg, 2008). In this paper, we investigate variations in the thermal history and exhumation mechanisms active during the Palaeoproterozoic Eburnean Orogeny of the WAC, focusing on newly documented high-grade domains of the Sefwi greenstone belt in southwest Ghana.

The Palaeoproterozoic Baoulé-Mossi domain (2300-2070 Ma) of the southern WAC is a vast (3 x 10 6 km 2 ) juvenile crustal domain, comprising Archaean-like tonalite-trondhjemite-granodiorite granite-gneiss domains and linear-to-arcuate metavolcanic and metasedimentary greenstone belts, bound by crustal-scale strike-slip shear zones (Bessoles, 1977). Widespread crustal reworking and the emplacement of more evolved, potassic granites is associated with terrane accretion and the collision of the juvenile Palaeoproterozoic domain with the Archaean Kénéma-Man nucleus during the 2150 -2070 Ma Eburnean Orogeny (Bonhomme, 1962;Feybesse, et al., 2006;Hirdes, et al., 1996;Kouamelan, et al., 1997;Ledru, et al., 1994;[START_REF] Milési | The metallogenic relationship between Birimian and Tarkwaian gold deposits in Ghana[END_REF]Pitra, et al., 2010). The Eburnean Orogeny represents one of the oldest Palaeoproterozoic orogens on Earth during a period of global orogenesis between 2100 Ma and 1800 Ma, and is likely to herald the onset of the amalgamation of the supercontinent, Nuna (or Columbia) (Rogers & Santosh, 2002;Zhao, et al., 2002;Zhao, et al., 2004).

There remains, however, disagreement regarding the tectonic processes responsible for the WAC assembly, which can be partially attributed to the limited number of studies with integrated metamorphic and geochronological data (e.g. Block, et al., 2015;Feybesse, et al., 2006;Kouamelan, et al., 1997;Pitra, et al., 2010). In this study we combine field observations, petrological and geochronological data to investigate spatial and temporal variation in the formation and exhumation of high-grade tectonometamorphic domains juxtaposed with low-grade supracrustal rocks in the south-eastern portion of the WAC. We address ambiguities in the relationship between high-grade gneissic domains, greenstone belts sensu stricto and metasedimentary provinces. The results are compared with those of Block et al. (2015) and we discuss the potential tectonic models proposed for the WAC. The findings have significant implications for understanding the enigmatic crustal architecture, including deciphering whether high-grade domains represent an older crustal basement or lower crustal equivalents of lowgrade metavolcanic and metasedimentary supracrustal rocks.

Geological Setting

West African Craton geology

The West African Craton (WAC) consists of the northern Reguibat Rise, the Kédougou-Kéniéba and Kayes inliers and the southern Leo-Man Rise (Bessoles, 1977;Rocci, et al., 1991), overlain by the Neoproterozoic to Devonian Taoudeni basin in the central part of the craton. The Leo-Man Rise (Fig. 4.1) comprises the western Archaean Kénéma-Man domain, flanked to the north and east by the Palaeoproterozoic Baoulé-Mossi domain, which are tectonically juxtaposed along the Sassandra Fault (Bessoles, 1977). Palaeoproterozoic rocks are characterised by elongate volcanic, volcano-sedimentary and sedimentary belts of the Birimian Supergroup and voluminous granite-gneiss terranes (Fig. 4.1).

The Birimian Supergroup is subdivided into basaltic to rhyolitic volcanic and volcano-sedimentary rocks of the Sefwi Group (Perrouty, et al., 2012) and the overlying volcaniclastic, volcano-sedimentary and rare chemical (Mn-rich) sedimentary rocks of the Kumasi Group (Adadey, et al., 2009;Perrouty, et al., 2012). Volcanic units display tholeiitic to calc-alkaline affinities (e.g. Abouchami, et al., 1990;Baratoux, et al., 2011;Boher, et al., 1992) emplaced across the craton between 2300 and 2160 Ma (Doumbia, et al., 1998;Hirdes & Davis, 1998;Hirdes, et al., 1996;Loh, et al., 1999;Pouclet, et al., 2006).

Deposition of the Birimian volcano-sedimentary units is constrained by detrital zircon, indicating a maximum deposition age of ca. 2150 Ma (Davis, et al., 1994;Oberthür, et al., 1998), continuing until late-depositional emplacement of a granodiorite pluton in the Kumasi Basin at 2136 ± 19 (Adadey, et al., 2009). Volcano-plutonic greenstone belts are commonly unconformably overlain by shallow water, fluvio-deltaic sedimentary rocks of the Tarkwaian Group (Kitson, 1918;Whitelaw & Junner, 1929) deposited in small fault-bounded basins, with detrital zircons constraining a maximum deposition age of c. 2130, or possibly as young as c. 2107 Ma (Davis, et al., 1994;Hirdes & Nunoo, 1994;Pigois, et al., 2003). Granite-gneiss terranes of the West Africa Craton formed during several protracted pulses of intense Na 2 O-and CaO-rich trondhjemite-tonalite-granodiorite (TTG) plutonic activity between ca.

2200 and 2130 Ma (Doumbia, et al., 1998;Egal, et al., 2002;Feybesse, et al., 2006;Gasquet, et al., 2003;Hirdes, et al., 1996;Oberthür, et al., 1998). Subsequent magmatism evolved towards potassic, peraluminous granites by ca. 2070 Ma (e.g. Block, et al., 2016a;Egal, et al., 2002;Gueye, et al., 2008;Hirdes, et al., 1992;Lompo, 2009).

Initial juvenile crustal formation is speculated to have evolved from an ocean plateau in association with mantle plume activity, with subsequent calc-alkaline plutonism and contemporaneous volcanism (Abouchami, et al., 1990;Boher, et al., 1992;Leube, et al., 1990) or alternatively from arc volcanism followed by micro-plate accretion (Baratoux, et al., 2011;Dampare, et al., 2008;Petersson, et al., 2016;Pouclet, et al., 1996;Sylvester & Attoh, 1992). This juvenile material was deformed during the c. 2150-1980 Ma Eburnean Orogeny (Bonhomme, 1962). Early crustal thickening was attributed by some authors to magmatic accretion similar to a modern plate tectonic setting, which was subsequently overprinted by thrust faults (e.g. Feybesse, et al., 2006;Feybesse & Milési, 1994;Milési, et al., 1992). Conversely, "Archaic-type" vertical tectonics that emphasised pluton diapirism and the relative downward movement of greenstone belts has also been proposed (Delor, et al., 1995;Pouclet, et al., 1996;Vidal, et al., 1996;Vidal, et al., 2009). Across the craton, regional metamorphism associated with the Eburnean Orogeny varies from lower-greenschist facies to lower-amphibolite facies (e.g. Béziat, et al., 2000;Feybesse, et al., 2006;Ganne, et al., 2012;Kříbek, et al., 2008). Upperamphibolite facies conditions are generally restricted to the contact aureoles of plutons (Debat, et al., 2003;Lobjoit, 1964;Pons, et al., 1995). The absence of HP-LT assemblages and significant lateral metamorphic breaks has been attributed by some authors to elevated geothermal gradients (>25 °Ckm -1 ) and homogenous uplift (Pons, et al., 1995;Vidal, et al., 2009).

Recent studies, however, have revealed heterogeneity in thermal gradients and metamorphic conditions (Block, et al., 2015;Ganne, et al., 2012). In southern Ghana, regional MP-MT amphibolite facies metamorphism (5-6 kbar, 500-600 °C) is recorded within the Sefwi and Ashanti belts (Galipp, et al., 2003;John, et al., 1999), and slightly higher anatectic conditions occur in the Winneba-Awutu migmatites on the Ghanaian southern coast ( >5 kbar, >600 °C) (Loh, et al., 1999;Opare-Addo, et al., 1993). High-pressure granulites are restricted to the tectonic contact between the Archaean Kénéma-Man shield and the Palaeoproterozoic Baoulé-Mossi domain (Fig. 4.1), where conditions reached ~13 kbar and 850 °C (Pitra, et al., 2010). Ganne, et al. (2012) documented an early, cold thermal regime (<10-15 °Ckm -1 ) and high-P greenschist to blueschist facies conditions in chlorite-phengite assemblages in greenstone belts in eastern Burkina Faso. Similar relic metamorphic conditions (8-10 kbar, 400-500 °C and 11-14 kbar, 520-560 °C) are preserved in volcano-sedimentary schists and paragneisses of northwest Ghana (Block, et al., 2015). Where observed, sharp metamorphic breaks between high-and low-grade domain are interpreted as the product of tectonic exhumation and juxtaposition of different crustal levels (Block, et al., 2015;Block, et al., 2016b;Hirdes, et al., 2007;Loh, et al., 1999;Opare-Addo, et al., 1993), which contradicts lateral, granitoid-related metamorphic gradients used to interpret high-and low-grade domains within the "hot" orogen models (Chardon, et al., 2009;Vidal, et al., 2009).

There are a number of models proposed for the tectono-metamorphic evolution of the West African Craton based on the interpretation of the relationship between high-grade metamorphic domains and low-grade volcano-sedimentary sequences. Small exposures of poly-deformed highgrade rocks in Burkina Faso and Ivory Coast (Boher, et al., 1992;de Kock, et al., 2011;2012), or polydeformed metavolcanics in SW Ghana (Perrouty, et al., 2012), have been interpreted as the basement upon which Birimian metasedimentary sequences were deposited. This sporadically recognised event is referred to as Eburnean I (Allibone, et al., 2002) or Eoeburnean (de Kock, et al., 2011;Perrouty, et al., 2012) in Ghana and western Burkina Faso (Baratoux, et al., 2011;Metelka, et al., 2011), and the Tangaean event in north-eastern Burkina Faso (Hein, 2010;[START_REF] Tshibubudze | The Markoye Shear Zone in NE Burkina Faso[END_REF]. It precedes the more widely recognised phase of the Eburnean (or Eburnean II) Orogeny (~2130 -1980 Ma), associated greenschist facies metamorphism and transcurrent tectonism (Allibone, et al., 2002;Eisenlohr & Hirdes, 1992;Feybesse, et al., 2006;Jessell, et al., 2012;Pouclet, et al., 2006;Pouclet, et al., 1996;Vidal, et al., 2009). Alternatively, a monocyclic evolution is proposed for the Eburnean Orogeny, during which high-grade gneisses and migmatites and low-grade rocks undergo coeval metamorphism at different crustal levels during a single progressive event and are subsequently tectonically juxtaposed (Block, et al., 2015;Block, et al., 2016b;Eisenlohr & Hirdes, 1992;Opare-Addo, et al., 1993).

Distinguishing between monocyclic and polycyclic models for the Eburnean orogeny is hampered by the limited amount of precise geochronological data constraining the time of high-grade metamorphic events, most notably lacking in southern Ghana. Amphibolite facies metamorphism in central Ivory Coast is dated at 2153 ± 93 Ma (Grt-WR Sm-Nd; Boher, et al., 1992) and 2144 ± 3 Ma in the Ivorian extent of the Sefwi belt (Pb-Pb Tnt; Hirdes, et al., 2007). In-situ ICP-MS and SHRIMP U-Pb monazite crystallisation ages, hosted by leucosomes and overprinting assemblages constrained anatexis at 2138 ± 7 Ma and overprinting amphibolite facies metamorphism at 2130 ± 7 Ma in migmatitic para-and orthogneissic units in NW Ghana (Block, et al., 2015). These ages are distinctly older than geochronological data revealed by syntectonic intrusions and metamorphic units from across the craton, which constrain Eburnean deformation and low-grade metamorphism between 2114 and 2080

Ma (de Kock, et al., 2011;Feybesse, et al., 2006;Hirdes, et al., 1996;Kouamelan, et al., 1997). The final tectono-metamorphic event records the collision of the Archaean and Palaeoproterozoic domains between ~2080 and 2030 Ma based on U-Pb zircon, monazite, titanite ages and Sm-Nd garnet ages (Kouamelan, et al., 1997;Pitra, et al., 2010). Whilst disparity in metamorphic ages recorded across the craton might suggest multiple metamorphic peaks between ca. 2150 and 2030 Ma, further integrated tectono-metamorphic and geochronological data are required to unravel the tectonic evolution and assembly of the WAC.

Geology of southwest Ghana

The Palaeoproterozoic rocks of southern Ghana and southeast Ivory Coast are characterised by NE-striking, parallel greenstone belts several hundred kilometres long and 20-60 km wide, with large, intervening metasedimentary domains. This study focuses on the fault-bounded, lithotectonic domains of the Sefwi greenstone belt and the adjacent Sunyani-Comoé and Kumasi-Afema metasedimentary domains (Fig. 4.2). Outcrop is exceedingly sparse and the field area is extensively covered by thick regolith, cocoa plantations and forest reserves.

The field area is dominated by the 280 km-long, 40-80 km-wide Sefwi Greenstone Belt, which is bounded to the north by the Kenyase-Yamfo shear zone and to the south by the Afema shear zone (Hirdes, et al., 2007) (Fig. 4.2). It is transected by the Ketesso High Shear Zone, which trends parallel to the Sefwi Belt structural grain (Hirdes, et al., 2007;Jessell, et al., 2012). In this study, the belt is subdivided into four litho-tectonic domains: the Kukuom-Juaboso domain (KJD), the Bechem domain (BD), the Hwidiem domain (HD) and the southern Sefwi-Wiawso domain (SWD) (Fig. 4.2). The central, rhomboidal Kukuom-Juaboso domain, is an 80 km long, elongate in a NEtrending domain that comprises amphibolite facies mafic to intermediate volcanic rocks, immature volcaniclastic greywackes, minor metasedimentary units and extensive granitoids intrusions (Galipp, et al., 2003;Hirdes, et al., 1993). The Bechem granitoid domain is characterised by layered intrusions of gabbro, norite and pyroxenite, strongly deformed amphibolites, large quartz-diorite and granodiorite plutons, and small, elliptical post-tectonic granites (Agyei Duodu, et al., 2009). The northern margin of the adjacent Bechem and Kukuom-Juaboso domains is marked by a ENE-striking shear zone, which contains slivers of migmatitic mafic orthogneisses (this study). Geochronological constraints for magmatism for these domains are limited to a rhyolite yielding a U-Pb zircon age of 2189 ± 1 Ma (Hirdes & Davis, 1998) and a hornblende-biotite granodiorite U-Pb zircon age of 2135

Ma (Amponsah, 2012). The NE-striking Ketesso shear zone transects the central region of the Sefwi belt, south of which is the low-grade Sefwi-Wiawso volcano-plutonic domain of the belt, comprising greenschist facies mafic to intermediate lavas and abundant granitoid intrusions that display pervasive chlorite-carbonate or quartz-sericite alteration with minor accessory sulphides. U-Pb zircon ages for granitoids in the Sefwi-Wiawso domain yield crystallisation ages between 2180 and 2140 Ma (Hirdes, et al., 1992;Hirdes, et al., 2007). or, in the case of the Ashanti belt, derived from the literature (e.g. Perrouty, et al., 2012). U-Pb zircons ages are sourced from: (1) Petersson, et al. (2016); (2) Hirdes, et al. (1992); ( 3) Delor, et al. (1992).

Towards the northeast of the study area, the Chiraa high-grade domain (formerly referred to as the Sunyani batholith) is a 25 x 35 km triangular shaped domain (Fig. 4.2), comprising strongly deformed, migmatitic paragneiss with abundant granite intrusions and pegmatite dykes, amphibolite enclaves and rare migmatitic mafic orthogneisses. Granitic intrusions often contain tectonic slivers of paragneiss and biotite-rich schlerien. Large exposures of paragneiss units comprise alternating pelitic and psammopelitic layers. Rare exposures of felsic orthogneiss display a gneissosity parallel with that of the paragneisses. This is in tectonic contact with greenschist facies volcano-sedimentary rocks of the Sunyani-Comoé basin along a NNE-striking fault. The eastern margin of the Chiraa high-grade domain is unconformably overlain by undeformed Neoproterozoic Voltaian sedimentary rocks (Kalsbeek, et al., 2008).

The Sunyani-Comoé Domain comprises low-grade chlorite-white mica schists, greywackes, argillites, banded iron stones, graphitic shales, volcaniclastic rocks and rare intercalated intermediate volcanic rocks of the Kumasi Group (Agyei Duodu, et al., 2009;Hirdes, et al., 2007;Hirdes, et al., 1993). Similar lithologies are found in the Kumasi-Afema Domain to the south of the study area, south of which lies the Ashanti Greenstone Belt (Agyei Duodu, et al., 2009;Perrouty, et al., 2012).

U-Pb zircon crystallization ages (ca. 2093 -2081 Ma) of leucogranites intruding the Sunyani-Comoé Domain parallel to the north-western margin of the Sefwi Belt (Fig. 4.2) (Agyei Duodu, et al., 2009;Hirdes, et al., 2007;Hirdes, et al., 1993;Petersson, et al., 2016).

Methods

Petrographic analysis and mineral chemistry

Mineral assemblages, textural context and microstructures were examined in polished thin sections under transmitted, plane polarised (PPL) and cross polarised light (XPL). Backscattered electron (BSE) imaging was performed at Monash Centre for Electron Microscopy at Monash University, Australia, using a JEOL JSM-7001F Field Emission Gun (FEG) Scanning Electron Microscope (SEM). Preliminary mineral chemistry examined by energy-dispersive x-ray spectroscopy (EDS)

using Oxford Instruments Aztec X-ray analysis system with 80mm 2 Silicon Drift Detector (SDD).

Conditions used for BSE and EDS mineral chemical composition characterization are: accelerating voltage of 15 keV, working distance (WD) of 10mm, and probe current of 10A, objective aperture of 3.

To image internal structures of monazites, accelerating voltage was increased to 20 keV with a probe current of 14. Further BSE imaging and chemical characterization was performed at Géoscience Environnement Toulouse, Université Paul Sabatier (Toulouse III), France, using a JEOL JSM 6360LV coupled to a PGT Sahara SDD EDS detector. Imaging conditions used accelerating voltage of 20 keV, WD of 10mm and spot size of 50µm. Mineral compositions were measured using a Cameca-SX-5 at Centre de Microcaractérisation Raimond CASTAING in Toulouse, France.

P-T calculations

To derive P-T conditions and metamorphic paths, pseudosections were calculated for the whole rock compositions, calculating minimum Gibbs free energy in the P-T space represented by a multiphase equilibrium diagram. Pseudosections were constructed using the computer software package Perple_X (version 6.6.8) [START_REF] Connolly | Multivariable phase diagrams; an algorithm based on generalized thermodynamics[END_REF][START_REF] Connolly | Computation of phase equilibria by linear programming: a tool for geodynamic modeling and its application to subduction zone decarbonation[END_REF] The bulk composition of each sample was obtained with X-ray fluorescence (XRF) at the Australian Research funded Centre of Excellence in Ore Deposits (CODES) at the University of Tasmania using a PANalytical Axios Advanced X-Ray Spectrometer [START_REF] Robinson | XRF analysis of flux-fused discs[END_REF][START_REF] Watson | Fast, simple method of powder pellet preparation for X-ray fluorescence analysis[END_REF]. CaO was corrected for apatite content using measured P 2 O 5 , and ferric iron was transformed into ferrous iron using the following equation:

M Fetotal = M FeO (wt%) = 0.9 × M Fe2O3 (wt %).

The internally consistent thermodynamic dataset of Holland and Powell (1998) (updated in 2002) was used for minerals and fluids (CORK model: [START_REF] Holland | A Compensated-Redlich-Kwong (CORK) equation for volumes and fugacities of CO2 and H2O in the range 1 bar to 50 kbar and 100-1600 C[END_REF]). Elemental fractionation during prograde garnet porphyroblast growth was considered for sample SB177 which has a garnet modal proportion of >1%, using the method according to [START_REF] Marmo | Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia[END_REF]. Solid solution models used for calculations of pseudosections for metapelitic samples were as follows:

Bio(TCC) for biotite [START_REF] Tajčmanová | A thermodynamic model for titanium and ferric iron solution in biotite[END_REF], Chl(HP) for chlorite (Holland, et al., 1998), St(HP)

for staurolite (Holland & Powell, 1998), Ctd(HP) for chloritoid (Holland & Powell, 1998), Gt(WPH) for garnet [START_REF] White | Calculation of partial melting equilibria in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (NCKFMASH)[END_REF], Ilm(WPH) for ilmenite, hCrd for cordierite, Mica(CHA1) for titanium-, magnesium-and iron-bearing white mica [START_REF] Auzanneau | Titanium in phengite: a geobarometer for high temperature eclogites[END_REF][START_REF] Coggon | Mixing properties of phengitic micas and revised garnet-phengite thermobarometers[END_REF], melt(HP) for haplogranitic melts (Holland & Powell, 2001), Pl(h) for plagioclase feldspar and Kf for potassic feldspar.

For the garnet amphibolite (sample SB344), ferric iron content in amphiboles was calculated using the spreadsheet of [START_REF] Locock | An Excel spreadsheet to classify chemical analyses of amphiboles following the IMA 2012 recommendations[END_REF] and was found to be negligible. The structural formulae and andradite content of garnet porphyroblasts were calculated according to the methods of [START_REF] Droop | A general equation for estimating Fe3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria[END_REF].

Given ilmenite is the only oxide observed in the sample and minimal ferric iron in the structural formula of garnet and amphibole, O 2 and Fe 2 O 3 were excluded from calculations. Sample SB344 was modelled using the following solid solution models:

Gt(WPH) for garnet [START_REF] White | Calculation of partial melting equilibria in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (NCKFMASH)[END_REF], GlTrTsPg for amphibole [START_REF] Wei | Phase relations in high-pressure metapelites in the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) with application to natural rocks[END_REF][START_REF] White | A mineral equilibria study of the hydrothermal alteration in mafic greenschist facies rocks at Kalgoorlie, Western Australia[END_REF], Act(M) for actinolite, Ilm(WPH) for ilmenite, melt (HP) for haplogranitic melts (Holland & Powell, 2001), Pl(h) for plagioclase feldspar, Opx(HP) for orthopyroxene and Omph(GHP2) for clinopyroxene [START_REF] Diener | Revised activity-composition models for clinopyroxene and amphibole[END_REF][START_REF] Green | An order-disorder model for omphacitic pyroxenes in the system jadeite-diopside-hedenbergite-acmite, with applications to eclogitic rocks[END_REF].

Geochronology

In-situ SHRIMP U-Th-Pb monazite dating

Monazite grains were initially located and investigated with transmitted and reflected light optical microscopy for textural location and optical features. The internal structure of each grain was further characterised by back-scatterede electron (BSE) imaging using a JEOL FEG 7001F scanning electron microscope at the Monash Centre for Electron Microscopy, Monash University, Melbourne, Australia.

Monazite grains were analysed in situ using the Sensitive High Resolution Ion Microprobe (SHRIMP II) at the Research School of Earth Sciences, the Australian National University, Canberra, Australia.

To ensure monazites were dated in the correct textural and petrogenetic context, polished thin sections were cut and mounted in a 30mm diameter SHRIMP "megamounts. " Large scale maps of the final mounts were constructed from reflected light photographic mosaics to target SHRIMP spot analysis. Monazite standard 44069, which has a U/Pb age of 425 Ma and U concentration of 2100 ppm [START_REF] Aleinikoff | Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite[END_REF] was mounted in each epoxy mount with thin section segments. The analyses were performed using a ~ 5nA negative O primary beam and a selected spot size of 15 μm. UO, regularly interspersed with analysis of the monazite standard. Data reduction was performed according to Williams (1998, and references therein), using SQUID 2 Excel macro of [START_REF] Ludwig | [END_REF] and Excel macro Isoplot 3 (Ludwig, 2003). Age calculations used decay constants recommended by the IUGS Subcommission on Geochronology (as given in [START_REF] Steiger | Subcommission on geochronology: convention on the use of decay constants in geo-and cosmochronology[END_REF] and corrections for common Pb were made using the measured 204 Pb and the relevant common Pb compositions (Stacey & Kramers, 1975). Uncertainties of 1s are given for individual U-Pb analyses (ratios and ages), however, uncertainties in the calculated weighted mean ages are reported as 95% confidence limits and include the uncertainties in the standard calibrations where appropriate.

Petrography and mineral chemistry

Field work was conducted throughout the Sefwi Greenstone Belt and the adjacent Sunyani-Comoé and Kumasi-Afema domains to collect samples of the major metamorphic rocks and igneous suites within distinct litho-tectonic domains. Samples were selected based on the geographic location, structural context and informative mineral assemblages for geothermobarometry. Sample location, peak metamorphic assemblages and whole rock bulk composition for key samples are presented in Tables 4.1 and 4.2. Table 4.3 contains representative mineral compositions described in the text.

Mineral abbreviations are after Whitney and Evans (2010). [START_REF] Kohn | Retrograde net transfer reaction insurance for pressure-temperature estimates[END_REF]Spear, 1993). Garnet cores contain fine-grained, pluri-mineralic White mica inclusions have high potassic white mica content (Ms = 100 x (K/K + Ca + Na) of Ms=88-92 and elevated TiO 2 content of 0.68-1.02 wt %. It has a composition of Si = 3.11-3.13 a.p.f.u, Fe 2+ = 0.07-0.08 a.p.f.u. and Mg = 0.03-0.04 a.p.f.u. (11O). This composition indicates partial phengitic and Ti substitution by Si + R 2+ = Al vi + Al iv and Al vi + Si = Ti + Al iv , consistent with high-P 2M 1 phengites [START_REF] Auzanneau | Titanium in phengite: a geobarometer for high temperature eclogites[END_REF][START_REF] Cibin | The octahedral sheet of metamorphic 2M1phengites: A combined EMPA and AXANES study[END_REF][START_REF] Massonne | Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz[END_REF], and is, therefore, denoted as phengite1 (Ph1). The euhedral shape and preservation of crystal faces of plagioclase2 (Fig. 4.4c)

suggests crystallisation from a melt [START_REF] Vernon | Igneous microstructures in migmatites[END_REF]. Plagioclase2 shows a small decrease in [30][31][32][33][34][35][36]. A second assemblage (B) is, therefore defined as Grt + Bt1 + Ph1 + Pl2 + Rt + Qz ± L (melt) associated high amphibolite facies metamorphism (e.g. Spear, 1993), potentially corresponding with high-P amphibolite facies conditions. The transition from metamorphic assemblage (A) in the garnet core to metamorphic assemblage (B) in the garnet mantle is interpreted as the product of two reactions. The first is the chlorite dehydration reaction (R1) corresponding to increasing pressures and temperatures:

Chl + Kfs + tk → Ph1 + Bt + Qz + H 2 O (R1),
after [START_REF] Simpson | Phase relations, singularities and thermobarometry of metamorphic assemblages containing phengite, chlorite, biotite, K-feldspar, quartz and H2O[END_REF], whereby tk is an additional Tschermak component [Al 2 (Fe, Mg) -1 Si -1 ] exchanged with garnet [START_REF] Coggon | Mixing properties of phengitic micas and revised garnet-phengite thermobarometers[END_REF]. The absence of Kfs inclusions in garnet may indicate its total consumption as a reactant, typical of average Al-rich, K-poor metapelites bulk compositions [START_REF] Simpson | Phase relations, singularities and thermobarometry of metamorphic assemblages containing phengite, chlorite, biotite, K-feldspar, quartz and H2O[END_REF]Spear, 1993). The shift from assemblage (A) to assemblage (B) indicates partial melting of the rock and transition to suprasolidus conditions. The third assemblage (C) is defined by phases in the matrix and the appearance of kyanite. Garnet porphyroblasts are observed in contact with matrix biotite (Bt2) and subhedral, matrix plagioclase (Pl3) (An 27-36), displaying overlapping An content to Pl2 inclusions in garnet. Kyanite has two grain morphologies in the matrix indicating multiple stages of growth. Millimetric, irregular anhedral kyanite grains containing rare monazite grains are observed as irregular inclusions parallel to elongate matrix biotite (Bt2), with a composition of #Mg = 42-45 and Ti vi = 0.12-0.17 a.p.f.u (based on 11 oxygen). Bt2 defines the penetrative metamorphic foliation in SB398. The more abundant form of kyanite is represented by millimetric to centimetric, near euhedral, poikilitic porphyroblasts. These contain inclusions of matrix biotite (Bt2), quartz, rutile, monazite and graphite aligned parallel or subparallel to the c-axis (Fig. 4.4a, e, f), thus kyanite is interpreted as syntectonic. Poikiloblastic kyanite is not observed in equilibrium contact with garnet porphyroblasts. Production of peritectic kyanite during anatexis is interpreted as the product of near total consumption of phengite, attributed to muscovite dehydration reaction 2 (R2) in suprasolidus conditions:

Ms (Ph2) + Pl + Qtz → Ky + melt (L) (R2),
consistent with reactions observed in low-temperature (<800 °C) anatectic pelites in the Greenland Caledonides [START_REF] Kalsbeek | Geochemistry and petrogenesis of S-type granites in the East Greenland Caledonides[END_REF].

Assemblage (C) comprises the phases Grt + Bt2 + Ky + Pl3 + Ph2 + Rt + Qz ± L. Remaining phengite in the matrix (Ph2) has a similar, although more variable, composition to phengite1 inclusions in the garnet with a composition of Si= 3.06-3.19 a.p.f.u, Al iv = 0.81-0.83 a.p.f.u., Al vi = 1.81-1.95 a.p.f.u. and Mg = 0.04-0.08 a.p.f.u. (11O). Fe 2+ in phengite2 displays a significant decrease from ~ 0.07 a.p.f.u. to ~ 0.05 a.p.f.u.. Biotite dehydration in SB398 is limited by its coexistence with rutile, thus extending its stability under higher temperatures [START_REF] Guidotti | Micas in metamorphic rocks[END_REF][START_REF] Stevens | Melt production during granulite-facies anatexis: experimental data from "primitive" metasedimentary protoliths[END_REF]. Furthermore, the inclusion of rutile grains in kyanite and as single grains in the matrix indicates that the transition from assemblage (B) to assemblage (C) remained at temperature and pressure conditions above the stability fields of ilmenite, at the amphibolite-granulite facies transition.

Early retrogression in sample SB398 is indicated by small muscovite overgrowths on kyanite rims (Fig. 4.4f), also overgrowing the biotite foliation associated with the back reaction of fluid produced during crystallisation of small volumes of melt, as defined by reaction 3: suggesting partial resorption along garnet rims [START_REF] Kohn | Retrograde net transfer reaction insurance for pressure-temperature estimates[END_REF]Spear, 1993).

Ky + Pl + H 2 O → Ms + Qz (R3).

Sample SB389 (migmatitic biotite-sillimanite-garnet paragneiss)

Sample SB389 is from a migmatitic biotite-sillimanite paragneiss with relict garnet porphyroblasts, Petrographic relationships and mineral chemical compositions are used to constrain three metamorphic assemblages (A-C). Garnet is preserved as relict, millimetric rounded-toglobular, anhedral porphyroblasts, with quartz, plagioclase (Pl1) and biotite (Bt1) inclusions (Fig.

located

4.7a

), representing a modal proportion of less than 1%. Garnet often appears as disaggregated porphyroblasts, truncated along overprinting biotite foliations. Garnet composition is strongly zoned with a core composition of , with increasing Sps (16-20) and decreasing Prp (10-11) towards garnet rim indicate strong garnet resoption and element diffusion [START_REF] Kohn | Retrograde net transfer reaction insurance for pressure-temperature estimates[END_REF]Spear, 1993). We note that the combined evidence of anhedral grain morphology and garnet resorption indicate that garnet compositions must be treated with caution.

Anhedral biotite (Bt1) inclusions observed in garnet have a composition of #Mg = 43-45 and Ti vi = 0.12-0.14 a.p.f.u. Plagioclase inclusions (Pl) in garnet have An(26-29), within the range observed for plagioclase in the matrix (An23-31). Muscovite observed in contact with garnet porphyroblasts has a composition of Ms = 93-97, Si = 3.04-3.12 a.p.f.u. and Al vi = 1.68-1.80 a.p.f.u., with Fe 2+ = 0.02-0.16 a.p.f.u. and Fe 3+ = 0.08-0.16 a.p.f.u., calculated using charge balance between charge ideal nd charge real, indicating partial ferric iron-rich celadonite substitution, according to [START_REF] Tischendorf | A new graphical presentation and subdivision of potassium micas[END_REF].

The composition is comparable to muscovite inclusions in subhedral matrix plagioclase. Metamorphic assemblage (A) is defined as Grt + Bt1 + Pl + Qz ± Ms, however, we note that microstructural evidence for the prograde assemblage associated with garnet porphyroblasts in this sample is often ambiguous due poor preservation.

Relict kyanite porphyroblasts occur in the matrix with extensively corroded rims and never in equilibrium contact with garnet. Figures 4.7a and 4.7c show a kyanite porphyroblast with a subrounded quartz core and an interstitial cuspate film of plagioclase (An 29-31) pseudomorphing melt, suggesting kyanite was forming in suprasolidus conditions and respresents a peritic phase [START_REF] Holness | On the pseudomorphing of melt-filled pores during the crystallization of migmatites[END_REF]. Kyanite is interpreted as the product of muscovite dehydration melting during reaction 2 (R2) as the rock crossed the solidus. Potassium feldspar was observed only as rare, round inclusions in plagioclase in thin section, suggesting limited production during reaction 2, total consumption during subsequent retrograde reactions or extraction during melt segregation and transportation.

Reaction 2 occurs at the transition from assemblage (A) to assemblage (B) Grt + Bt2 + Ky + Pl + Qtz + L at suprasolidus, high-pressure amphibolite facies conditions. Kyanite porphyroblasts are embayed in thick muscovite overgrowths containing small round inclusions of quartz and biotite (#Mg = 43-44; Ti vi = 0.17-0.18 a.p.f.u.). Muscovite composition is indistinguishable from that of muscovite in contact with garnet. The relationship between kyanite and muscovite overgrowths is interpreted the product of two possible reactions: first, as a rapid metasomatic back reaction between kyanite and water released during in situ melt crystallization (R3) occurring as the rock returned below the solidus, and second, the following closed-system sillimaniteforming, decompression, chain reaction (R4):

Ky + Bt + Qz → Ms → Sil + Bt +Qz (R4),
involving mica as a catalyst [START_REF] Foster | The role of biotite as a catalyst in reaction mechanisms that form sillimanite[END_REF][START_REF] Gervais | Linking metamorphic textures to U-Pb monazite in-situ geochronology to determine the age and nature of aluminosilicate-forming reactions in the northern Monashee Mountains[END_REF]. This reaction produces distinct replacement of kyanite by muscovite with Bt-free mantles and sillimanite growth after biotite with Ms-free mantles forming fine-grained, fibrolitic sillimanite nodules, as described by [START_REF] Foster | The role of biotite as a catalyst in reaction mechanisms that form sillimanite[END_REF]

Sample SB344 (garnet-bearing amphibolite)

Sample SB344 is sourced from an outcrop located 5 km south of the Ketesso shear zone, 40km south of Juaboso (Fig. 4.2). The location of this garnet amphibolite is interpreted as the southernmost extent of the amphibolite-facies Kukuom-Juaboso domain based on its metamorphic grade and structural information. Exposures of chlorite-actinolite bearing, weakly-foliated metabasalts and unmetamorphosed granodiorite plutons are found less than 5 km to the southeast of the garnet amphibolite sample (SB344), therefore, we infer a significant lateral metamorphic break lies to its south and southeast, likely coinciding with a poorly exposed tectonic contact. The penetrative metamorphic foliation in this sample is defined by aligned subhedral to euhedral, elongate amphibole grains and minor plagioclase and quartz-rich bands, wrapping around millimetric garnet porphyroblasts. Strain shadows of garnets primarily consist of quartz. Accessory apatite is visible throughout the sample.

Garnet porphyroblasts are subhedral to euhedral, 1 -3 mm in diameter with aligned inclusions of elongate quartz and magnesio-ilmenite, which preserve a relict foliation (S 1 ) oblique to the penetrative amphibole and plagioclase foliation of the matrix, denoted as S 2 (Fig.

Sample SB164 (garnet-bearing amphibolite)

Sample SB164 comes from the western margin of the amphibolite facies Kukuom-Juaboso domain in the Sefwi greenstone belt, about 10 km north of the township of Juaboso. The domain is separated from the chlorite-white mica schists of the Sunyani-Comoé metasedimentary province by a north-northeast striking shear zone (Fig. 4.2). The rock contains a high modal proportion (~30%) of subhedral garnet porphyroblasts (2-7 mm) and a penetrative foliation. intertectonic [START_REF] Passchier | Microtectonics[END_REF], the mineral assemblages and mineral chemical compositions are too similar to distinguish multiple events without additional geochronological constraints (Vernon, et al., 2008).

Sample SB177 (garnet-clinopyroxene bearing migmatitic gneiss)

Sample SB177 is from a raft of migmatitic, calcic metabasalt surrounded by extensive granite outcrop

Hbl1 + Qz → Grt1 + Cpx1 + Pl1 + H 2 O (R5).
The bulk rock composition has a low Na/Ca ratio, thus prolonging the hornblende-out reaction.

Anatexis and peritectic growth of garnet and clinopyroxene occur during continued hornblende dehydration (R6) as the rock crosses the solidus:

Hbl1 + Pl1 + Qz = Grt2 + Cpx2 + L (or V) (R6)
The peak metamorphic assemblage corresponds with key high-pressure granulitic associations in mafic rocks, forming at the boundary between amphibolite/high-pressure granulite metamorphic conditions (700-850 °C, 10-14 kbar) [START_REF] O'brien | High-pressure granulites: formation, recovery of peak conditions and implications for tectonics[END_REF]. The absence of orthopyroxene in garnet-clinopyroxene-bearing mafic granulites is indicative of partial melting occurring above 10 kbar (Pattison, 2003). Growth of retrograde hornblende (Hbl2) on garnet rims indicates the back reaction (R7) between peritectic minerals and melt, common in rocks with minimal melt extraction:

Grt2 + Cpx2 + melt (L) → Hbl2 + Pl2 (R7)
Hbl2 has #Mg = 34-41, Ca/Na = 5-6, (Ca+Na) B = 1.95-1.99 a.p.f.u. and (Na+K) A = 0.48-0.53 a.p.f.u.

Continued hydration of garnet, and hornblende results in the final retrograde assemblage (C) of marked by the appearance of actinolite, chlorite, epidote, albite and calcite on late fracture networks and grain boundaries of porphyroblasts (Fig. 4.8f) and is defined as Hbl2 + Pl + Act + Ep +Chl + Ab.

The secondary retrograde reaction (R8) is typical of greenschist facies metamorphism:

Grt2 + Hbl2 + Pl + H 2 0 → Act + Chl + Ab + Ep + Calcite (R8).

H 2 O-saturated melting

Partial melting of mafic protoliths occurs extensively along the Hwidiem shear zone and in the core The retrograde path is more poorly constrained. Phases associated with retrogression are limited in modal proportion, evident in small muscovite overgrowths on kyanite rim, the appearance of ilmenite after rutile in the matrix, often in assocaition with muscovite and decrease in the #Mg of structural formulae for garnet and amphibole showed negligible ferric iron content, therefore, ferric iron was not incorporated in pseudosection calculations and epidote is represented by zoisite in the pseudosections.

Results

P-T conditions and

Peak pressure and temperature conditions of 10.0 -10.5 kbar and 580 -640 °C are deduced from grossular and pyrope content of garnet porphyroblasts, overlapping with a calculated assemblage of Grt + Amp + Pl + Ilm + Qz, consistent with the observed assemblage (A). These metamorphic conditions correspond with a slightly cool apparent geothermal gradient of ~17 °C km -1 in the upper amphibolite facies. Following the formation of assemblage (A), the rock crossed hydration reaction 1, into the stability domain of chlorite, as indicated by the partial hydration and growth of chlorite in the fracture network of garnet and along the foliation plane. A further decrease in pressure and temperature crossing out of the stability domain for ilmenite and into that of titanite during reaction 2. The measured #Mg of amphibole and anorthite content of matrix plagioclase constrain greenschist facies conditions of 6.5-9 kbar and 450-500 °C, corresponding with a calculated assemblage of Chl + Amp + Pl + Grt + Zo + Ttn + Qtz and the observed matrix and retrograde assemblage.

Samples SB164 (garnet amphibolite) and SB177 (garnet-clinopyroxene bearing migmatitic gneiss)

Whilst it is mineralogically similar to sample SB344, attempts to model pressure and temperature condition for garnet amphibolite SB164 using Perple_X were unsuccessful. All pseudosections calculated for the bulk composition of SB164 yielded unresolvable discrepancies between observed mineral assemblages and measured and calcluated mineral compositions. Pseudosections calculated for the garnet-clinopyroxene-bearing mafic migmatite (SB177) indicated unrealistic clinopyroxene stability down to greenschist facies. This is interpreted as the result of elevated CaO content in the bulk composition. Whole rock CaO wt% was corrected for calcite content, however, isopleths derived from the pseudosections with adjusted whole-rock CaO significantly underestimated Ca content relative to the measured Ca composition in the Ca-bearing phases (Grt, Cpx, Pl). Whilst we are confident the petrological and textural evidence in SB177 indicates high grade metamorphism of a mafic to intermediate metavolcanic rock, thermodynamic calculations of P-T for SB177 are deemed unreliable.

Geochronology

Sample SB398 (garnet-kyanite-bearing paragneiss)

Eighteen U-Th-Pb SHRIMP analyses were performed on 14 individual monazite grains from sample SB398 (mounts Z6388 and Z6391). All monazites are unzoned and hosted in biotite and kyanite porphyroblasts or as isolated grains in the matrix. Monazites in this sample are subhedral to anhedral, often featuring prominent elongation. Rare inclusions of quartz occur in larger grains. Some monazites 4.4). The ca. 2073 Ma intercept age is interpreted to record the timing of the growth of kyanite, with metamorphism occurring at the transition from amphibolite to granulite facies immediately above the solidus.

Sample SB389 (biotite-sillimanite gneiss)

Monazites in sample SB389 occur in the matrix commonly associated with biotite and aligned with the penetrative tectonic fabric (Fig. 4.4). Monazites in all textural locations yielded identical dates. As the monazites in SB389 were not shielded in porphyroblasts, this age likely represents the timing of the host rock passing through the closure temperature of monazite as the rock returned to subsolidus conditions during cooling and exhumation.

Discussion

Metamorphic evolution of the study area

The culmination of an extensive, regional-scale field mapping campaign, petrological studies and thermodynamic calculations of P-T conditions and P-T path determinations are presented in Figure 4.14, revealing distinct tectono-metamorphic domains and significant, lateral metamorphic breaks along the major shear zones bounding the Sefwi greenstone belt and normal-sense detachment faults.

In this section, we summarise the progression of metamorphism within the study area, revealing higher pressures and a more complex metamorphic history than previous studies suggest.

1. We interpret the earliest prograde metamorphic event from phases preserved as plurimineral aggregates (Chl + Pl + Rt + Qz ± Bt) in garnet cores of the Grt-Ky-Rt paragneiss sample (SB398) from the Chiraa high-grade domain. This is interpreted as the partial preservation of an early prograde assemblage at temperatures below ~590 °C, in the stability field of rutile (P > 10 kbar). Whilst precise pressures and temperatures could not be constrained, this assemblage is associated with epidote-amphibolite facies conditions and a cool apparent geothermal gradient of <15 °C km -1 . SB398 is the only sample to record such conditions in the early stage of the metamorphic evolution of the study area.

This event is the earliest metamorphic event, however, it is relatively poorly constrained and is therefore denoted as M1e.

2. Subsequent prograde high-pressure amphibolite facies conditions (10.0-11.5 kbar, 600-650 °C) are deduced from phases in the garnet mantle of SB398 (Fig. 4.10b, 4.10d) in the Chiraa high-grade domain and from garnet porphyroblast compositions of the garnet amphibolite (SB344; Fig. 4.12b), ~150 km southwest, in the adjacent Kukuom-Juaboso medium-grade domain. From these two samples, we interpret metamorphism associated with an apparent geothermal gradient of 15-17 °Ckm -1 that was contemporaneous across the two domains and part of the same metamorphic event (M1). The Opx-free, Grt + Cpx + Pl ± Hbl mafic granulite (SB177) on the southern margin of the Chiraa domain indicates higher, but poorly constrained, high pressure granulite facies metamorphic conditions (>10 kbar, >700 °C), associated with an apparent geothermal gradient of < 20 °Ckm -1 . This likely reflects metamorphism at the M1/M2 transition.

3.

In contrast to the samples above, sample SB389 in the Chiraa high-grade domain, ~20km east of SB398, indicates different initial conditions. Relict garnet porphyroblasts in biotite sillimanite paragneiss sample SB389, indicate initial conditions of 5-8 kbar, 560-640 °C. These conditions indicate a higher apparent geothermal gradient than those of M1, however, due to evidence of extensive diffusional reequilibration, we suggest that such constraints are not significantly robust for further interpretation.

4. Following M1, the next metamorphic event, M2, was characterized by metamorphic conditions of 8-10 kbar, 650-700 °C, establishing a new apparent geothermal gradient of 20-30 °C km -1 . This conditions are associated with anatexis recorded only in samples from the Chiraa high-grade domain, best reflected in samples SB389 and SB398 (Fig.

4.10c, 4.11c). P-T conditions for these two samples converge during M2 metamorphism.

Sample SB398 records well the P-T path, evolving from M1 to M2 through heating during minor decompression (<2 kbar) (Fig. 4.10). This compares with sample SB389 (Fig. 4.11) 20km east, that evolves from initial medium-pressure stages and converges towards these same conditions by heating and burial (Fig. 4.9). If these two paths are assumed to be contemporaneous it suggests that the entire terrane is heating up while parts of it are being exhumed (SB398) and others being buried (SB389).

5.

In-situ monazite U-Pb SHRIMP ages of ca. 2073 Ma were yielded by both paragneiss samples (SB398 and SB389), hosted in kyanite porphyroblasts, matrix biotite and as individual grains within the matrix. The consistency of the U-Pb dates indicates relatively uniform monazite crystallisation within the Chiraa Domain. This suggest convergence of P-T paths discussed in point 4. This age post-dates zircon U-Pb crystallization ages of ca.

2092 Ma for two mica granites and pegmatites within the domain (Agyei Duodo et al., 2009;Petersson, et al., 2016), supporting the concept of zircon ages tending to be older than monazite ages in the same rock, commonly observed in high-grade metasediments (Kelsey, et al., 2008). We propose that the uniform monazite ages records the monazite closure temperature at the beginning of the retrograde paths thus recording the initialtion of colling and exhumation. The determined metamorphic history is significantly more complex than that deduced by Galipp, et al. (2003), and indicates much higher pressures than metamorphic conditions (490-580 °C, 4-6 kbar) of Galipp, et al. (2003) derived from garnet-biotite thermometry and plagioclaseamphibole thermobarometry for a schist and a mafic meta-granitoid, located ~ 35km north-northeast of SB344 in the Kukuom-Juaboso domain. Galipp, et al. (2003) infer a clockwise P-T path from an overprinting retrograde assemblage of Chl + Ep + Ab + carbonate, similar to petrological observations the retrograde assemblage in mafic granulite sample SB177.

Regional context

Geochronological constraints of metamorphism and magmatism in the Sefwi belt are severely limited, hindering correlation with similar greenstone or high-grade terranes of the West African Craton.

Emplacement of hornblende-biotite bearing granitoids in domains of the Sefwi belt is restricted to ~2180 -2150 Ma (Delor, et al., 1992;Hirdes, et al., 1992;Hirdes, et al., 2003;Hirdes, et al., 2007), with late high-K granites and granodiorites emplaced at ca. 2136 Ma (Amponsah, 2012;Hirdes, et al., 2007). These ages are significantly older than metamorphic ages revealed in this study and, therefore, are unlikely to be correlated. A single metamorphic date exists in the Ivorian extent of the Sefwi belt, with a metamorphic titanite in a dioritic amphibolite yielding a Pb-Pb age of 2144 ± 3 Ma (Hirdes, et al., 2007). Again, this age is significantly older than monazite ages detailed in this study, however, in the absence of a structural context, no further correlation can be made.

U-Pb zircon crystallisation ages of 2092 ± 2 Ma (Agyei Duodu, et al., 2009), 2093 ± 2 Ma and 2092 ± 4 Ma (Petersson, et al., 2016) were yielded by exposures of two-mica granite and pegmatite within the Chiraa high-grade domain, obtained using SIMS analysis. Leucogranites in the adjacent Sunyani-Comoé domain were emplaced parallel to the northwest margin of the Sefwi belt in upper crustal, greenschist facies metasedimentary rocks between 2088 ± 1 Ma (Agyei Duodu, et al., 2009;Hirdes, et al., 1992) and 2081 ± 6 (Hirdes, et al., 2007) In the context of southern Ghana, MP-MT metamorphic conditions and peak temperatures of ~550 °C in the neighbouring Ashanti greenstone belt occurred at ca. 2100 Ma, with post-peak amphibolite facies metamorphism constrained by a hydrothermal xenotime crystallisation SHRIMP U-Pb age of 2063 ± 9 Ma (Pigois, et al., 2003). Staurolite-grade, amphibolite facies metamorphism of the Tarkwa Phyllite in the Damang gold deposit, in the southern central region of the Ashanti belt, was constrained by EMPA U-Pb monazite analysis, which yielded an age of 2005 Ma [START_REF] White | Constraints on the timing of late-Eburnean metamorphism, gold mineralisation and regional exhumation at Damang mine, Ghana[END_REF], however, the structural context of the monazite may be obscured by textures related to late dyke intrusion. Further south, migmatitic orthogneisses are documented in the Kibi-Winneba belt and in the far southeast of the Ashanti belt (Fig. 4.2) with calculated peak metamorphic conditions of 5 -8.5 kbar, ~600 °C, which are as yet undated (Feybesse, et al., 2006;Opare-Addo, et al., 1993).

Exposed elsewhere in the West African Craton, high-grade terranes comparable to the Chiraa domain reveal metamorphic ages significantly older than ages in southern Ghana. Garnet-whole rock Sm-Nd isochron ages constrain the timing of the amphibolite facies, Yaouré ortho-and paragneisses in central Ivory Coast at 2153 ± 13 Ma, directly west of the study area (Boher, et al., 1992). North of the study area, paragneisses and chlorite-white mica schists in NW Ghana and eastern Burkina Faso, respectively, reveal relic blueschist facies mineral assemblages, matching low apparent geothermal gradients (10 -15 °Ckm -1 ) (Block, et al., 2015;Ganne, et al., 2012). The timing of subsequent anatexis at the amphibolite-granulite transition and a late amphibolite facies overprint in the high grade rocks of NW Ghana occurred between ca. 2140 and 2110 Ma, as indicated by zircons and monazite ages yielded by magmatic and metamorphic units (Block, et al., 2015;2016b). Block, et al. (2015) elucidate a metamorphic history analogous to that of this study, however, the discrepancy in the timing of metamorphism suggests high-grade metamorphism occurred ~40-70 m.y. earlier in NW Ghana than in SW Ghana. High-pressure granulite facies assemblages in western Ivory Coast post-date Ghanaian high-grade metamorphism, forming at c. 2030 during the terminal collision between the Archaean and Palaeoproterozoic domains of the WAC along the Sassandra Fault (Fig. 4.2) (Grt-WR Sm-Nd age, Kouamelan, et al., 1997;Pitra, et al., 2010). Whilst further geochronological data is required to fully understand craton-wide metamorphic relationships, the considerable discrepancy in the timing of high-grade metamorphism and lower crustal exhumation between NW Ghana, SW Ghana and western Ivory Coast suggests a diachronous metamorphic history for the Eburnean Orogeny during the assembly of the West African Craton.

Burial and exhumation

This study reveals an initial period of crustal thickening and burial of supracrustal rocks in high-grade domains to depths greater than 35 km (10-11 kbar) (Fig. 4.14). M1 is characterised by intermediate high-P/T conditions, matching a cool apparent geothermal gradients of 15-17 °km -1 , suggesting rapid burial of sediments during crustal thickening. It has been well established that similar low apparent geothermal gradients (10-20 °Ckm -1 ) record the thermal conditions associated with burial of crustal material to significant depths during the early stages of continental collision [START_REF] England | The influence of erosion upon the mineral fades of rocks from different metamorphic environments[END_REF]England & Thompson, 1984). Given the paucity of outcrop and the extensive overprinting by subsequent transcurrent tectonism, evidence for the crustal thickening mechanism of D 1 is largely obscured. However, based on the ubiquitous bedding-parallel metamorphic foliations and rare, moderately to steeply, north and south pitching down-dip mineral stretching lineations on E-W to ENE-WSW-striking early shear zones, D 1 is likely characterised by N-S to NNW-SSE shortening and thrust-related thickening in a convergent tectonic setting. Similar crustal thickening and rapid burial of metasediments is recorded in northwest Ghana (Block, et al., 2015).

Investigation of negative dP/dT segments of P-T-t-D paths aids in the understanding of the evolution of orogens. They lend additional insight into the uplift or exhumation mechanism of metamorphic terranes, including exhumation of high-pressure and ultra-high-pressure (HP and UHP) rocks via erosion, extrusion, corner flow, buoyancy and extension [START_REF] Franěk | Model of syn-convergent extrusion of orogenic lower crust in the core of the Variscan belt: implications for exhumation of high-pressure rocks in large hot orogens[END_REF][START_REF] Guillot | Exhumation processes in oceanic and continental subduction contexts: a review[END_REF][START_REF] Platt | Exhumation of high-pressure rocks: A review of concepts and processes[END_REF][START_REF] Teyssier | Gneiss domes and orogeny[END_REF], each recording a unique P-T-t path. Combined metamorphic and structural data presented in this study indicate a progressive transition from D 1 to D 2 under continued NNW-SSE shortening. Following peak-P conditions, sample SB398 of the Chiraa domain underwent heating and small amount of exhumation (<2 kbar) to suprasolidus conditions, with the negative dP/dT segment of the clockwise P-T path likely recording thermal relaxation of the crust (England & Thompson, 1984) and the termination of crustal thickening.

Stromatic leucosomes, parallel with the primary gneissosity or bedding-parallel S 1 foliations, feed into NNE-striking dilational zones, indicating late D 1 -syn-D 2 partial melting. D 2 structures are characterised by ~E-W to NE-SW striking, upright to slightly inclined, tight F 2 folds, seen refolding S 0 /S 1 at an outcrop scale and D1 shear zones a regional scale (Fig. 4.15). The L 2 mineral stretching lineation is variably defined by kyanite and amphibole/biotite in the Chiraa and Kukuom-Juaboso domains, respectively. Both F 2 fold axes and L 2 mineral stretching lineations predominantly pitch shallowly W to WSW, or alternatively, shallowly to the east due to later deformation. The dip direction of L 2 and F 2 is oblique to the NE-SW orientation of the major shear zones bounding the Sefwi Belt.

The coaxial relationship of F 2 and L 2 stretching lineations, indicating constrictional folding, and their obliquity to the regional shear zones are characteristic of transtension tectonic regimes (Fossen, et al., 2013). The orientation of the major tensional axis of the strain ellipsoid during D 2 , is therefore interpreted as ENE-WSW, marking the transition from a NNW-SSE compressive regime to a ENE-WSW tensile regime.

Metamorphic breaks coincide with the Kenyase-Yamfo and the Ketesso high-strain zones, with a regional decrease in metamorphic grade from the northeast to the southwest. The most significant metamorphic breaks, however, occur along NNE-striking detachment faults. We propose that anatectic lower crustal rocks of the Chiraa high-grade domain and amphibolite facies, middle crust of the Kukuom-Juaboso domain were differentially exhumed under sinistral transtension and tectonically juxtaposed against the greenschist facies metasediments of the Sunyani-Comoé domain.

The development of a kilometric drag fold rotating the tectonic fabrics in the Chiraa high-grade domain is consistent with sinistral displacement along the high strain zone marking the southern boundary of the domain (Fig. 4.15). Syn-orogenic partial melting of the lower crust has been recognised as the catalyst for the rheological weakening of the crust, resulting in crustal flow in response to either tectonic or gravitational stresses (Brown, 2010;Royden, et al., 2008;Vanderhaeghe, 2009;Vanderhaeghe & Teyssier, 2001).

Evidence of partial melting in the lower crustal rocks of the Chiraa domain during M 2 /D 2 , likely contributed to rheological weakening of the over-thickened crust, facilitating the transition to the D 2 transtensional regime orthogonal to the shortening regime of D 1 , after which exhumation occurred. Concurrent cooling and decreasing pressure after peak temperatures is evident in steep negative dP/dT segments of P-T paths for samples from the Chiraa domain. The gradient of their retrograde paths crosses steady state geotherms suggesting exhumation exceeded thermal reequilibration, thus precluding homogeneous uplift and erosion of the terrane. In Phanerozoic orogenic belts, gravitationally-driven orogenic collapse is associated with anatectic dome structures, downdip mineral stretching lineations and isothermal decompression in suprasolidus segments of P-T paths [START_REF] Teyssier | Gneiss domes and orogeny[END_REF]Whitney, et al., 2004), similar to those documented in high-grade domains of Palaeoproterozoic NW Ghana (Block, et al., 2015;Block, et al., 2016b). The shallow to moderate pitches of L 2 stretching lineations and the association of small temperature decreases with decreasing pressure during exhumation observed in SW Ghana distinguishes transtension-related exhumation from coupled extension and gravitational collapse driving exhumation in NW Ghana.

We note that gravitational forces acting on the over-thickened crust likely assisted for the transition from a compressive to tensile regime, resulting in orogen parallel flow and late orogenic exhumation reminiscent of modern orogenic processes, however, we are reticent to invoke gravitational collapse based on the discussed metamorphic constraints.

Transpressional dextral reactivation and transposition of pre-existing tectonic contacts to sub-

Implications for Palaeoproterozoic tectonics

The Palaeoproterozoic crust exposed in SW Ghana and SE Ivory Coast comprises small high grade domains, metamorphosed at high-pressure amphibolite to transition amphibolite-granulite facies metamorphic conditions. These domains are tectonically juxtaposed against low-grade, upper crustal metasediments along detachments faults and regional scale shear zones. The presence of strong metamorphic breaks does not conform to the spatially uniform metamorphic conditions and homogeneous uplift proposed by some authors in Precambrian accretionary or hot orogens (Cagnard, et al., 2006;[START_REF] Gapais | Mountain building and exhumation processes through time: inferences from nature and models[END_REF], attributed to warmer, weak lithosphere (Cagnard, et al., 2011).

The metamorphic record of the study area instead indicates early heterogeneous crustal thickening, associated with cool apparent geothermal gradients, reminiscent of modern incipient collisional orogenesis.

Numerical modelling of post-accretionary deformation in wide, hot orogens displays lateral, orogen parallel flow is produced during bulk constriction, common to Precambrian hot orogens and modern orogenic plateaux (Cagnard, et al., 2006;Chardon, et al., 2009;Chardon, et al., 2011;Cruden, et al., 2006). Here, however, we illustrate the development of constrictional structures occurred under moderate geothermal gradients of 20-30 °Ckm -1 resulting in differential exhumation of anatectic midto lower crustal blocks, reminiscent of modern processes. Such evidence suggests that transtension was instead driven by combined boundary forces, likely associated with a period of oblique plate divergence, with the assistance of gravitational forces acting on the thickened, partially molten orogenic crust.

Similar crustal mechanical behaviour in the Palaeoproterozoic West African Craton is documented only in NW Ghana (Block, et al., 2015). Palaeoproterozoic lower crustal exhumation in SW Ghana is analogous to the Devonian transtension-related exhumation of ultra-high-pressure rocks of the Western Gneiss Region of the Scandinavian Caledonides (Fossen, et al., 2013;Krabbendam & Dewey, 1998). These orogenic processes significantly contrast those observed in other Palaeoproterozoic accretionary orogens such as those of the southern Finnish Svecofennides (Cagnard, et al., 2007).

Eclogites and high-P metamorphism have been documented in a number of Palaeoproterozoic orogens, including the Usagaren Orogen, Tanzania, at ca. 2000 Ma (Collins, et al., 2004) and, more recently, in the Trans-Hudson Orogen at ca. 1800 Ma (Weller & St-Onge, 2017). These authors argue that this indicates modern subduction-related orogenic processes were active early in the Earth's history.

The findings of this study do not reveal direct evidence of subduction-related processes. Instead, we propose that the combined evidence of HP-MT assemblages, low initial apparent geothermal gradients, heterogeneous crustal thickening and the generation and emplacement of leucogranitic plutons parallel to the northwest margin of the Sefwi belt is the ancient expression of a Palaeoproterozoic collisional orogen suturing two micro-continental fragments represented by southern Ghana and central Ghana/ central Ivory Coast.

Our findings, in addition to other recent studies (e.g. Block, et al., 2015;Ganne, et al., 2012), document diverse geothermal gradients within the West African Craton, indicating a more heterogeneous tectono-metamorphic evolution than previously assumed. These results contrast with the high geotherms recorded during collisional orogenesis involving the juvenile crust of Palaeoproterozoic Southern Svecofennian Arc Complex, southern Finland (Cagnard, et al., 2007). It must be noted, however, that the in sporadic preservation of HP-LT or HP-MT metamorphic conditions within the WAC, pressures never exceed 14kbar, potentially indicating the maximum amount of crustal thickening supportable by the juvenile Palaeoproterozoic crust. Diverse geothermal gradients are associated with terrane accretion in the Archaean Barberton granitoid-greenstone terrain (Moyen, et al., 2006) and in throughout the eastern Yilgarn Craton (Goscombe, et al., 2009). Whilst granulite facies metamorphism is ubiquitous throughout Earth's history, low geothermal gradients (10-20 °Ckm -1 ) associated with eclogite-high pressure granulite (E-HPG) metamorphism first appeared in the Neoarchaean, with an increasing number of occurrences in the Palaeoproterozoic and into the Phanerozoic (Brown, 2007, and references therein). Coupled with the occurrence of ultra-high temperature (UHT) metamorphism after the Archaean-Proterozoic boundary, Brown (2007;2009) proposes that changes to global geodynamic processes created a proto-tectonic regime during the Neoarchaean, associated with an increased diversity in tectono-metamorphic environments. In this context, the assorted thermal regimes and tectonic processes recorded in the West African Craton, in the absence of extreme pressure metamorphism attributed to modern subduction-related orogenic processes, supports the notion of a transitional Proterozoic plate tectonic regime. Furthermore, this research is complementary to the growing body of research that suggest Palaeoproterozoic orogens forming between 2100 and 1800 Ma document growing heterogeneity in thermal regimes, whilst also displaying spatial and temporal variations in metamorphism and strain accommodation between individual Palaeoproterozoic provinces.

Conclusions

The metamorphic record of juxtaposed domains in the Palaeoproterozoic Sefwi greenstone belt of the West African Craton reveals a far more complex metamorphic history than previously recognised. We document low apparent geothermal gradients and clockwise P-T-t paths with heterogeneous prograde segments. Sharp lateral metamorphic breaks are interpreted as the product of tectonic exhumation of crustal slices from different depths. Exhumation is associated with strain localisation along cratonscale ductile shear zones and normal sense detachments faults during oblique divergence, likely assisted by partial melting in the lower crust. Palaeoproterozoic rocks in southwest Ghana provide excellent insight into the link between high grade metamorphism within greenstone belts and the craton-scale shear zones that dominate the West African Craton. These results contrast the weak, hot orogen model proposed for some areas of the craton. The metamorphic record of the study area is interpreted as the product of monocyclic orogenesis. The discrepancies in the timing of high grade metamorphism throughout the craton, however, suggests episodic collisional orogenesis during the Eburnean Orogeny was ultimately responsible for craton assembly, superimposed on the accretionary orogenic system of the West African Craton.

0 Introduction

The previous research chapters explored the tectonic style and exhumation mechanism of the Eburnean Orogeny as preserved in SW Ghana. This chapter focuses on the magmatic evolution of the Sefwi Greenstone Belt in order to understand the geodynamic setting of crustal growth. It presents new

Introduction

The Archaean and Proterozoic rock record reveals temporal changes in the mechanisms and processes associated with crustal growth and preservation. These changes reflect the secular cooling of the Earth, the differentiation of the Earth's crust and evolving plate dynamics (e.g. [START_REF] Davies | Dynamic Earth: Plates, plumes and mantle convection[END_REF]Hawkesworth & Kemp, 2006a;Herzberg, et al., 2010). Crustal growth models indicate that more than 50% of the cumulative volume of continental crust had formed by 3000 Ma (Belousova, et al., 2010;Cawood, et al., 2013;Dhuime, et al., 2012;Hirdes, et al. 1992;Taylor & McLennan, 1985), with continuous additions hypothesised from the early Earth through to the present day. However, major changes are recorded in the continental rock record between 3000 and 2500 Ma, including the geochemical evolution of volcanic and plutonic rocks (Keller & Schoene, 2012;Laurent, et al., 2014;Moyen & Martin, 2012), the diversification and secular evolution of the metamorphic record (Brown, 2006;Brown, 2007a) and changes in orogenic strain patterns, crustal architecture and rheology (Cagnard, et al., 2011;Chardon, et al., 2009;Rey & Houseman, 2006). This multi-disciplinary body of evidence signals significant changes in global geodynamics, often interpreted as the commencement of some form of plate tectonics [START_REF] Condie | Episodic zircon age spectra of orogenic granitoids: The supercontinent connection and continental growth[END_REF][START_REF] Condie | The Archean-Proterozoic boundary: 500 my of tectonic transition in Earth history[END_REF]Keller & Schoene, 2012;Sizova, et al., 2010;[START_REF] Windley | The Archaean-Proterozoic boundary[END_REF]. Indeed the onset of plate tectonics is likely responsible for the transition from net crustal growth to major lithospheric recycling and reworking as part of the Wilson cycle, with the destruction or consumption of tectonic plates in subduction zones (e.g. Cawood, et al., 2013;Dhuime, et al., 2012).

A number of datasets within the geological record illustrate distinct clusters of magmatic and tectonic activity during the evolution of the early Earth. Episodic crustal growth is suggested by peaks in the juvenile crust record at 2700, 1900 and 1200 Ma (Cawood, et al., 2009;[START_REF] Condie | Episodic ages of Greenstones: A key to mantle dynamics[END_REF]Condie, 1998); the orogenic record displays discrete events at ca. 3800-3500, 2900-2600, 1900-1600, and 1200-900 Ma (e.g. Zhao, et al., 2002), whilst the U-Pb detrital zircon ages define peak clusters at 2700, 2500, 2010, 1840, 1650, 1150and 600 Ma (Condie, et al., 2005;Condie, et al., 2011). These peaks often coincide with supercontinent formation (e.g. [START_REF] Condie | Episodic zircon age spectra of orogenic granitoids: The supercontinent connection and continental growth[END_REF][START_REF] Nance | The supercontinent cycle[END_REF]Rogers & Santosh, 2004), however, they all represent periods during which conditions were favourable for preservation of the crust (Hawkesworth, et al., 2009;2010). Falling between these maxima, cratonic domains of the West African-Sao Luis Craton and Guiana Shield record a period of significant juvenile crustal growth during the early Palaeoproterozoic (2300-2070 Ma), representing one of the youngest cratonic provinces on Earth (Abouchami, et al., 1990;Boher, et al., 1992;Gasquet, et al., 2003;Gruau, et al., 1985;Klein, et al., 2005;Taylor, et al., 1992). Production and preservation of juvenile crust is attributed to the Eburnean-Trans-Amazonian Orogen (Abouchami, et al., 1990;Boher, et al., 1992;Bonhomme, 1962), forming one of the earliest orogenic events of the 2100-1800 Ma period of global orogenesis that heralded the rise of the Palaeoproterozoic supercontinent Nuna (also called Columbia (Betts, et al., 2016;Rogers & Santosh, 2002;Zhao, et al., 2002). Juvenile Palaeoproterozoic rocks of the southern portion of the West African Craton, forming the Baoulé-Mossi Domain, comprise narrow, elongate-to arcuate greenstone belts alternating with parallel sedimentary basins or vast granite-gneiss domains of tonalite-trondhjemite-granodiorite plutons, bearing striking lithological and architectural similarities with Archaean granite-greenstone provinces, including the Superior Province of the Canadian Shield (Card, 1990a;Condie & Benn, 2006;Daigneault, et al., 2002;[START_REF] Percival | Minto block, Superior province: Missing link in deciphering assembly of the craton at 2.7 Ga[END_REF], the Kaapvall Craton and the Limpopo Belt (de Wit, et al., 1992;[START_REF] Poujol | A review of the geochronological constraints on the evolution of the Kaapvaal Craton, South Africa[END_REF]Zeh, et al., 2009;[START_REF] Zeh | Juvenile crust formation in the northeastern Kaapvaal Craton at 2.97 Ga-Implications for Archean terrane accretion, and the source of the Pietersburg gold[END_REF], and the Kola and Karelian provinces of the Baltic Shield (Sorjonen-Ward & Luukkonen, 2005, and references therein). Parallel greenstone belts, striking between N-and NE, occur across the craton and are often fault-bounded by regional to craton-scale shear zones.

The use of in-situ isotopic analysis of zircon, including both U-Pb and Lu-Hf systematics yield both the crystallisation age and the timing of extraction of the source material from the mantle (Hawkesworth & Kemp, 2006b;[START_REF] Patchett | Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth[END_REF]. This is a well-established method used to understand crustal evolution, crustal architecture and the geodynamic setting of crustal formation (e.g. Belousova, et al., 2010;Cawood, et al., 2013;Condie, et al., 2005;Dhuime, et al., 2012;[START_REF] Griffin | Archean crustal evolution in the northern Yilgarn Craton: U-Pb and Hf-isotope evidence from detrital zircons[END_REF][START_REF] Griffin | The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites[END_REF]Hawkesworth & Kemp, 2006b;[START_REF] Kemp | Magmatic and crustal differentiation history of granitic rocks from Hf-O isotopes in zircon[END_REF].

This study reports new whole-rock geochemical data for twenty-nine igneous samples and ion microprobe (SHRIMP II) U-Pb and LA-MC-ICP-MS Lu-Hf zircon data for four samples. The selected samples are believed to be representative of the major magmatic suites exposed in the Sefwi Greenstone Belt and Sunyani-Comoé of southwest Ghana. The aim of this study is to examine the magmatic suites emplaced in southern Ghana and elucidate the crustal evolution of an archetypal Palaeoproterozoic greenstone belt. Furthermore, we discuss the implications of our results, combined with published data, within the context of the crustal architecture of the West African Craton and the geodynamic setting responsible for the formation and preservation of the juvenile crustal domain.

Palaeoproterozoic geology of West Africa

The West African Craton (WAC) encompasses the northern Reguibat Rise and the sub-Saharan Leo-Man Rise, each comprising 3600-2600 Ma Archaean provinces to the west and 2300-2070 Ma Palaeoproterozoic domains to the east (Abouchami, et al., 1990;Potrel, et al., 1998;Rollinson, 2016;[START_REF] Thiéblemont | A 3.5 Ga granite-gneiss basement in Guinea: further evidence for early archean accretion within the West African Craton[END_REF]. The domains are separated by the overlying Mesoproterozoic-Palaeozoic Taoudeni Basin. The Archaean Man Shield is juxtaposed with Palaeoproterozoic rocks of the Baoulé-Mossi Domain along the Sassandra Shear Zone (Fig. 5.1) [START_REF] Attoh | The West African Shield[END_REF]Bessoles, 1977), with the Palaeoproterozoic Kayes and Kédougou-Kéniéba Inliers lying to the northwest. The Baoulé-Mossi domain is dominated by rocks of the Birimian Supergroup (2300 -2125 Ma), forming linear to arcuate, N-to NE-trending volcanic greenstone belts and broad volcano-sedimentary domains, often bounded by craton-scale shear zones and separated by vast granite-gneiss terranes (Hirdes, et al., 1996;Junner, 1940;Leube, et al., 1990). Volcanic belts are commonly unconformably overlain by fluvio-deltaic sequences of the Tarkwa Group and equivalents (2107-2097 Ma) (e.g. Baratoux, et al., 2011;Perrouty, et al., 2012;Pigois, et al., 2003;[START_REF] Zitzmann | Geological, geophysical and geochemical investigations in the Bui Belt area in Ghana[END_REF]. The Palaeoproterozoic rocks were accreted and cratonised during the 2150-2070 Ma Eburnean Orogeny (Bonhomme, 1962;Feybesse, et al., 2006;Oberthür, et al., 1998).

Volcanic rocks of the Birimian Supergroup range in composition from basalt to rhyolite, displaying tholeiitic to calc-alkaline affinities (e.g. Baratoux, et al., 2011;Hirdes, et al., 1993;Senyah, et al., 2016;Sylvester & Attoh, 1992), with subordinate intercalations of volcaniclastic or immature sedimentary rocks. Volcano-sedimentary domains comprise deformed sequences of greywacke, shale, argillitic and volcaniclastic rocks (pyroclastic and epiclastic), and rare chemical sedimentary rocks and calc-alkaline volcanic sequences (Hirdes, et al., 1993;Leube, et al., 1990). The volcanic and volcano-sedimentary rocks are extensively intruded by voluminous plutonic rocks forming a wide range of magmatic suites, with diorite to tonalite-trondhjemite-granodiorite (TTG), high-K granite, and leucogranite compositions. Prior to the advent of precision geochronological methods, plutonic rocks in Ghana were classified as "belt-type" or "basin-type", based on their host terranes. In southern Ghana, based on the predominant ferromagnesian mineral and alkali index classification, further subdivision of felsic plutons resulted in the four following suites: Dixcove, Winneba, Cape Coast and Bongo (Leube, et al., 1990). The Dixcove granitoids are variably foliated, metaluminous, hornblendebearing plutons which intrude volcanic belts, whilst the biotite-and/or muscovite-bearing Winneba and Cape Coast granites and granodiorites are restricted to volcano-sedimentary domains or basins (Leube, et al., 1990). Finally, the K-rich Bongo granites intrude Tarkwaian-like sediments in NW Ghana [START_REF] Abitty | Geochemistry and petrogenesis of the K-rich 'Bongo-type' granitoids in the Paleoproterozoic Bole-Nangodi greenstone belt of Ghana[END_REF]Leube, et al., 1990). Milési, et al., 2004).

Mafic to intermediate volcanism occurred across the craton between 2300 and 2180 Ma (e.g.

Agyei Duodu, et al., 2009;Delor, et al., 1995;Feybesse, et al., 2006;Hirdes & Davis, 1998;Lompo, 2009), approximately coeval with the earliest pulse of TTG magmatism (de Kock, et al., 2011;Gueye, et al., 2007;Hirdes, et al., 1992;Oberthür, et al., 1998;Siegfried, et al., 2009). The most significant volumes of syn-to late-tectonic granitoid intrusions, however, were emplaced near-continuously between 2150 and 2100 Ma, yet preserved no clear lateral age gradients (Agyei Duodu, et al., 2009;Doumbia, et al., 1998;Egal, et al., 2002;Gasquet, et al., 2003;Hirdes, et al., 1992;Hirdes, et al., 1996;Oberthür, et al., 1998;Tapsoba, et al., 2013;Tshibubudze, et al., 2013). Granitoids and lavas emplaced between 2100 and 2060 Ma are largely restricted to the Kédougou-Kéniéba Inlier and along the Archaean-Palaeoproterozoic contact (Egal, et al., 2002;Gueye, et al., 2007;Hirdes & Davis, 2002a), with a significant, but localised population documented in southern Ghana along the northwest margin of the Sefwi Belt (Agyei Duodu, et al., 2009;Hirdes, et al., 1992;Hirdes, et al., 2007).

Many authors liken the evolution from bimodal tholeiitic volcanism to calc-alkaline volcanism with the development of a modern-day oceanic island arc setting (Ama [START_REF] Salah | Evolution d'un arc insulaire océanique birimien précoce au Liptako nigérien (Sirba): géologie, géochronologie et géochimie[END_REF][START_REF] Arndt | Mafic and ultramafic magmatism[END_REF]Baratoux, et al., 2011;Béziat, et al., 2000;Dampare, et al., 2008;[START_REF] Soumaila | REE patterns, Nd-Sm and U-Pb ages of the metamorphic rocks of the Diagorou-Darbani greenstone belt (Liptako, SW Niger): implication for Birimian (Palaeoproterozoic) crustal genesis[END_REF]Sylvester & Attoh, 1992). Other authors attribute the wide-spread tholeiitic basaltic volcanism and associated plutonic activity to the plume-related formation of an oceanic plateau (Abouchami, et al., 1990;[START_REF] Augustin | Paleoproterozoic plume-related basaltic rocks in the Mana gold district in western Burkina Faso, West Africa: Implications for exploration and the source of gold in orogenic deposits[END_REF]Boher, et al., 1992;Lompo, 2009). Alternatively, authors have invoked intra-cratonic rifting leading to the opening of oceans and the formation of accretionary systems (Leube, et al., 1990) or a transtensional back-arc basin setting [START_REF] Vidal | The palaeoproterozoic (Birimian) of Haute-Comoé in the West African craton, Ivory Coast: a transtensional back-arc basin[END_REF]. Isotopic

studies of the juvenile terranes of the Baoulé-Mossi domain that formed between 2200 and 2100 Ma reveal Nd-model ages within ~300 Myrs of their magmatic crystallisation ages, recording crustal residence ages between 2500 and 2200 Ma (Abouchami, et al., 1990;Boher, et al., 1992;Gasquet, et al., 2003;Pawlig, et al., 2006;Peucat, et al., 2005). Nd and Hf-model ages reported in a small number of studies from far south-eastern Ghana and in the western portion of the Baoulé-Mossi domain, however, suggest a greater Archaean crustal influence or reworking of older Archaean crust in some portions of the domain (Parra-Avila, et al., 2016;Petersson, et al., 2017;Petersson, et al., 2016;Taylor, et al., 1992).

The crustal architecture and structural evolution of the West African Craton are attributed to lateral shortening across the craton during the 2150-2070 Ma Eburnean Orogeny (e.g. Bonhomme, 1962;Eisenlohr & Hirdes, 1992;Feybesse, et al., 2006). Following the extensive granitoid emplacement, an early phase of deformation is characterised by thrust-related tectonism, unit stacking and highgrade metamorphism attributed to crustal thickening (Allibone, et al., 2002;Block, et al., 2015;Block, et al., 2016b;Feybesse, et al., 2006;Galipp, et al., 2003;Ganne, et al., 2012;John, et al., 1999;Milési, et al., 1992). An alternative hypothesis suggests that the early crustal architecture defined dome and basin strain patterns, attributed to the diapiric uprising of granitoid bodies (Delor, et al., 1995;Lompo, 2010;Vidal, et al., 1996;Vidal, et al., 2009). The widely recognised second phase of the Eburnean Orogeny (ca. 2110-2070 Ma) is characterised by the development of craton-scale strike-slip shear zones (Allibone, et al., 2002;Feybesse, et al., 2006;Jessell, et al., 2012;Pouclet, et al., 2006). Proponents of polycyclic orogenesis interpret high-grade terranes as older, deformed gneissic basement upon which discordant sequences of Birimian sediments were deposited during a crustal extension event which

were subsequently deformed during the Eburnean Orogeny (e.g. de Kock, et al., 2012;Hein, 2010;Perrouty, et al., 2012). In contrast, authors who invoke monocyclic orogenesis suggest that Birimian volcanic and sedimentary sequences formed pene-contemporaneously, underwent coeval deformation and metamorphism, and were subsequently juxtaposed during a single progressive deformation event during the Eburnean Orogeny (Block, et al., 2015;Block, et al., 2016b;Eisenlohr & Hirdes, 1992;Hirdes, et al., 2007). Discrepancies in the geochemical, geochronological, isotopic evolution and tectono-thermal histories between terranes in the West African Craton have been used as evidence of suturing of independent cratonic fragments and collision orogenesis within the craton, (Block, et al., 2016b;Parra-Avila, 2015;Parra-Avila, et al., 2017).

Main lithologies and stratigraphy

The crustal architecture of southern Ghana and southeast Ivory Coast is defined by four parallel, elongate, narrow volcanic greenstone belts, the Kibi-Winneba, Ashanti, Sefwi and Bui belts (from southeast to northwest), separated by the Cape Coast, Kumasi-Afema and Sunyani-Comoé volcanosedimentary basins (Agyei Duodu, et al., 2009;Hirdes, et al., 1993;Jessell, et al., 2012) (Fig. 5.2). The greenstone belts define the prevailing northeast-striking structural trend and are bounded by regionalto craton-scale shear zones. The stratigraphy of southern Ghana was recently re-characterised by Adadey, et al. (2009) and . The lower Sefwi Group, as defined by Perrouty, et al. (2012), comprises basalt and andesite sequences intercalated with minor volcaniclastic and sedimentary layers, with a minimum emplacement age of 2174 ± 2 Ma, interpreted from a syn-tectonic granitoid intrusion (U-Pb zircon age; Oberthür, et al., 1998). The overlying Kumasi Group (2154 -2125 Ma) is predominantly made of phyllites and volcaniclastic rocks, with syn-depositional andesitic volcanism dated in the Kumasi-Afema Domain at 2142 ± 24 Ma (Adadey, et al., 2009). Detrital, fluvio-deltaic sedimentary rocks of the Tarkwa Group unconformably overlie the Sefwi Group volcanic rocks in the Ashanti belt, which were deposited between 2107 and 2097 Ma (Perrouty, et al., 2012;Pigois, et al., 2003). Existing geochronological data, including methodologies, for southern Ghana is summarised in Appendix D. Hirdes, et al. (2007); (2): Adadey, et al. (2009); (3) Hirdes, et al. (1992); ( 4) : Petersson, et al. (2017); ( 5) Hirdes and Davis (1998); [START_REF]= 0.01-0.22), associated with moderately negative Rb, weakly positive Ba and mixed Sr anomalies (Sr = 94-460 ppm). Furthermore, only pyroxenites and gabbros samples display positive V and Sc anomalies with[END_REF] : Petersson, et al. (2016);(7): Chapter 4this study;(8): Agyei Duodu, et al. (2009).

The study area focuses on plutonism and volcanism throughout the Sefwi Greenstone Belt and the volcano-sedimentary Sunyani-Comoé Domain (Fig. 5.2). These domains are bounded by the northeaststriking Sefwi Shear System (Jessell, et al., 2012), including the Kenyase-Yamfo, the Ketesso and the Kumasi-Afema shear zones, from north to south. Results of the geochemical and geochronological analysis presented in this study are used to better understand the magmatic evolution of the study area, rather than redefining previously established stratigraphic column for the Ashanti Belt, located southeast of the Sefwi Belt.

Sampling and analytical methods

Twenty-nine fresh samples representative of the dominant lithological units, including nineteen granitoid samples and ten basaltic to rhyolitic volcanic and volcanoclastic rock samples, were collected from sub-domains within the Sefwi Greenstone Belt and the Sunyani-Comoé Domain for petrological and geochemical analysis. Samples were collected from the tectonic-metamorphic domains defined in Chapter 3, including the southern greenschist facies, volcano-plutonic Sefwi-Wiawso Domain, the central amphibolite facies Kukuom-Juaboso and Hwidiem domains, the Bechem Granitoid Domain and the north-eastern, migmatitic Chiraa Domain. Several additional samples were collected from volcanoclastic and plutonic units in the low-grade Sunyani-Comoé volcano-sedimentary domain. The locations of the samples are shown in Fig. 5.2 and coordinates are given in Table 5.1.

Whole rock geochemistry

Major oxide and trace element geochemistry was collected for all twenty-nine igneous samples.

Powder concentrates for each sample were produced at the School of Earth, Atmosphere and Environment, Monash University. Major element oxide analysis was performed at the School of Earth Sciences, CODES, University of Tasmania (samples marked with asterisk in Table 5.1) and at the ALS Geochemical Laboratory, Brisbane, Australia, using X-ray Fluorescence (XRF) analysis according to ALS method ME-XRF26 (ALS, 2006a). Major element oxides analysed by XRF include SiO 2 , TiO 2, Al 2 O 3 , Fe 2 O 3 , MnO, MgO, CaO, Na 2 O, K 2 O, BaO, Cr 2 O 3 and P 2 O 5 , with a lower detection limit of 0.01%.

Loss on ignition was determined gravimetrically. Trace element concentrations, including rare earth elements, were obtained by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) at the ALS Geochemical Laboratory, Brisbane, Australia according to ALS geochemical procedure ME-MS61 (ALS, 2006b). Replicate analyses and Monash rhyodacite (DAND) and basalt (BNB) reference standards were included to ensure reproducibility and accuracy. Major and trace element concentrations for all twenty-nine samples are presented in Table 5.1.

Zircon U-Pb dating

Four samples were selected for in-situ determination of the U-Pb age and Hf-isotope composition using Sensitive High-Resolution Ion Microprobe (SHRIMP) and Laser Ablation Multi Collector-Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICP-MS) analysis, respectively.

Zircon separation

Zircon concentrates were obtained for five samples following the procedures described by [START_REF] Claoué-Long | Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40 Ar/ 39 Ar analysis[END_REF]. Zircon extraction followed the established techniques using magnetic fractionation using a hand magnet and a Franz Isodynamic magnetic separator, followed by heavy liquid (diiodomethane) separation. Zircons were hand-picked and mounted in epoxy resin discs, 5 mm in thickness and 25 mm in diameter. Samples were mounted with the following reference standards: M257 (561.3 Ma and 840 U ppm; Nasdala, et al., 2008), Temora II (416.8 Ma;Black, et al., 2004), OGC1 (3465 Ma;[START_REF] Stern | Measurement of SIMS instrumental mass fractionation of Pb isotopes during zircon dating[END_REF] and silicate glass NBS610. Reference standards were used for instrument set-up, common lead ( 204 Pb) correction, Pb/U fractionation correction and to monitor instrument drift at the accuracy of results. Epoxy mounts were polished to expose a cross section through the internal structure of the grains. Back-scatter electron (BSE) and cathodoluminescence (CL) imaging of zircons was performed at the Monash Centre for Electron Microscopy (MCEM), Monash University using the JEOL JSM-7001F Field Emission Gun Scanning Electron Microscope (FEG SEM) and the FEI Nova NanoSEM 450 FEG SEM, respectively. Further BSE and CL imaging was performed at the Centre for Microscopy, Characterisation and Analysis at the University of Western Australia using the VEGA 3 Tescan SEM. BSE and CL images were used to identify internal structures, growth patterns and damage zones, and subsequently guide grain analysis.

U-Th-Pb SHRIMP II zircon dating

In-situ zircon U-Th-Pb analysis was conducted using a Sensitive High Resolution Ion microprobe (SHRIMP II) at the John de Laeter Centre for Isotope Research at Curtin University, Perth, Australia.

Instrument conditions are detailed by [START_REF] Kennedy | The performance characteristics of the WA SHRIMP II ion microprobe[END_REF] and De Laeter and [START_REF] De Laeter | A double focusing mass spectrometer for geochronology[END_REF]. The analytical procedures for zircon dating with the SHRIMP ion microprobe are outlined in [START_REF] Claoué-Long | Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40 Ar/ 39 Ar analysis[END_REF] and Williams (1998). The mass-filtered, 10keV primary O 2-beam is focussed to an elliptical spot, ~25 μm in diameter, with an ion current of mass-filtered, of 2.2 -2.0 nA. For each spot, the dataset includes six cycles (scans) through the mass range of 196 Zr 2 O + , 204 Pb + , background (204.1), 206 Pb + , 207 Pb + , 208 Pb + , 238 U + , 248 ThO + and 254 UO + , as outlined by [START_REF] Nasdala | Zircon M257-a Homogeneous Natural Reference Material for the Ion Microprobe U-Pb Analysis of Zircon[END_REF]. Data reduction was performed using the MS Excel add-in programs, SQUID version 2.5 and ISOPLOT v3.0 (Ludwig, 2003), according to [START_REF] Compston | U-Pb geochronology of zircons form lunar Breccia 73217 using a sensitive high mass-resolution ion microprobe[END_REF][START_REF] Claoué-Long | Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40 Ar/ 39 Ar analysis[END_REF] and Williams (1998). Applied common-Pb corrections were based on the Stacey and Kramers (1975) model for isotopic ratios of terrestrial lead. Analytical cut offs were applied prior to data interpretation in order to exclude zircons that had potentially undergone disruption of the U-Pb systematics. In this study, the reported U-Pb ages are derived from concordant or near-concordant grains with accepted discordance values between -5% and +15% in order to minimise the number of inversely concordant values. In addition, analyses that yielded U concentrations over 1000 ppm or included more than 1.0% of non-radiogenic Pb ( 204 Pb) were rejected. All ages are reported as 207 Pb/ 206 Pb ages with 1σ error.

In-situ Lu-Hf analyses

Lutetium-Hafnium (Lu-Hf) isotope analyses were performed on grains from samples dated using U-Pb zircon SHRIMP II method. Analyses were conducted using a Resonetics S-155-LR 193nm excimer laser ablation system attached to a Nu Plasma II multi-collector ICP-MS at the John de Laeter Centre, Curtin University. Sited adjacent to or "on-top-of " SHRIMP spots, circular laser spots with diameters of 33 μm were drilled with a laser pulse repetition of 5 Hz and an energy density of ~2 J/ cm 2 during 30 seconds of data acquisition, following an initial baseline period of 30 seconds. The isotopes 173 Yb, 174 (Yb, Hf), 175 Lu, 176 (Yb, Lu, Hf), 177 Hf, 178 Hf, and 179 Hf were measured on Faraday cups in a static sequence. Isobaric interference of 176 Yb was corrected according to the method of [START_REF] Nowell | Simultaneous acquisition of isotope compositions and parent/daughter ratios by non-isotope dilution-mode Plasma Ionisation Multi-collector Mass Spectrometry (PIMMS)[END_REF] whereby JMC475 Hf standard solutions were doped with Yb. From this, the true value of 176 Yb/ 173 Yb was derived using a regression, with 176 Yb subsequently subtracted from the 176 peak.

Correction for interference of 176 Lu on 176 Hf used the measured interference-free 175 Lu isotope and a value of 0.02653. Corrections were applied during each analysis after an inverse mass bias correction using the measured 179 Hf/ 177 Hf ratio, this allows for absolute differences between the Hf and Yb mass bias. The 178 Hf/ 177 Hf and 180 Hf/ 177 Hf stable isotope ratios were used to monitor data quality.

All data were adjusted relative to the primary reference zircon 91500. Analyses of reference zircons (MUN1, 3, 4,Fisher, et al., 2011;R33, Fisher, et al., 2014;GJ-1, Morel, et al., 2008;Plešovice, Sláma, et al., 2008;91500, Wiedenbeck, et al., 1995;Mudtank, Woodhead & Hergt, 2005) were conducted every twenty spots throughout the session to verify accuracy and ensure reproducibility.

Standard results are presented in Appendix E and comparison of measured standards to accepted values is shown in Appendix F.

The measured 176 Lu/ 177 Hf ratios for each of the zircons analysed were used to calculate initial 176 Hf/ 177 Hf ratios. There are a number of proposed decay constants for 176 Lu (e.g. Bizzarro et al., 2003;[START_REF] Blichert-Toft | The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system[END_REF][START_REF] Scherer | Calibration of the lutetium-hafnium clock[END_REF]Soderlund et al., 2004). A decay constant (λ) for 176 Lu of 0.01867 for all Hf isotope calculations (Soderlund et al., 2004, after Scherer et al., 2001) is used in this study. εHf values are calculated relative to modern CHUR values, using the chondritic values of 176 Lu/ 177 Hf = 0.282785 and 176 Hf/ 177 Hf = 0.0336 (Bouvier, et al., 2008, after Blichert-Toft andAlbarède, 1997) are used for calculating εHf and model ages. Hf model ages (T DM-1 ) are calculated for a linear 4.56 Ga depleted mantle model with modern compositions of 176 Lu/ 177 Hf = 0.0384 and 176 Hf/ 177 Hf= 0.28325 [START_REF] Griffin | Zircon chemistry and magma mixing, SE China: in-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes[END_REF]. Two-stage model ages, which yield more robust model age estimates, are calculated with the same depleted mantle parameters with average crustal 176 Lu/ 177 Hf = 0.0150 (Condie, et al., 2005) and a 176 Lu decay constant of 0.01867 [START_REF] Söderlund | The 176 Lu decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions[END_REF]after Scherer et al., 2001). Initial 176 Hf/ 177 Hf and εHf (t) were calculated for using individual zircon crystallisation ages to observe the array of zircon age relative to their Hf signature, as well as comparative εHf (t) plots of the study area relative to West African and global zircon Hf databases.

Results

Lithological and petrological descriptions

Granitoids

In order to simplify the presentation and the description of the lithologies sampled in this study, analysed granitoid samples have been grouped into four main lithologies, based on petrological composition and geochemical analysis, detailed in section 5.1 and 5.2, respectively. The four main groups are: i) tonalite, trondhjemite and granodiorite (TTG); ii) quartz diorite and diorite; iii) granite and quartz monzonite, and; iv) pyroxenite and gabbro norite.

Within the study area, there are two series of plagioclase-rich tonalite and trondhjemite plutons.

The first group comprises large, composite, medium-to coarse-grained, leucocratic trondhjemite and tonalite plutons that contain biotite as the main ferro-magnesian mineral with subordinate to rare hornblende (Figs. 5.3a and 5.3b) and rare interstitial microcline. The most significant exposures are located in the Sefwi-Wiawso and Bechem Granitoid domains (Fig. 5.2). Accessory phases include euhedral magmatic epidote, euhedral titanite, apatite and zircon. Deformed plutons display biotite alignment and sub-grain rotation recrystallisation of large, subhedral to euhedral plagioclase phenocryst with irregular, bulging grain boundaries. Plagioclase shows zoning of some grains suggesting changes in anorthite content and is variably sericitized. Minor chlorite replacement of biotite indicates late, weak greenschist facies metamorphic overprint, increasingly pervasive proximal to pluton boundaries.

The second group comprises smaller, elliptical to elongate, mesocratic granodiorite and trondhjemite plutons that are emplaced proximal to the Bibiani and Afema shear zones in the south of the Sefwi Belt and often contain small, round, mafic enclaves (Fig. 5.3c). The plutons are medium-to coarsegrained with an equigranular texture, containing hornblende as the primary ferro-magnesian mineral, subhedral to euhedral plagioclase and quartz. Minor biotite and microcline are often observed with accessory epidote, magnetite, apatite and hematite.

Quartz diorite and diorite plutons are medium-to coarse-grained and contain abundant hornblende phenocrysts and interlocking, subhedral to euhedral plagioclase often strongly altered by sericite, and minor quartz (Fig. 5.3d). Clinopyroxene phenocrysts often display extensive replacement rims of secondary hornblende and quartz. In the Hwidiem Domain, large irregular plutons are often weakly deformed displaying a foliation of aligned hornblende, potentially associated with retrograde metamorphism. Exposure of a mesocratic to melanocratic layered plutonic complex in the Bechem Granitoid Domain contains quartz diorite through to gabbro diorite, bounded to the south by the Ketesso High-Strain Zone. The plutons are variably deformed with foliations defined by metamorphic hornblende, seen wrapping around magmatic hornblende phenocrysts and are often structurally associated with strongly sheared metabasalts. Accessory minerals include magnetite, ilmenite, apatite and zircon with retrograde chlorite growth on hornblende rims and metamorphic epidote.

Three granite samples are grouped together based on their elevated potassium feldspar content, however, they each have distinct petrological assemblages and are presented together for comparative purposes.

Volcanic rocks

Volcanic rocks analysed in this study range from basaltic to rhyolitic in composition. These basalts, metabasalts and basaltic andesites contain magmatic and/or metamorphic hornblende (Fig. 5.4c).

Foliations in the metabasalts and meta-basaltic andesites are defined by aligned hornblende in higher grade samples, or chlorite and actinolite in lower grade samples. The deformed dacitic volcanoclastic rocks (Fig. 5.4d) contain relic plagioclase and quartz phenocrysts and exhibit chlorite-muscovite foliations with rare preservation of foliation aligned biotite porphyroblasts. A number of quartz-phyric pink rhyolite outcrops are documented, observed stratigraphically overlying garnet-hornblende paragneisses in the Kukuom-Juaboso domain (Fig. 5.4e). Late andesite dykes contain hornblende and zoned plagioclase phenocrysts, observed in the field as a cross-cutting dyke, intruding a folded amphibole-bearing para-amphibolite (Fig. 5.4f). Intermediate and felsic lavas commonly display sericite alteration, including growth of fine grained white mica.

Whole rock geochemistry

Samples

Samples were collected from most geological domains in the study area, including volcano-plutonic and volcanic layers in sedimentary domains, based on stratigraphic and structural relationships, and their potential significance for unravelling the early crustal evolution. A total of twenty-nine magmatic rocks were sampled for this study. They comprise nineteen plutonic samples, including four mafic to ultramafic plutons, and ten volcanic samples. Sample locations are shown in Fig. 5.2 and the results are presented in Table 5.1.

Plutonic rocks

Sampled plutonic rocks record a wide range of silica contents. Plutons in the granite and TTG groups have high silica contents of 70-76 wt% SiO 2 , with the exception of the quartz monzonite sample with 68 wt% SiO 2 (Fig. 5.5a). Diorite and quartz diorite rocks range from 55 to 61 wt% SiO 2 (Fig. 5.5a), whilst pyroxenites and gabbros contain < 52 wt% SiO 2 (Table 5.1 -not shown in Fig. 5.5).

Excluding the mafic and ultramafic suites, the silicic, plagioclase-rich granitoids are plotted on an An-Ab-Kfs normative ternary diagram (fields from Barker, 1979;[START_REF] O'connor | A classification for quartz-rich igneous rocks based on feldspar ratios[END_REF]

Volcanic rocks

Samples from volcanic units across the study area are plotted on the TAS diagram [START_REF] Le Maitre | A classification of igneous rocks and glossary of terms: Recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks[END_REF] displaying a range of compositions from basalt to rhyolite (Fig. 5.7a). Nine of thet en samples plot in the sub-alkaline field, whilst a quartz-feldspar-phyric rhyolite sample (SB076) is slightly alkaline. In the AFM diagram , all samples plot (Fig. 5.7b) in the calc-alkaline field except for a metabasalt (SB164) and one meta-dacite (SB016), both from the Kukuom-Juaboso Domain within the Sefwi Belt. All [START_REF] Irvine | A guide to the chemical classification of the common volcanic rocks[END_REF][START_REF] Kuno | Basalts: The Poldervaart treatise on rocks of basaltic composition[END_REF] where A is Na Twenty-six analyses were performed on twenty-five zircons. Six analyses were rejected: one because it has a discordancy greater than +10%; one analysis that had a 204 Pb corr % 206 Pb greater than 1%; three that had 1σ greater than 25 Ma and one analysis which had U content over 1000 ppm. Twenty accepted analyses from sample SB248 yielded a single age population and a weighted mean average 207 Pb/ 206 Pb age of 2155 ± 5 Ma (MSWD of 1.15) interpreted to be the crystallization age due to the smaller error Thirteen U-Pb analyses were performed on thirteen zircons with from both oscillatory zoned and unzoned zircons, which defined a single zircon age population. Analyses yielded a weighted average 207 Pb/ 206 Pb age of 2158 ± 6 Ma (MSWD = 1.02, n = 13) (Fig. 5.9c), interpreted as the crystallisation age of this rock, whilst a less precise upper intercept age of 2159 ± 14 Ma (MSWD = 1.11) was calculated from the concordia plot (Fig. 5.9d). Thirteen Hf analyses were performed on dated grains yielding 176 Lu/ 177 Hf of < 0.0028, 176 Yb/ 177 Hf of < 0.09 and 176 Hf/ 177 Hf range from 0.281464 to 0.281567 (± 0.000043 -0.000073 2SE). A juvenile, radiogenic Hf character is indicated by a wider range of suprachondritic εHf (t) values between +1.7 and +5. for the average continental crust (second stage) (Condie, et al., 2005) and a depleted mantle 176 Lu/ 177 Hf and 176 Hf/ 177 Hf of 0.0384 and 0.28325 [START_REF] Griffin | Zircon chemistry and magma mixing, SE China: in-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes[END_REF].

2 O = K 2 O, F is

Hornblende quartz diorite (SB092)

Zircons for sample SB092 are subhedral to euhedral and clear to pale rose-coloured, with length to width ratio commonly between 1:2 and 1:3. Two morphologies are noted for SB092, one is more equant (80-120 µm long and 80-100 µm wide) and the other slightly more elongate and near prismatic (120-200 µm long and 50-80 µm wide). Inclusions of ilmenite needles are visible in some elongate grains.

CL and BSE images show little evidence of metamictization. Most zircons have a large, uniform centre succeeded by irregular oscillatory zoning, a characteristic common to both morphological populations (Fig. 5.8c). Sixteen zircon U-Pb analyses were performed on 16 zircons from both morphological groups. The two groups yielded indistinguishable ages, indicating a single zircon age population that yielded a weighted average 207 Pb/ 206 Pb age of 2155 ± 9 Ma (MSWD = 1.17, n =16) (Fig. 5.9e), which is interpreted as the crystallisation age. The Concordia plot yields an upper intercept age of 2144 ± 10 (MSWD = 0.81, n = 16) (Fig. 5.9f), however, a number of grains show reverse concordance, artificially lowering the intercept age.

Fifteen Hf analyses on fifteen grains yielded 176 Lu/ 177 Hf of < 0.003, 176 Yb/ 177 Hf of < 0.04 and 176 Hf/ 177 Hf range from 0.281475 to 0.281554 (± 0.000046 -0.000074 2SE). These analyses yield a relatively radiogenic Hf signature, with suprachondritic εHf (t) values ranging from +2.0 to +5.5 (mean εHf (t) = 3.9 ± 1. [START_REF] Davies | Dynamic Earth: Plates, plumes and mantle convection[END_REF][START_REF] Foley | Growth of early continental crust controlled by melting of amphibolite in subduction zones[END_REF]Moyen, 2011;Moyen & Stevens, 2006a;[START_REF] Xiong | Rutile stability and rutile/melt HFSE partitioning during partial melting of hydrous basalt: implications for TTG genesis[END_REF]. The ubiquitous negative Nb-Ta, P and Ti anomalies coupled with positive Pb anomalies and variable LILE enrichment for all samples analyses in this study, excluding one meta-gabbro and one metabasalt, are consistent with the trace element patterns of the present-day bulk continental crust (Hawkesworth & Kemp, 2006a;[START_REF] Hofmann | Mantle geochemistry: the message from oceanic volcanism[END_REF][START_REF] Rudnick | Composition of the continental crust[END_REF]. Such features typify arc magmas and crust formation in a subduction setting (e.g. Arculus, et al., 1999), and are therefore consistent with the onset of "modernstyle" subduction-related processes after 3.0 Ga (e.g. Condie & Benn, 2006;Halla, et al., 2009;Laurent, et al., 2014;Smithies & Champion, 2000). Magmatic rocks sampled in this study share a number of geochemical similarities with late-Archaean granitoids generated within nascent subduction-related geodynamic settings, which are characterised by: (i) sodic tonalite-trondhjemite-granodiorite (TTG) granitoids derived from partial melting of hydrous metabasalts; (ii) mantle-derived, incompatible element-rich sanukitoids; (iii) biotite and two-mica granites derived from crustal sources; and, (iv) hybrid granitoids sharing petrological and geochemical similarities and sources to the first three groups (Halla, et al., 2009;Laurent, et al., 2014;Moyen, 2011;Moyen & Martin, 2012), suggesting somewhat comparable petrogenetic processes.

The two TTG groups, defined in section 5.1, are both characterised by a silicic, plagioclaserich assemblage, distinguished from one another on the basis on the dominance of either biotite or hornblende as the primary ferromagnesian mineral. They show tholeiitic to calc-alkaline affinities and low K 2 O/Na 2 O ratios, with higher ratios in the hornblende-bearing TTGs reflecting slightly higher modal proportions of potassium feldspar. Based on experimental melt compositions (Laurent, et al., 2014, and where magma is primarily derived through metasomatism of mantle peridotite by fluids released during dehydration of the subducting slab (Martin, et al., 2009). The complementary trace element patterns of the two groups of TTGs suggests a link in their magmatic history, but contrasting petrogenetic processes. (Drummond & Defant, 1990;Moyen, 2011); and (f) granite discrimination plot for tectonic setting of magma generation (Pearce, et al., 1984a).

REE patterns of the biotite-dominant TTG group display a strongly fractionated REE pattern, with a significant depletion in HREE indicating residual garnet and/or hornblende in the magma source.

High Sr/Y ratios and Y content also reflect a garnet-bearing residuum (Fig. 5.12c), often characterising deep melting at a convergent margin (Condie, 2008;[START_REF] Martin | The mechanisms of petrogenesis of the Archaean continental crust-comparison with modern processes[END_REF]. The La N /Yb N vs Yb N binary diagram (Fig. 5.12d) indicate high modal proportions of garnet in residuum of the garneteclogite source for the biotite-rich TTGs, compared to the garnet-free amphibolite indicated for the hornblende-dominant TTGs (Drummond & Defant, 1990;Laurent, et al., 2014;Moyen, 2011). These high-pressure TTG melts display positive Sr and Eu anomalies (Fig. 5.12e), a characteristic commonly documented in slab-derived tonalite and trondhjemite melts involving subduction of young, hot oceanic crust due to the release of Sr and Eu during plagioclase breakdown at the garnet amphiboliteeclogite transition (Defant & Drummond, 1990;[START_REF] Drummond | Petrogenesis of slab-derived trondhjemite-tonalitedacite/adakite magmas[END_REF]Drummond & Defant, 1990), occurring at pressures between 1.5 and 2.0 GPa (Moyen & Stevens, 2006b;[START_REF] Rapp | Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites[END_REF].

As such, these magmas are considered analogous to adakites generated through the partial melting of a hot, down-going slab in an oceanic environment (Defant & Drummond, 1990;[START_REF] Drummond | Petrogenesis of slab-derived trondhjemite-tonalitedacite/adakite magmas[END_REF]Smithies, 2000). Their unusually low Mg# and high SiO 2 and elevated concentrations of Sc, V and Co suggest that evidence of interaction and assimilation with peridotite in the overlying mantle wedge has been partially obscured by subsequent amphibole fractionation [START_REF] Rapp | Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa[END_REF][START_REF] Wang | Petrogenesis of Cretaceous adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province (eastern China): implications for geodynamics and Cu-Au mineralization[END_REF][START_REF] Woodhead | High field strength and transition element systematics in island arc and back-arc basin basalts: evidence for multi-phase melt extraction and a depleted mantle wedge[END_REF].

Predominantly low-K mafic sources are indicated for hornblende-bearing TTGs (Fig. 5.12a), however, compared to the biotite-bearing TTGs, they display lower Al 2 O 3 contents, low Sr concentrations, weakly fractionated REE patterns with high HREE concentrations and slightly negative Eu anomalies. These characteristics indicate that magma production occurred within the stability field of plagioclase, indicating either plagioclase fractionation or plagioclase as a residual phase in a garnet-free or garnet poor-residuum at pressures of <1.0 GPa (Moyen & Stevens, 2006b). These characteristics suggest a shallow, low-pressure source, such as partial melting of garnet-free basaltic crust, interpreted as analogous to the low-pressure TTG group of Moyen (2011).

Contrasting TTG magmas, both the pyroxenite-gabbro and the diorite suites follow the calc-alkaline differentiation trend in the Na-K-Ca ternary diagram (Fig. 5.5). They display low SiO 2 contents, high FeO T , MgO and Mg#, and enrichment in transitional metals including Cr and Ni, thus precluding melt derivation from a purely crustal source. This is further highlighted by REE patterns which display less than an order of magnitude enrichment in HREE and MREE relative to primitive mantle values. Trace element patterns do, however, indicate moderate LILE and Pb enrichments for the quartz diorite suite, suggesting a crustal influence [START_REF] Brown | The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources[END_REF]Hawkesworth & Kemp, 2006a). The pyroxenite suite potentially represents the primary magma from which the dioritic magmas evolved through fractional crystallisation during differentiation, generated thorough partial melting of an enriched mantle source.

Depletion of HFSE elements, including Nb, Ta, Zr and Hf, in diorite and quartz diorite samples is typical of island arc magmas generated during dehydration melting with a rutile-bearing residuum [START_REF] Foley | Growth of early continental crust controlled by melting of amphibolite in subduction zones[END_REF]. Despite similarly low SiO 2 , elevated ferromagnesian (FeO T + MgO) content and Sr and Ba concentrations > 500 ppm, the diorite and quartz diorite samples are not considered to be analogous to the sanukitoids series frequently recorded at the Archaean-Palaeoproterozoic boundary, due to lower MgO concentrations, Mg# and weaker REE fractionation (Martin, et al., 2009;Martin, et al., 2005;[START_REF] Shirey | Mantle-derived Archaean monozodiorites and trachyandesites[END_REF][START_REF] Stern | Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province[END_REF].

Three samples plot in the high-K calc-alkaline field, including the biotite-hornblende quartz monzonite and two granites. The quartz monzonite sample displays a similar composition to the highpressure TTGs in terms of REE distribution and a positive Sr anomaly, but features a higher K 2 O/ Na 2 O ratio, greater concentrations of Rb, Ba and K, as well as lower SiO 2 and no Eu anomaly. It may represent the mixture of magmas from a metasomatised mantle source and partial melting of preexisting high-pressure TTG sources (Feng & Kerrich, 1992). It contrasts the two-mica granite and the muscovite-bearing leucogranite samples, which display higher SiO 2 contents with elevated K 2 O/ Na 2 O (0.93-1.01), reflecting higher modal proportion of potassium feldspar. These granites display strongly fractionated REE patterns, substantial enrichment in LILE and negative Eu anomalies (Fig. 5.12e), indicating plagioclase fractionation or melt generation within the stability field of plagioclase, consistent with geochemical characteristics of S-type granites generated through partial melting of metasedimentary rocks [START_REF] Clemens | S-type granitic magmas-petrogenetic issues, models and evidence[END_REF][START_REF] Clemens | Constraints on melting and magma production in the crust[END_REF]. These granites are interpreted as the product of shallow partial melting of enriched crustal sources, including sedimentary packages, further supported by field observations (Fig. 5.3g), and calculated metamorphic conditions for anatectic terranes detailed in Chapter 4. Within the Sefwi Belt, the extremely low REE content of a sampled rhyolite (SB076) is a reflection of its petrological assemblage which contains no ferromagnesian phases, however, field relationships indicate that this rhyolite non-conformably overlies a garnet-amphibolebearing paragneiss and potentially represents a felsic melt due to partial melting of a nearby crustal source. Figure 5.12f shows that the two-mica and muscovite bearing granites plot in the collisional granite field (Pearce, et al., 1984a), whilst both TTG groups and the diorite groups plot in the volcanic arc granite field due to lower LILE enrichment.

Timing of juvenile crust formation, recycling and basin deposition

The temporal evolution of magmatism in the study indicates partly contemporaneous emplacement of high-pressure TTGs derived from melting of the subducting, young, hot, oceanic lithosphere, and quartz diorite melts derived from an enriched mantle source. These granitoids are emplaced into volcanic rocks of the Sefwi Belt, which have crystallisation ages between ca. 2189 and 2164 Ma (Hirdes & Davis, 1998;Hirdes, et al., 2007). The dacitic volcaniclastic sample from the Sunyani Basin yielded a zircon U-Pb crystallisation age of 2158 ± 6 Ma and displays similar trace element patterns to the lowpressure TTGs, and is therefore interpreted as their volcanic equivalent. Hornblende-bearing tonalites emplaced in the Ivorian extent of the Sefwi-Wiawso domain at 2162 ± 1 Ma display moderate LILE enrichment relative to LREE and weak to intermediate REE fractionation patterns (Hirdes, et al., 2007), and are also considered analogous to the hornblende-bearing TTG suite of this study. The timing of emplacement of the pyroxenite and gabbro suite remains poorly understood however, samples documented in this study are consistent with pyroxenites documented in south-eastern Ivory Coast also indicating a LILE-enriched upper mantle source (Hirdes, et al., 2007).

Approximately 20 to 25 Myrs after TTG and diorite emplacement, high-K quartz monzonites yield zircon crystallisation ages of 2136 ± 7 Ma, interpreted as the product of remelting of existing high-pressure TTGs by mantle derived magmas. Quartz monzonite magmatism in the study area coincides with the emplacement of monzonitic and arfvedsonite-bearing albite granite magmatism between ca. 2145 and 2135 Ma in the Ivorian extent of the Sefwi-Wiawso Domain (Hirdes, et al., 2007).

Finally, two-mica and muscovite granitoids are emplaced in the Sunyani-Comoé Domain along the north-western margin between ca. 2092 and 2081 Ma (Agyei Duodu, et al., 2009;Hirdes, et al., 1992;Hirdes, et al., 2007;Petersson, et al., 2016), associated with crustal thickening and anatexis of felsic or supracrustal rocks, as detailed in Chapters 3 and 4. The muscovite granite sample (SB010) is taken from the margin of a muscovite-bearing granitoid that yielded Pb-Pb ages of 2088 ± 1 Ma (Hirdes, et al., 1992) and the two-mica granite (SB370) was sampled from the Wenchi pluton dated at 2092 ± 2 Ma (Agyei Duodu, et al., 2009), suggesting that true granite magmas occur significantly later in the magmatic history of the study area.

The evolution of magmatic suites in the Neoarchaean terranes defines the following progression: (1) TTG magmatism, generated through the partial melting of mafic rocks; (2) crust-derived biotite or two-mica granitic magmatism; (3) sanukitoids derived from a hybridized incompatible element-rich source and mantle peridotite, and (4) high-K hybrid granites displaying characteristics of the three previous suites (Feng & Kerrich, 1992;Laurent, et al., 2014;Smithies & Champion, 2000). Laurent, et al. (2014) contend that the distinct evolution and diversity of magmatism in Archaean granitoids represents an ancient manifestation of subduction and tectonic collision. Despite the absence of sanukitoid-like rocks in the study area, a similar diversity and evolution is hypothesised for southern Ghana, involving early subduction-related TTG magmatism from both the subducting slab and shallow partial melting of a basaltic source with coeval dioritic magmatism derived from the metasomatised mantle wedge, emplacement of minor hybrid, high-K plutons, followed by production of crust-derived peraluminous granites during collision of continental fragments in the early Palaeoproterozoic.

The results of U-Pb dating of magmatic zircons constrain a period of magmatic activity in the Sefwi Greenstone Belt and the Sunyani-Comoé Domain yield two inherited zircon ages of 2250 ± 28 and 2270 ± 21 Ma and crystallisation ages between ca. 2158 and 2136 Ma. Previous studies record a wider range of U-Pb crystallisation ages, between ca. 2222 and 2081 Ma, both within the study area and in the immediately adjacent Palaeoproterozoic domains (e.g. Davis, et al., 1994;Delor, et al., 1992;Hirdes & Davis, 1998;Hirdes, et al., 2007;Oberthür, et al., 1998;[START_REF] Siméon | Carte géologique de la Côte-d'Ivoire 1/200000, feuille Abidjan. Direction des mines et de la géologie[END_REF]. The oldest preserved crystallisation age in the study area, yielded by a meta-gabbro in the Hwidiem Domain, for which K-Ar dating of amphibole indicating cooling below 550°C at 2222 ±32 Ma (Feybesse, et al., 2006), within error of inherited zircon cores dated in this study. Andesitic and rhyolitic flows and volcaniclastic rocks within the Sefwi Belt yield U-Pb and Pb-Pb zircon ages between ca. 2189 and 2164 (Hirdes & Davis, 1998;Hirdes, et al., 2007), partly contemporaneous with the emplacement of "belt-type" or "Dixcove-type" TTG granitoids into volcanic rocks of the Sefwi Belt between ca. 2179 and 2154 Ma (Hirdes, et al., 1992;Hirdes, et al., 2007;Petersson, et al., 2017).

On the northern margin of the Sefwi Greenstone Belt, the timing of intermediate volcanism and sedimentation in the Sunyani-Comoé Domain (also referred to in the literature as the Sunyani Basin, or an extension of the Comoé Basin; Agyei Duodu, et al., 2009;Feybesse, et al., 2006;[START_REF] Vidal | The palaeoproterozoic (Birimian) of Haute-Comoé in the West African craton, Ivory Coast: a transtensional back-arc basin[END_REF]) is constrained by syn-depositional dacitic sample SB173B, which yielded an 207 Pb/ 206 Pb crystallisation age of 2158 ± 6 Ma. This dacite is found intercalated with low-grade volcano-sedimentary rocks and provides a new maximum age for the commencement of sedimentation for the domain, previously constrained by post-depositional emplacement of elongate leucogranites between ca. 2092 and 2081 Ma (Agyei Duodu, et al., 2009;Hirdes, et al., 1992;Hirdes, et al., 2007). Furthermore, this age indicates that volcanism and sedimentation in the Sunyani-Comoé Domain was contemporaneous with plutonism in the Sefwi Belt, suggesting coeval crustal formation with an unknown initial spatial relationship. Comparison of the timing of deposition of volcano-sediments in the Kumasi-Afema Domain, bracketed between ca. 2135 and 2122 Ma (Adadey, et al., 2009) or between ca. 2154 to 2136 Ma (Perrouty, et al., 2012) Zircons from magmatic rocks analysed in this study display suprachondritic εHf values between +1.2 and +8.0 at the time of zircon crystallisation reflecting a highly radiogenic source and the juvenile nature of the newly formed crust. Furthermore, the juvenile Hf signature of all samples suggests that older, Archaean crustal material was absent or distal during juvenile crust production, consistent with the findings of previous studies of Palaeoproterozoic rocks in the West Africa Craton that yielded positive εNd between +1.2 and +4.3 (Abouchami, et al., 1990;Boher, et al., 1992;Gasquet, et al., 2003;Taylor, et al., 1992) (Condie, et al., 2005) and a mafic crust evolution with 176 Lu/ 177 Hf = 0.0210 [START_REF] Kramers | Hierarchical Earth accretion and the Hadean eon[END_REF]. The two evolutionary trends proposed by different authors [START_REF] Blichert-Toft | Depleted mantle sources through time: evidence from Lu-Hf and Sm-Nd isotope systematics of Archean komatiites[END_REF][START_REF] Griffin | Zircon chemistry and magma mixing, SE China: in-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes[END_REF] -Avila, et al., 2016). Younger inheritance is recorded by the Ghanaian Gondo granite gneiss, dated at ca. 2876 Ma, [START_REF] Thomas | Geological Map Explanation[END_REF] and the Infantayire granite gneiss with inherited ages between ca. 2390 and 2260 Ma (Siegfried, et al., 2009), and the 2312 ± 12 Ma inherited zircons age from the Dabakala tonalite, Ivory Coast (Gasquet, et al., 2003). By comparison, the oldest recorded crystallisation ages within the Baoulé-Mossi Domain is 2265 ± 17 Ma, yielded by a granodiorite in the Goren/Po-Tenkodogo-Yamba of the Banfora Belt, et al. (2016a), Eglinger, et al. (2017), Parra-Avila, et al. (2016) and Petersson, et al. (2016;2017).

The most significant implication of this data is realised when it is displayed as an overlay of the geological map of the Baoulé-Mossi Domain (Fig. 5.15), highlighting the spatial distribution of juvenile versus reworked crustal material. Whilst we acknowledge the relative sparsity of the data, it may be noted that the highly radiogenic Hf signatures of ca. 2155 Ma samples from the Sefwi Belt (this study; Petersson, et al., 2017) separate the sub-chondritic Hf signature of the Kibi Belt from the sub-chondritic to supra-chondritic rocks of the Maluwe and Sawla Suite of NW Ghana (Petersson, et al., 2017), and the more distal mixed Hf signatures near the Archaean-Palaeoproterozoic boundary in southern Mali (Parra-Avila, et al., 2016) and in Palaeoproterozoic rocks in Guinea external to the Archaean domain (Eglinger, et al., 2017). : Block, et al., 2016a;Eglinger, et al., 2017;Parra-Avila, et al., 2016;Petersson, et al., 2017;Petersson, et al., 2016) We, therefore, propose that the juvenile nature of crust exposed in the study area represents an isotopic boundary within the crustal architecture of the WAC. Testing of this hypothesis, however, requires significantly more data, especially in the Ivory Coast. This data supports the conclusions of a number of authors who suggest that isotopic signatures of magmatic rocks in southern Ghana require a second, as yet unknown, Archaean crustal source that lay southeast of the Baoulé-Mossi domain during crust formation (Petersson, et al., 2017;Taylor, et al., 1992). Further isotopic mapping of internal boundaries within the juvenile terranes of the WAC is somewhat limited by the capability of Hf analyses to recognise the reworking of recently created crust (Payne, et al., 2016, and references therein), thus impeding our understanding of the crustal architecture through Hf-isotopes alone.

A geodynamic model for the southern West African Craton

Palaeoproterozoic greenstone belts and granite-gneiss domains in the West African Craton have been likened to Archaean provinces based on lithological and structural associations (e.g. Delor, et al., 1995;Lompo, 2009;Lompo, 2010;Vidal, et al., 1996;Vidal, et al., 2009), whilst others consider greenstone belts in West Africa as analogous to modern volcanic arcs (e.g. Baratoux, et al., 2011;Dampare, et al., 2008;Senyah, et al., 2016;Sylvester & Attoh, 1992). New geochemical and isotopic data presented in this study supports subduction-related magma generation, with most of the magmatic suites analysed from the Sefwi Greenstone Belt bearing geochemical signatures characteristic of arc environments.

The proposed arc type environment is consistent with a growing body of evidence for volcanic arcs generated in a subduction setting within the West African Craton (Ama [START_REF] Salah | Evolution d'un arc insulaire océanique birimien précoce au Liptako nigérien (Sirba): géologie, géochronologie et géochimie[END_REF]Béziat, et al., 2000;Block, et al., 2016a;[START_REF] Lambert-Smith | Palaeotectonic setting of the south-eastern Kédougou-Kéniéba Inlier, West Africa: New insights from igneous trace element geochemistry and U-Pb zircon ages[END_REF]Parra-Avila, et al., 2017;Petersson, et al., 2017;Petersson, et al., 2016;Sylvester & Attoh, 1992;Tapsoba, et al., 2013).

The two end-member models generally considered for the production of TTGs through partial melting of hydrous metabasalts are distinguished by two contrasting geodynamic settings. The first geodynamic setting for TTG production is through partial melting of a basalt underplating thickened crust (Albarède, 1998;Atherton & Petford, 1993;Bédard, 2006;de Wit, 1998;Smithies, et al., 2005). The second setting is through the partial melting of a young, hot subducting slab, rather than dehydration melting [START_REF] Foley | Growth of early continental crust controlled by melting of amphibolite in subduction zones[END_REF][START_REF] Kenneth | Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas[END_REF]Martin & Moyen, 2002;[START_REF] Rapp | Dehydration melting of metabasalt at 8-32 kbar: implications for continental growth and crust-mantle recycling[END_REF][START_REF] Rapp | Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites[END_REF], which generates melts that more closely resemble the geochemical characteristics of the high-P, low-HREE TTGs occurring in the Sefwi Belt.

Juvenile magmatism documented in the study area is likely sourced from both the metasomatised mantle wedge as well as the subducting slab. This suggests that the geodynamic setting for crust production was hot enough to generate slab-derived melts, more than 250 Myrs younger than the 2500 Ma end-date of slab derived melts, proposed by Martin, et al. (2009), reflecting a decrease in the Earth's heat production. We note the diversity and chronological evolution of magmatism in the study area, including subduction-related magmatism followed by emplacement of syn-collisional crustal melts and hybrid granitoids, which somewhat resembles those recorded in juvenile terranes from the late Archaean, including the Pietersburg Block of the Kaapvaal Craton (Laurent, et al., 2014) and the Kola and Karelian Craton (Halla, et al., 2009), elucidated by similar petrogenetic processes and broadly comparable geodynamic settings. In contrast to these systems however, the crustal evolution documented here in southern Ghana occurs in <100 Myrs, as opposed to the 1000 Myrs evolutionary period proposed for Archaean terranes [START_REF] Laurent | A linear Hf isotope-age array despite different granitoid sources and complex Archean geodynamics: Example from the Pietersburg block (South Africa)[END_REF].

Whilst magmatism in the Sefwi Belt and the adjacent domain is consistent with the peak magmatic activity for the eastern block of the West African Craton (Parra-Avila, et al., 2017), there are multiple avenues of evidence which suggest southern Ghana docked onto central Ghana and the central east Ivory Coast at ca. 2090 Ma. Evidence includes collision-related crust-derived granitic magmatism along the north west margin of the Sefwi Belt between ca. 2092 and 2081 Ma (this study ;Hirdes, et al., 1996;Hirdes, et al., 2007) as well as high-pressure, intermediate-temperature metamorphism and anatexis (Ch. 4) with retrograde (post-peak T) metamorphism constrained by in-situ monazite U-Pb ages dated at ca. 2073 Ma. Within the broader context of the Palaeoproterozoic Baoulé-Mossi Domain of the WAC, diachroneity in the geochronological record suggests additional crustal boundaries coincide the boundary located between the Banfora and Bagoe belts of western Burkina Faso and southern Mali, respectively, as identified by Parra-Avila, et al. (2017). Block, et al. (2016b) recently proposed a collisional event at ca. 2130 Ma between NW Ghana and western Burkina Faso/Ivory Coast associated with peak metamorphism and crustal anatexis. This coincides with peak magmatism and the first phase of the westward migration of magmatism from the eastern most portions of the Baoulé-Mossi domain to the central crustal boundary (Hirdes, et al., 1996;Parra-Avila, et al., 2017). Parra-Avila, et al. (2017) note a second westward migration of magmatism at ca. 2090 Ma, with subsequent synchronous subduction-related juvenile crust formation of the Kédougou-Kéniéma Inlier (Hirdes & Davis, 2002a;[START_REF] Lambert-Smith | Palaeotectonic setting of the south-eastern Kédougou-Kéniéba Inlier, West Africa: New insights from igneous trace element geochemistry and U-Pb zircon ages[END_REF] and high-K magmatism along the northern margin of the Kénéma Man Shield (Egal, et al., 2002;Eglinger, et al., 2017) during the collision of the Archaean and Palaeoproterozoic domains. This protracted westward migration of magmatism and subsequent cessation of magmatism is incompatible with the timing of collision between the Kibi Belt and an Archaean fragment at ca. 2140 Ma (Petersson, et al., 2017), and the subsequent collision along the NW margin of the Sefwi Belt between southern Ghana and central Ghana/eastern Ivory Coast between 2100 and 2080 Ma (this study). The complex formation and terrane accretion of discrete crustal blocks therefore requires the invocation of a second driving force during the amalgamation of the craton, likely represented by a second subduction zone interacting with the larger, west migrating system. As such, we proposed that the amalgamation of the West African Craton was the product of multiple, diachronous docking or collisional events between discrete juvenile arc terranes. In the case of the Ivory Coast, NW Ghana and western Burkina Faso, the arc terranes may have developed within an oceanic plateau with subsequent collision causing intense structural imbrication of the buoyant oceanic arc lithosphere, analogous to the Belingwe Greenstone Belt and associated terranes in the southern Zimbabwe Craton [START_REF] Kusky | Remnants of an Archean oceanic plateau, Belingwe greenstone belt, Zimbabwe[END_REF], partially obscuring the early collisional history.

Juvenile crust of the West African Craton represents a unique episode in the geological history of the early Earth, immediately following a hypothesised shut down of magmatism and plate tectonics between 2400 and 2200 Ma [START_REF] Condie | Evidence and implications for a widespread magmatic shutdown for 250 My on Earth[END_REF], and coinciding with the supposed break up of Archaean cratons [START_REF] Bleeker | The late Archean record: a puzzle in ca. 35 pieces[END_REF]. Figure 5.16 shows the spectrum of εHf (t) values yielded by magmatic zircons from the study area relative to the global detrital zircon record [START_REF] Roberts | The zircon archive of continent formation through time[END_REF], demonstrating that the juvenile crustal growth in southern Ghana occupies the upper part of the εHf spectrum for the global record between ca. 2500 and 2000 Ma. Furthermore, in the global magmatic zircon Hf record, Condie, et al. (2011) demonstrated a peak in crustal reworking and a minima in juvenile crust production at 2100 Ma, which significantly contrasts with the crustal record of the study area and the West African Craton. We propose that high juvenile production and preservation recorded in West Africa is the product of the interaction of two northwest-dipping subduction zones. The central/northern subduction zone interacted with an oceanic plateau, as proposed by Boher, et al. (1992), associated with the westward migration and cessation of magma attributed to multiple collisional events within the craton and highly varied TTG and magma petrogenetic processes [START_REF] Goldfarb | West Africa: The World's Premier Paleoproterozoic Gold Province[END_REF]Hirdes & Davis, 2002b;Hirdes, et al., 1996;Parra-Avila, et al., 2017). A smaller subduction zone to the southeast, responsible for the generation of the juvenile crust of southern Ghana (Sefwi, Ashanti and Kibi belts) and the collision of the unknown south-eastern Archean block with the Kibi Belt, with the subsequent collision along the Sefwi Belt. Similar crustal evolution and growth models are proposed for the Archaean Belingwe greenstone belt of the Zimbabwe Craton [START_REF] Kusky | Remnants of an Archean oceanic plateau, Belingwe greenstone belt, Zimbabwe[END_REF] and the Mesoarchaean Rio Maria granite-greenstone terrane of the Amazonian Craton Lateral accretion, age gradients and horizontal shortening are predominantly interpreted as the result of subduction, with several authors proposing such features have characterised accretionary orogens since throughout the majority of Earth's history [START_REF] Betts | Palaeoproterozoic accretion processes of Australia and comparisons with Laurentia[END_REF]Card, 1990b;Cawood, et al., 2009;Cawood, et al., 2006;[START_REF] Kusky | Accretion of the Archean Slave province[END_REF][START_REF] Percival | Minto block, Superior province: Missing link in deciphering assembly of the craton at 2.7 Ga[END_REF]. The amalgamation of the West African Craton requires multiple subduction zones to elucidate the widespread magmatism, likely interacting with an oceanic plateau representing the recycled proto-crust, as well as the diachronous, distributed collision of multiple terranes. The collisions or intra-terrane sutures do not preserve evidence of modern collisional zones, such as ophiolite sequences or ultra-high pressure metamorphism associated with the subduction of continental crust (Chopin, 2003;Pearce, et al., 1984b), however, evidence of low apparent geothermal gradients, high-pressure-low/medium-temperature metamorphism and tectonic exhumation of lower orogenic crust has been demonstrated in a number of areas in the craton (this study ;Block, et al., 2015;Block, et al., 2016b;Ganne, et al., 2012). We, therefore, propose that the craton represents the ancient expression of a subductioncollision systems, with intermediate characteristics of late Archaean and Phanerozoic accretionary orogenic systems, supporting the concept of the secular evolution and development of modern style plate tectonics during the Palaeoproterozoic (Brown, 2006;Brown, 2007b).

Conclusions

The Palaeoproterozoic domains of the West African Craton, comprising greenstone belts, large granitegneiss domains and wide meta-sedimentary basins share a number of characteristics with Archaean provinces. The proto-crust was likely extracted from the depleted mantle between ca. 2650 and 2260 Ma, subsequently completely recycled prior to the generation of crustal material preserved in the Sefwi Belt between ca. 2222 Ma and 2080 Ma. Geochemical evidence presented in this study indicates the generation of diverse plutonic suites, including TTG granitoids, which share geochemical affinities with classical volcanic arc rocks whilst displaying striking similarity to Neoarchaean magmatic suites. Major and trace element geochemistry indicates predominantly low-K mafic sources and near ubiquitous positive Pb and Ba anomalies and negative Nb-Ta, P and Ti anomalies, in agreement with models of intra-oceanic subduction-related crustal growth for the craton. Two groups of TTGs as well as dioritic granitoids are emplaced synchronously from contrasting melt sources and melt depths, including partial melting of a young, hot subducting slab, dehydration melting of the metasomatised mantle wedge and shallow partial melting of a mafic source. The subsequent emplacement of quartz monzonites bearing hybrid characteristics and mantle derived magmas and partial melting of preexisting TTGs potentially represents a late extension event, associated with the opening of the Kumasi-Afema Domain between the Sefwi and the Ashanti belts. The emplacement of syn-tectonic two-mica and muscovite granites associated with crustal anatexis is associated with the collision of discrete crustal fragments the cessation of magmatism in southern Ghana. The diversity of magmatic suites documented in the study area resembles that of late Archaean terranes suggesting comparable petrogenetic processes, linked to the global onset of subduction-collision cycles, with Neoarchaean and Palaeoproterozoic terranes indicating the onset occurred locally and diachronously between different cratons (Laurent, et al., 2014). εHf (t) values from analysed samples suggest the Sefwi Greenstone Belt represents an isotopic boundary within the West African Craton, separating reworked or inherited Archaean crustal material in southeast Ghana from those in the northwest of the Baoulé-Mossi domain. This finding has two implications: (1) that distinct terranes within the craton may potentially be isotopically mapped, allowing us to better understand the crustal architecture; and, (2) that a second Archaean craton lay southeast of the WAC during the Palaeoproterozoic accretion of juvenile crust as indicated by subchondritic εHf values in the Kibi Belt, supporting the hypothesis of Petersson, et al. (2016). The most significant geodynamic implications of these findings is that we, therefore, have geochemical and isotopic evidence of intraoceanic subduction operating between two Archaean cratons, suggesting that subduction processes were driving tectonic activity in the WAC during the early Palaeoproterozoic. Furthermore, evidence of multiple subduction zones simplifies models that must reconcile rapid generation of multiple volcanic arcs, their diachronous accretion and preservation of such large areas juvenile crust.

Discussion

Introduction

This thesis investigated the tectonic, metamorphic and crustal evolution on the volcano-plutonic and volcano-sedimentary rocks exposed in SW Ghana in the Palaeoproterozoic south-eastern portion of the West African Craton. The major aims of this thesis were to: 1) understand the structural and metamorphic evolution of the Sefwi Greenstone Belt;

2) characterise the tectonic setting and driving forces of the Palaeoproterozoic Eburnean Orogeny; and, 3) investigate the geodynamic setting and processes responsible for the formation, amalgamation and preservation of large volumes of juvenile Palaeoproterozoic crust of the West African Craton, and how this reflects global geodynamic changes and the thermal evolution of the Earth.

In order to explore these broad aims, this thesis encompassed a multi-scale and multi-disciplinary approach combining field mapping, structural geology, structural geophysics, metamorphic petrology and P-T modelling, whole rock major and trace element geochemistry, SHRIMP U-Pb zircon and monazite geochronology, LA-MC-ICP-MS Lu-Hf zircon analysis and LA-ICP-MS U-Pb detrital zircon geochronology. Regional lithological, structural and metamorphic mapping was conducted throughout the 60 000 km 2 study area, from which we were able to develop a structural and metamorphic framework. This framework, combined with field observations and regional lithological databases, allowed us to interpret regional geophysical datasets to generate new litho-tectonic and metamorphic maps of the Sefwi Greenstone Belt and adjacent volcano-sedimentary domains. In this discussion, we summarise the tectonic and metamorphic history of the study area and potential tectonic models for the Eburnean Orogeny and discuss the insight this provides into Palaeoproterozoic orogenesis.

In addition, we discuss the crustal evolution of southern WAC, and how this conceptual model aids our understanding of the evolution of global geodynamics since the Palaeoproterozoic relative to the modern plate tectonic regime. Finally, we highlight the opportunities derived from the thesis for further research in the West African Craton and other Palaeoproterozoic provinces.

Evolution of the West African Craton

Tectonic and metamorphic evolution

This thesis provides a new litho-structural and metamorphic map of the Sefwi Greenstone Belt, as well as parts of the Sunyani-Comoé and the Kumasi-Afema domains of southwest Ghana. The work combines a detailed analysis of the metamorphic conditions and thermal regimes recorded by tectono-metamorphic domains within the study area, revealing a far more complex metamorphic history than previously recognised. The thesis presents evidence of an otherwise undocumented migmatitic domain on the north-eastern margin of the Sefwi Belt, tectonically juxtaposed against high-pressure amphibolite facies rocks within the central region of the Sefwi Belt and the low-grade metamorphosed volcano-sedimentary rocks of the Sunyani-Comoé Domain. Documented initial low apparent geothermal gradients (~15°C/km) and clockwise P-T paths are associated with thrust-related crustal thickening, subsequent partial melting in the lower crust and shallow-crustal emplacement of syn-tectonic leucogranites between ca. 2092 and 2081 Ma, parallel to the north-western margin of the Sefwi Belt. Furthermore, high-grade metamorphism and exhumation are attributed to monocyclic orogenesis during Eburnean deformation in the study area, representing the tectonic accretionary phase of the orogen. Combined structural and metamorphic evidence is consistent with collisional orogenesis [START_REF] England | Some thermal and tectonic models for crustal melting in continental collision zones[END_REF]Thompson & England, 1984).

Late-orogenic exhumation along normal detachments and shear zones during regional transtension resulted in the tectonic juxtaposition of mid-crustal and lower-crustal rocks of the Sefwi Belt with the low-grade, upper-crustal rocks of the Sunyani-Comoé Domain. Constrictional deformation and exhumation of the lower crust are consistent with simple shear-dominated transtension during oblique orogenic extension or collapse (Fossen, et al., 2013), reminiscent of processes recorded in the Early Devonian Scandinavian Caledonides [START_REF] Dewey | Extensional collapse of orogens[END_REF]Krabbendam & Dewey, 1998). Petrological and metamorphic evidence of orogenic collapse includes symplectite textures, supra-solidus isothermal decompression [START_REF] Duchêne | Exhumation of eclogites: insights from depth-time path analysis[END_REF] and formation of anatectic domes [START_REF] Rey | Gravitational collapse of the continental crust: definition, regimes and modes[END_REF]Vanderhaeghe, 2009;Vanderhaeghe & Teyssier, 2001a;Whitney, et al., 2004). This contrasts with the metamorphic record of the study area, which is characterised by concurrent decreases in pressure and temperature indicating cooling during exhumation, juxtaposing both migmatitic and amphibolite facies rocks with greenschist facies supracrustal sequences. Postcollisional extension, exhumation and transcurrent tectonism is favoured over orogenic collapse in SW Ghana. It is interpreted on the basis of previous studies of elongate granites (Jessell, et al., 2012), coupled with macro-geometries of tectono-metamorphic domains, that D 2 deformation in the Sefwi shear system is characterised by major normal, left-lateral movement. We suggest that extensional strain was localised along the NW margin of the Sefwi Belt due to the presence of a partially molten lower crust along the collisional front, which would have resulted in the mechanical weakening of the lithosphere [START_REF] Vanderhaeghe | The thermal-mechanical evolution of crustal orogenic belts at convergent plate boundaries: a reappraisal of the orogenic cycle[END_REF]Vanderhaeghe & Teyssier, 2001a;Vanderhaeghe & Teyssier, 2001b).

Our understanding of the driving force of transtension, however, is hindered by the destruction of the Palaeoproterozoic plate margin. Transtension may either be interpreted as the localisation of strain due to a change in boundary conditions, such as rollback of a far-field subduction zone, or, alternatively, it may represent lateral extrusion along a narrow orogenic front (Molnar & Tapponnier, 1975;Sperner, et al., 2002;Thompson, et al., 1997b). In terranes that have undergone extrusion, peak pressures along P-T-t paths coincide with peak temperatures (Thompson, et al., 1997a), however, the P-T-t paths derived for metamorphic rocks in the study area yield results that are inconclusive in this respect due to poor preservation of prograde assemblages.

As this thesis aimed to explore the evolution and style of tectonic processes during the Palaeoproterozoic, we must also consider the results in light of the alternative tectonic models. In assessing the applicability of a gravitationally-driven tectonic regime, it was established in the introductory chapter of this thesis that sagduction of dense, cold greenstone cover into a mechanically weak felsic lower crust could provide a potential mechanism for the rapid burial of supracrustal rocks (e.g. [START_REF] François | Burial and exhumation during Archean sagduction in the East Pilbara granite-greenstone terrane[END_REF], Ganne, et al. 2014), with corresponding lateral shortening and low apparent geothermal gradients. All of these features are observed in the Sefwi Greenstone Belt, recorded by migmatitic paragneisses of the Chiraa Domain, found in association with voluminous peraluminous granite intrusions, and in garnet amphibolites in the core of Sefwi Belt in the Kukuom-Juaboso Domain. Exhumation of high-grade rocks in the vertical tectonic model requires the diapiric ascent of buoyant granitoids. Whilst this could potentially explain exhumation in the Chiraa Domain, the same can't be said for the garnet amphibolite in the southwest of the Kukuom-Juaboso Domain, which is located within the major Ketesso Shear Zone, distal to any major intrusions. Alternatively, if the entire Sefwi Belt was sagducted, it would require the diapiric emplacement of vast granitoid domains immediately adjacent to the belt. This is not reflected in the crustal architecture of southwest Ghana nor in the isotopic record. Other features observed in the field area that contradict the vertical tectonic model include: dominantly lateral deformation and a near-total absence of steeply down-diachronous nature of deformation and metamorphism raises new questions for future research in the craton. For example, have all episodes of deformation and metamorphism been detected or are there additional internal collisions and what are the implications for the progression of the Eburnean Orogeny?

Juvenile crustal growth

The radiogenic Palaeoproterozoic continental crust exposed in Sefwi Belt was derived from a mafic proto-crust extracted from the depleted mantle between 2.26 and 2.65 Ga, indicative of short crustal residence times with minimal reworking of Archaean-aged cratonic material (Ch. 5, this study;Petersson, et al., 2017;Petersson, et al., 2016). The results of coupled geochronological and geochemical studies indicate the coeval production of TTG-like and calc-alkaline magmas, derived from both partial melting of mafic crust at contrasting depths, as well as a LILE-enriched mantle source, with late emplacement of crust-derived felsic magmas. New U-Pb and Lu-Hf analyses indicate the recycling of juvenile mafic crust, with inherited zircon cores dated at ca. 2250 and 2270 Ma.

Petrological and geochemical analyses inform us about the petrogenetic processes active during crust formation, including high-pressure partial melting of the subducting slab and low-pressure melting of mafic material generating high SiO2, sodic TTG granitoids with contrasting REE patterns, partial melting of the metasomatised mantle wedge forming dioritic magmas, and late-stage crust-derived granitic plutons. This magmatic evolution is somewhat analogous to Neoarchaean provinces in the Karelian and Kola cratons (Halla, et al., 2009), the Abitibi and Pontiac sub-provinces of the Superior Craton (Feng & Kerrich, 1992), the Rio Maria granite-greenstone terrane of the Carajás Province, Brazil [START_REF] Almeida | Zircon geochronology and geochemistry of the TTG suites of the Rio Maria granitegreenstone terrane: Implications for the growth of the Archean crust of Carajás Province, Brazil[END_REF] and the Limpopo Belt and Kaapvaal Craton of southern Africa (Laurent, et al., 2014). New geochronological constraints presented in this study indicate contemporaneous emplacement of magmas from a range of sources within a volcanic arc setting, with late emplacement of magma from shallow crustal sources, interpreted as the product of collision between two crustal blocks. As such, geochemical and isotopic evidences support crustal growth in a subduction setting followed by subsequent arc-arc collision during continental accretion, suturing the arc material of southern Ghana with the arc material of central Ghana and the Ivory Coast, which likely developed within, or in association with, an oceanic plateau. The magmatic evolution of the Sefwi Greenstone Belt falls within the Eoeburnean event of Perrouty, et al. (2012), whilst the proposed collision (this study) corresponds with the Eburnean sensu stricto. I suggest that division of the Eburnean Orogeny into multiple orogenies on the basis of pulses of magmatism must be carefully considered. Whilst the two peaks of magmatic activity documented by Perrouty, et al. (2012Perrouty, et al. ( ) at ca. 2160Perrouty, et al. ( and ca. 2090 Ma are likely separated by the deposition of the Kumasi Group, they most clearly demonstrate a change in magma chemistry from calc-alkaline and sodic tonalitic and trondhjemitic magma to more peraluminous, crust-derived magma, thus demonstrating a similar petrogenetic evolution to modern accretionary-collisional orogens.

The proposed collision between southern Ghana and central Ghana/Ivory Coast is contemporaneous with volcanic arc magmatism in the far west of the West African Craton in the Kedougou-Kéniéba Inlier of eastern Senegal (Hirdes & Davis, 2002). As these areas are more than 1000 km apart, we propose that the formation and accretion of juvenile crustal domains in the West African Craton is associated with the interaction between two coeval, west-dipping subduction zones operating simultaneously, and ultimately driving episodic collision between discrete arc terranes. Greenstone belts of southern Ghana are the product of the smaller subduction zone operating independently of a larger, westward migrating subduction zone driving multiple collisional events during interaction with an oceanic plateau in the Ivory Coast, NW Ghana and northern Burkina Faso (Abouchami, et al., 1990;Boher, et al., 1992), and subsequent terminal collision with the Archaean Kénéma-Man Domain at ~2050 to 2030 Ma. This is evident in localised age gradients preserved in both the magmatic and metamorphic record and proposed sutures within the craton (Block, et al., 2016;Eglinger, et al., 2017;Hirdes & Davis, 2002;Kouamelan, et al., 1997;Parra-Avila, et al., 2016;Parra-Avila, et al., 2017;Petersson, et al., 2017;Petersson, et al., 2016;Pitra, et al., 2010). This hypothesis is consistent with the distribution of greenstone belts, the voluminous TTG magma emplacement in the central regions of the southern West African Craton and the diachronous nature of high-grade metamorphism and deformation in the craton. In addition, the rapid formation and accretion of such terranes provides a plausible mechanism for the stabilisation and preservation of such a vast area of juvenile crust.

Finally, the geochemical, isotopic, metamorphic and structural history for SW Ghana presented in this study highlights the striking similarity between the Sefwi Greenstone Belt and the Paleoproterozoic Ile de Cayenne complex of French Guiana. They display coeval juvenile arc-related magmatism between 2174 and 2144 Ma with positive consistently εNd values, with subsequent crustal thickening and anatexis resulting in the emplacement of small syn-tectonic granite plutons along major shear zones, dated at 2093 ± 8 and 2083 ± 8 Ma (Delor, et al., 2003;Vanderhaeghe, et al., 1998). The final correlation between the two terranes comes from the timing of crustal extension and formation of granulite facies metamorphism is in the Guiana Shield at 2070 to 2060 Ma (Delor, et al., 2001), Archaean-Proterozoic transition is coupled with a cooling and strengthening of continental crust, after which, the continental crust should theoretically support thrust-related crustal thickening and elevated topography. This study supports a growing body of research in the West African Craton that demonstrates thrust-related crustal thickening and structurally-controlled exhumation in a relatively strong lithosphere (Block, et al., 2016;Feybesse, et al., 2006;Hirdes, et al., 2007). The tectonic style and orogenic model proposed for SW Ghana bears a number of similarities to collisional orogenesis documented in the Archaean Barberton terrain (Diener, et al., 2013;Moyen, et al., 2006) and the Archaean Abitibi Sub-province (Daigneault, et al., 2002;Mueller, et al., 1996) and the Early Devonian Western Gneiss Region of the Scandinavian Caledonides [START_REF] Dewey | Extensional collapse of orogens[END_REF]Krabbendam & Dewey, 1998). In the Palaeoproterozoic rocks of southern Ghana, the absence of UHP rocks, akin to those found that characterise the Scandinavian Caledonides, highlights a conundrum in Neoarchaean to Mesoproterozoic terranes eloquently elucidated by Brown (2007). Brown poses the question as to whether the absence of such rocks reflects: a) the inactivity of deep subduction and generation of UHP rocks at this time in the Earth's history, or, b) the inability of the early Earth lithosphere to exhume rocks from such depths, on the basis of elevated geotherms resulting in a rheologically weaker lithospheric plate. We suggest that the crustal record of the West African Craton represents nascent subductionrelated plate tectonics, tempered by elevated lithospheric temperatures during the Palaeoproterozoic relative to today. This is supported by coeval generation of slab-derived high-pressure TTGs, which requires elevated geotherms in the downgoing slab, and calc-alkaline dioritic magmas characteristic of a modern arc terrane. Furthermore, the metamorphic record reveals cold apparent geothermal gradients and HP-LT and HP-MT metamorphic assemblages similar to modern collisional orogens (this study ;Block, et al., 2015;Ganne, et al., 2012). The absence of UHP assemblages likely reflects the inability of the slightly warmer and weaker Palaeoproterozoic crust to generate or exhume such assemblages, supporting the hypothesis of Sizova, et al. (2014), who propose a secular evolution of collisional orogenesis during the Proterozoic. The hybrid Archaean-Phanerozoic characteristics of subduction-related magmatism and subduction-collision-related metamorphism suggests that the evolution of geodynamic processes may have been a smooth progression, with gradual adoption of modern characteristics and the appearance of classic tectonic indicators occurring in the Palaeoand Mesoproterozoic. If this is the case, Palaeoproterozoic terranes around the world may provide the missing link between archaic and modern tectonic regimes, thus warranting further geological research.

Recommendations for future research

Based on the findings of this thesis, more thorough, suite-specific geochemical and isotopic analysis would further improve our understanding of the temporal evolution of magmatism and petrogenetic processes within the Sefwi Greenstone Belt. The expansion of the TerraneChron® program (Parra-Avila, 2015, and references therein) may facilitate wider scale isotopic mapping of the craton and to examine the possibility of variance in the age of the 2300-2200 Ma proto-crust being recycled in the WAC, as evidenced by the limited inherited zircon data. Alternatively, continued systematic, regional Lu-Hf, Sm-Nd and O isotope analysis may provide greater insight into the timing of mantle contribution and reworking of existing crust (e.g. Hawkesworth & Kemp, 2006). In addition, this may help better resolve questions regarding the mono-or polycyclic history of the Eburnean Orogeny by informing us about the true basement of the West African Craton.

Continued high-resolution mapping within the belt at a domain-scale may provide additional insight into deformation within each domain as well as information regarding the tectonic nature of domain boundaries. Such studies could be complemented by structural mapping and geochronological studies conducted across the width of the Sunyani-Comoé and Kumasi-Afema domains to better constrain the relative timing of volcano-sedimentary deposition and deformation of each domain, as well as potentially elucidating the stratigraphic and tectonic relationships with the Bui and Ashanti greenstone belts. Lu-Hf dating of garnet growth from throughout the Sefwi Belt would elucidate the timing of HP metamorphism and D1 deformation for better correlation between domains within the belt. Additional geochronological constraints on the timing of high-grade metamorphism and mineralisation in both the Sefwi Belt and Ashanti Belt would greatly improve our understanding of the relative timing of deformation between the two greenstone belts, and their spatial association during the Eburnean Orogeny.

It is recommended that future research focuses on continued high-resolution mapping of high-grade terranes, particularly along shear zones within central and south-western Ivory Coast, in order to investigate other potential sutures within the craton. This may be supplemented with isotopic studies to investigate other potential isotopic boundaries in order to better understand the crustal architecture of the West African Craton. Additional analysis of O-isotope systematics in zircons from the WAC would greatly complement Hf data by providing insight into the geodynamic setting of crust formation as well as information on the interaction of mantle-derived melts and supracrustal material exposed to near-surface processes, such as hydrothermal alteration [START_REF] Valley | Oxygen isotopes in zircon[END_REF]. Structural studies should be complemented by quantitative calculations of metamorphic P-T paths and direct, high-precision dating of deformation and metamorphism. There should be a degree of caution when considering correlating craton-wide magmatic and tectonic events, as this leads to an oversimplification of stratigraphic and tectonic relationships and a loss of detail, which is detrimental to the understanding of the growth and amalgamation of the craton. Furthermore, we suggest that the Eoeburnean vs Eburnean relationship be reassessed in a more detailed, spatial context in order to better understand lateral age gradients and tectono-thermal relationships between adjacent terranes.

Conclusion

The Palaeoproterozoic crust of the West African Craton provides a natural laboratory to explore the geodynamic and orogenic links between the Archaean and the Proterozoic, preserving a unique period of juvenile crustal growth in the global rock record. The magmatic, tectonic and metamorphic evolution of the Palaeoproterozoic West African Craton reflects a nascent horizontal plate tectonic regime, highlighting the progressive changes in the geochemical and metamorphic record. Within this thesis, I established a new magmatic framework for the Palaeoproterozoic Sefwi Greenstone Belt, which indicated juvenile crust production and collision of subduction-related intraoceanic arcs. Striking similarities exist between the petrogenetic processes and magmatic evolution of SW Ghana and Neoarchaean terranes as well as modern arc systems. The geochemical and isotopic record of the region reflects the gradual adoption of modern petrogenetic and crustal processes. This is supported by the tectonic and metamorphic history of the region, characterised by initial thrustrelated crustal thickening and generation of HP-MT metamorphic conditions which is interpreted to represent the collision of arc terranes. The subsequent tectonic exhumation of the lower crust is evidence of strengthening of the lithosphere, inconsistent with the proposed hot-orogen model for the early Proterozoic. The implication of this proposed geodynamic and tectonic evolution for the West African Craton informs our understanding of the global tectonic regime and secular cooling of the early Earth. (Shuttle Radar Topographic Mission, SRTM, 2000) with an overlay of a ternary image of gamma-ray signal from three channels, K, eTh, and eU, highlighting the partial coverage of radiometric data of the study area.

  Atmosphere and Environment, Monash University), le Prof. Mark Jessell (Centre for Exploration Targeting, University of Western Australia/ Géosciences Environnement Toulouse/ L'Institut de Recherche pour le développement), le Prof. Peter Betts (School of Earth, Atmosphere and Environment, Monash University), le Dr. Lenka Baratoux (GET/IRD) and le Dr. (HDR) Jérôme Ganne (GET/IRD). De proches collaborations ont pu être établie avec Sylvain Block (GET), Nicholas Thébaud (CET, UWA), Luis Parra-Avila (CET/UWA), Robin Armit (School of Earth, Atmosphere and Environment, Monash University) et Christopher Spencer (Earth Dynamics Research Group, Curtin University). Ce projet a été financé par AMIRA International et les commanditaires de l' industrie, y compris AusAid et le projet ARC Linkage LP110100667, qui a bien voulu soutenir le projet WAXI (P934A). Une part importante de la recherche a été financée par le School of Earth Atmosphere and Environment, Monash Research Initiative Fundà travers le Dr Laurent Ailleres et des contributions du Dr Robin Armit et du professeur Peter Betts, ainsi que par l'Institut de Recherche pour le Développement (IRD), à travers les fonds de recherche du Dr Jérôme Ganne. xv Chapitre III Résumé Ce chapitre se concentre sur les lithologies et les séquences de déformation préservées dans la ceinture de roches vertes du Sefwi et dans les domaines tectono-métamorphiques adjacents au sud-ouest du Ghana. Nous présentons de nouvelles cartes litho-structurales et métamorphiques de la zone d'étude, qui sont le produit de l'intégration de la cartographie sur le terrain, des données pétrophysiques obtenues sur des lithologies prélevées localement et des données géophysiques aéroportées obtenues à l'échelle régionale. L'étude structurale multi-échelle et l'intégration de plusieurs ensembles de données géologiques sur une région où les affleurements géologiques se font rares, permet de mieux appréhender l'histoire tectonique de ce craton. Nous considérons ici le style et l'histoire de la déformation dans le contexte des modèles tectoniques existants pour le craton ouest-africain et des modèles orogéniques proposés pour la Terre primitive, au regard de ceux opérant aujourd'hui. La pétrologie métamorphique, les estimations quantitatives du métamorphisme et celle de la dation U-Pb des monazites métamorphiques mentionnés dans le présent chapitre sont discutés en détail au chapitre 4. Ce chapitre est destiné à être soumis à la revue Precambrian Research début 2018. Cette étude présente des cartes litho-structurales et métamorphiques nouvellement interprétées sur la ceinture de roches vertes de Sefwi et des domaines volcano-sédimentaires d'âge Paléoprotérozoïque (2300-2070 Ma). Cette ceinture se situe dans la partie sud-ouest du Ghana, marquant la limite orientale du craton ouest-africain. L'utilisation intégrée des observations sur le terrain, la cartographie structurale et l'interprétation des données géophysiques aéroportées régionales fournissent de nouvelles informations sur l'architecture, la séquence de déformation, les relations de superposition tectonique et l'évolution tectonique de la région exposée à l'époque paléoprotérozoïque (2150-2070 Ma). Cette étude met en lumière des domaines tectoniquement juxtaposés, marqués par des grades métamorphiques contrastés, comprenant des roches métamorphisée dans le faciès des schistes verts et amphibolites, des roches volcaniques et volcanoclastiques mafiques à felsiques, des paragneiss de haut grade métamorphique et des ensembles volcano-sédimentaires faiblement métamorphisés. Ces roches para-et ortho-dérivés sont très largement recoupés par plusieurs générations de granitoïdes. La première déformation se caractérise par une foliation métamorphique omniprésente S1, parallèle à la stratification originelle des roches. Cette foliation précoce est présente sur l'ensemble de la zone d'étude. Elle est plan axial de plis isoclinaux mal préservés Le xvi raccourcissement D1, d'orientation NNO-SSE, est associée à un métamorphisme de haut grade impliquant de la fusion partielle au stade finaux de son évolution. Il est interprété comme lié à un événement d'épaississement de la croûte birimienne, générant des granites à deux micas qui se mettent en place entre 2092 et 2081 Ma, ce qui permet de fixer un âge minimum à cette déformation D1. Les structures prenant place lors de l'événement tectonique ultérieur (D2) sont caractérisées par un pendage moyen, orienté OSO-ENE à NE-SO et un plan de clivage S2 portant une minéralogie métamorphique. La nature transtensive de la déformation est révélée par la nature coaxiale des plis F2 dont les axes sont parallèles à celle de la linéation métamorphique L2, s'orientant ENE-OSO à NE-SO, légèrement oblique par rapport aux grandes zones de cisaillement observées régionalement. Au sein de ce régime transtensif, l'exhumation différentielle tardi-orogénique de la croûte moyenne et inférieure se produit le long des plans extensifs orientés NNE, associant des mouvements en transpression sénestre le long des zones de cisaillement d'échelle régionale orientés NE-SO. La chronologie de l'exhumation est contrainte par des âges métamorphiques U-Pb sur monazites situés autour de ca. 2073 Ma. Le raccourcissement tectonique E-O qui vient se surimposer à l'édifice structurale est interprété comme lié à une modification des conditions aux limites (régionales). Il est associé à la réactivation dextre des zones de cisaillement d'orientation NE-SO et au plissement des structures antérieures, présentes pourtant en position distale par rapport à ces zones de cisaillement majeures. Nous proposons que la limite nord-ouest de la ceinture de Sefwi représente une zone de collision entre d'une part le sud du Ghana et d'autre part, les parties centrales et nord-ouest du Ghana / Côte d'Ivoire. Ces deux domaines présentent des histoires tectoniques contrastées. En outre, nous suggérons que l'accrétion du craton ouest-africain soit le résultat d'une succession d'épisodes d'accrétion au sein de l'orogène éburnéenne, ce qui nous offre de nouvelles perspectives sur l'évolution des processus orogéniques au Paléoprotérozoïque.

  La province d'âge Paléoprotérozoïque du Craton ouest-africain fournit un laboratoire naturel pour explorer les liens géodynamiques et orogéniques entre l'Archéen et le Protérozoïque. Ce craton préserve en son coeur un épisode unique de croissance crustale, qui n'a pas d'équivalent xxi connu au niveau mondial. L'évolution magmatique, tectonique et métamorphique du Craton ouest-africain au Paléoprotérozoïque reflète un régime de Tectonique des Plaques naissant, où s'opère des changements géochimiques et métamorphiques progressifs et majeurs. Dans le cadre de cette thèse, j'ai établi un nouveau cadre magmatique pour la ceinture de roches vertes de Sefwi au Paléoprotérozoïque, mettant en lumière une production intense de croûte juvénile et la collision d'arcs intra-océaniques au sein d'un régime de subduction. Des similitudes frappantes existent entre les processus pétrogénétiques et l'évolution magmatique des terranes du sud-ouest du Ghana et ceux d'âge Néoarchéen ou de leurs homologues plus modernes, issus des systèmes d'arcs actuels. Les données géochimiques et isotopiques de la région reflètent l'adoption progressive des processus pétrogénétiques et crustaux modernes. Ceci est soutenu par l'histoire tectonique et métamorphique de la région, caractérisée par un épaississement crustal initial en contexte chevauchant puis la génération de conditions métamorphiques de HP-MT, le tout étant interprétée comme la conséquence d'une collision progressive entre terranes d'arc. L'exhumation tectonique ultérieure de la croûte inférieure est la preuve d'un durcissement de la lithosphère sous la province birimienne il y a 2 Ga. Ce comportement mécanique de la lithosphère apparaît incompatible avec le modèle « d'orogène chaud » proposé pour le Protérozoïque. L'implication de cette évolution géodynamique et tectonique proposée pour le craton ouest-africain améliore notre compréhension sur l'évolution du régime tectonique global et du refroidissement séculaire de la Terre primitive depuis 2.5 milliards d'années.

Fig. 2

 2 Fig. 2.1. Simplified geological map of the West African Craton, showing key geological provinces (modified after Ennih & Liégeois, 2008).

  2. What were the characteristics of Palaeoproterozoic deformation and metamorphism and what were the possible driving forces? Furthermore, what insight does this provide into the tectonic style of the Eburnean Orogeny and what are the implications for Palaeoproterozoic orogenesis?

Fig. 3 .

 3 Fig. 3.1: a) The southern portion of the West African Craton showing the study area in the southeast, (modified after Milési, et al., 2004), with inset showing location of Archaean and Palaeoproterozoic domains of the West African Craton, Dark and light green colours represent mafic volcanic rocks and sedimentary rocks intercalated with intermediate to felsic volcanoclastic rocks, respectively. Yellow units comprise unconformably overlying fluvio-deltaic sediments in faultbounded basins; b) Simplified map from this study showing dominant lithologies, domain names used within the text and major shear zones transecting the SW Ghana. CD: Chiraa Domain; HD: Hwidiem Domain; BGD: Bechem Granitoid Domain; KJD: Kukuom-Juaboso Domain; SWD: Sefwi-Wiawso Domain; SZ: Shear Zone.

  acquired by the Geological Survey Department of Ghana and GoldenStar Resources Pty Ltd between 1984 and 2005 and national Ivory Coast aeromagnetic data collected by Kenting Earth Sciences in 1976. The Ghanaian data is a composite grid of five individual surveys, the details and locations of which are shown in the supplementary information (Fig. A1SI). Four surveys have line spacings of 200-500m and flight altitudes of 70-80 m, depending on the individual survey. Data was collected along E-W flight lines with the data from the south-eastern-most survey (outside mapping area) collected along N-S flight lines. The national airborne magnetic data of Ivory Coast was collected along N-S flight

  minor intercalated felsic to intermediate volcanic rocks metamorphosed under sub-greenschist to greenschist facies conditions, with elevated metamorphic grade limited to narrow contact aureoles of intruding granitoids. It forms large proportions of the Sunyani-Comoé and Kumasi-Afema domains. Metamorphic assemblages included quartz-chlorite-white mica-feldspar in the Sunyani-Comoé Domain, and quartz-chlorite-actinolite-feldspar meta-greywackes and sericitic argillites in the Kumasi-Afema Domain. Airborne aeromagnetic data covering the metasedimentary rocks of the Kumasi-Afema Domain reveals a smooth low wavelength, low-to moderate amplitude magnetic signal (-690 to -605 nT), indicating homogeneous distribution of magnetite. Higher magnetic responses and dipole magnetic anomalies are observed proximal to shear zones, associated with localised magnetite formation during deformation. In contrast, the volcano-sedimentary schists and intermediate volcanic rocks in the Sunyani-Comoé Domain are characterised by a prominent bimodal magnetic signal. Smooth, wide and elongate, low-amplitude signals associated with larger exposures of nonmagnetic rocks (-816 to -684 nT) immediately north of Kenyase and then again north of Sunyani, whilst volcano-sedimentary schists along strike of intruding leucogranites often have a high magnetic response characterised by strong ENE-striking magnetic fabrics. These fabrics are characterised by high amplitude (-620 to -545 nT) anomalies with high frequency, which we attribute to the production of magnetite during prograde metamorphic reactions in granitoid aureoles, coupled with structurally controlled fluid flow. Measured magnetic susceptibility values are consistently low (0.02-1.30 x 10 -3 SI); however, we note that the surface exposures of this unit are often extensively weathered. The radiometric signal reveals K and Th enrichment relative to U in the Kumasi-Afema Domain; however, the airborne radiometric data indicates extensive regolith cover in the Sunyani-Comoé Domain. The volcano-sedimentary schists display pervasive chlorite-sericite-ankerite alteration in proximity to shear zones.

  3.2a).The north-western margin of the Sefwi Belt is defined by a ~250km long, curvilinear NEstriking magnetic lineament, known as Kenyase-Yamfo Shear Zone, which separates amphibolite facies tectono-metamorphic domains of the belt from low-grade metasedimentary packages of the Sunyani-Comoé Domain. The central, rhomboidal Kukuom-Juaboso Domain is characterised by a strong, E-W to ENE-WSW trending magnetic fabric, predominantly associated with metamorphosed volcaniclastic and greywacke sequences. Magnetic fabrics of the Kukuom-Juaboso Domain are truncated to the west and to the east along NNE-striking structures, interpreted as the domain boundaries. The northern margin of the Kukuom-Juaboso Domain is demarcated by the elongate, narrow, ENE-striking magnetic lineaments of the Hwidiem Shear Zone. Exposures within the shear zone comprise migmatitic meta-basalt and meta-diorites, which are associated with alternative low and moderate-to-high amplitude magnetic anomalies, indicating structurally controlled magnetite destruction and growth during deformation and metamorphism. To the north of the Hwidiem Shear Zone, the Hwidiem Domain is characterised by high, amplitude, high frequency, NE-striking magnetic fabrics intruded by multiple, low magnetic plutons.

Fig. 3 .

 3 Fig. 3.2: a) Composite image of the negative reduced to the equator (nRTE) residual magnetic intensity draped over the shaded first vertical derivative (1VD), comprising airborne magnetic surveys from SW Ghana and SW Ivory Coast (survey details in Fig. A1SI); b) Lithological and structural map of the Palaeoproterozoic rocks of SW Ghana. CD: Chiraa Domain; HD: Hwidiem Domain; BGD: Bechem Granitoid Domain; SWD: Sefwi-Wiawso Domain; SZ: Shear Zone.Shear zones are colour coded according to the last deformation event during which they were active. Darker colours of the same lithological unit represent more rocks with a higher magnetic response.

  sequences, display metamorphic banding dipping moderately (<60°) towards N330 to N350, bearing down-dip amphibole stretching lineations and top-to-the-SSE kinematic indicators (Fig. 3.3a). A rose diagram plotting all S 1 measurements for the Kukuom-Juaboso Domain displays two main trends, striking approximately E-W and N030, respectively (Fig. 3.3d). Amphibolite facies packages and large granitoid bodies of the Sefwi Belt are juxtaposed with the low-grade metasediments of the Sunyani-Comoé domain along the Kenyase-Yamfo Shear Zone; however, D 1 kinematic indicators are largely obscured by later deformation and poor exposure.

Fig. 3

 3 Fig. 3.3: (a, b, c) Representative D 1 structures in the study area showing key geometries and overprinting relationships (see Fig. 3.7 for outcrop locations). a) NNW-dipping gneissosity (S 1 ) in a meta-andesite (SB016) in the Kukuom-Juaboso Domain with foliation-parallel, coarsely crystalline, late-D 1 leucosomes; b) Illustration highlights asymmetry of S 1parallel leucosomes with steeply down-dip L 1 amphibole stretching lineations (not shown) indicating reverse top-to-the-

Fig. 3 .

 3 Fig. 3.4 Representative D 2 structures. a) Tight, SW plunging F 2 folds in a garnet-biotite paragneiss in the Chiraa domain (SB440), refolding pelitic and psammitic layers (S 0 ) and a bedding-parallel biotite foliation (S 1 ); b) Migmatitic orthopyroxene-free garnet-clinopyroxene meta-basalt (SB177) on the southern margin of the Chiraa Domain, displaying subparallel relationship between S 2 and transposed, refolded S 1 foliation and plagioclase-rich leucosomes; c) Elongate, megacrystic garnet and clinopyroxene porphyroblasts and plagioclase-rich leucosomes (SB177); d) Open, constrictional F 2 fold refolding S 1 gneissosity in a migmatitic meta-basalt (SB089) in the Hwidiem Shear Zone, displaying a prominent L 2 stretching lineation parallel to the F 2 fold axis; e) L>>S tectonite in the Ketesso Shear Zone displaying pervasive mineral elongation in a quartz dioritic garnet amphibolite; f) Southeast dipping S-C 2 fabrics in the footwall of the Kenyase-Yamfo Shear Zone, associated with steeply east plunge L 2 stretching lineations, indicating normal, left-lateral movement;

Fig. 3 .

 3 Fig. 3.5. Summary diagram of D 2 and D 3 structural data from domains within the study area. The first and third columns plot S 2 planes and L 2 and F 2 poles to highlight the coaxial, shallow-plunging relationship of L 2 and F 2 . The second and fourth columns highlight the relatively consistent expression of D 3 shortening throughout all domains.

  references therein), L>>S and L-tectonites are indicative of transtension and ductile deformation in the mid-to lower crust. D 2 deformation in the Sefwi-Wiawso, Sunyani-Comoé and Kumasi-Afema domains displays contrasting metamorphic grade and fold style. The NE-striking Kenyase-Yamfo Shear Zone separates the Sunyani-Comoé-Domain and amphibolite facies rocks of the Kukuom-Juaboso and Hwidiem domains. Foliations in the footwall of the shear zone dip moderately to the south (~48-64°) bearing east-pitching mineral stretching lineations (L 2 : 52° →N092) and normal, left-slip kinematic indicators

F

  2 folds. The truncation of ~E-W striking high amplitude magnetic fabrics in the domain defines the NNE-striking D 2 shear zone representing the western boundary of the domain. Along this shear zone, rocks of the Kukuom-Juaboso Domain are juxtaposed with the NE-striking, alternating short wavelength, high amplitude and smooth, low amplitude magnetic signals of volcano-sedimentary schists of the Sunyani-Comoé Domain. In the north of the field area, alternating NE-striking, high-ampitude and low amplutide magnetic signals of the Sunyani-Comoé Domain are trucated along the boundary of the patchy, weakly magnetic to strongly magnetic lithologies within the Chiraa Domain. Both S 2 foliation trajectories and magnetic anomalies preserved in rocks of the Chiraa Domain progressively rotate counter-clockwise in orientation from NE-striking in the north to E-W striking in the south. This rotation is mirrored by the magnetic fabric of the adjacent greenschist facies sediments of the Sunyani-Comoé Domain. The rotation of D 2 structures is consistent with drag folding associated with sinistral movement along the Kenyase-Yamfo Shear Zone. Additional kinematic indicators of sinistral movement are interpreted from the S-C magnetic fabric in the metamorphosed volcano-sedimentary rocks south of Kawtiego as well as the complex, δ-type shear sense indicated by magnetic units wrapping around a small pluton near Adzopé. In the south of the study area, truncation of early magnetic fabrics occurs along the Bibiani Shear Zone associated with normal movement during D 2 transtension. 3.6.1.3 D3 E-W directed shortening Early fabrics, folds and shear zones are overprinted by deformation during D 3 , characterised as a progressive E-W directed shortening event in a strike-slip setting. D 3 resulted in distributed accommodation of strain, later transitioning to simple shear-related deformation along narrow shear zones associated with a localised greenschist facies metamorphic overprint. The overprinting relationship between D 1/2 and D 3 is most clearly evident in internal areas of high-grade domains, with fabrics, shear zones and isograds oriented ~E-W, or approximately parallel to the D 3 shortening axis. In the Chiraa Domain, D 3 is characterised by sub-vertical, N-S striking crenulation cleavage with minor biotite recrystallization along crenulation axes, and open, gently N-plunging F 3 folds (Fig. 3.5). Microstructural evidence shows crenulation of kyanite-defined L 2 stretching lineations by the overprinting N-S S 3 foliation. The near orthogonal relationship between F 2 and F 3 fold axes has resulted in Type 1 fold interference patterns in both the Chiraa (Fig 3.6a) and Kukuom-Juaboso domains, distal to high-strain zones along domain boundaries. On the NNE-striking western boundary of the Chiraa Domain, earlier fabrics are transposed to a sub-vertical orientation during localised dextral strike-slip movement. The post-crystallisation dextral shearing of garnet-bearing leucosomes in migmatitic paragneisses (Fig. 3.6b), approximately 5km east of the domain boundary, indicate that D 3 deformation post-dates peak metamorphism. Overprinting relationships in the amphibolite facies, Kukuom-Juaboso domain are demonstrated by the refolding of sinistral drag folds along metre-scale D 2 shear zones by near-orthogonal NNE-striking upright F 3 folds (Fig. 3.6c).

Fig. 3 .

 3 Fig. 3.6: D 3 structures. a) Type-1 fold interference pattern in migmatitic paragneisses in the Chiraa Domain (SB404), illustrating the near orthogonal relationship of ENE-WSW oriented F 2 folds and ~N-S striking S 3 biotite foliations; b) Post-crystallisation dextral shearing of a garnet-bearing leucosome, sub-parallel to S 1 and S 2 foliations in a migmatitic paragneiss on the western margin of the Chiraa Domain (SB147); c) N-to NNE striking vertical S 3 foliation overprinting drag folds generated during D 2 sinistral strike-slip faulting in the southwestern regions of the Kukuom-Juaboso Domain (SB313); d) Prominently elongation of quartz clasts in Tarkwaian-type conglomerates and chlorite-white mica S-C 3 fabrics (SB418) indicating dextral shearing along the Ketesso Shear Zone.

  3.6.1.4 D4: N-S shortening -Brittle ductile deformation and NNE-SSW sinistral shear zonesD 4 is associated with NW-SE-directed shortening associated with brittle-ductile deformation. This deformation event did not significantly influence the macro-scale crustal architecture of the study area. D 4 generated late sinistral oblique reactivation of the NNE-SSW Bibiani and Chirano shear zones, deflected existing fabric and faults in the Kumasi-Afema Domain. Pseudotachylites in the northern areas of the Bibiani Shear Zone crosscut an earlier east-west mineral foliation in a meta dacite (SB051). Immediately west of the Chiraa Domain, NE-SW striking structures are truncated by a narrow N-S sinistral shear zone, with C 4 -S fabrics developed in a graphitic schist. In the aeromagnetic data, NNS striking D 4 faults, including the reactivated Bibiani Shear Zone, truncate the Ketesso Shear Zone immediately to the north, whilst a smaller N-S shear zone west of Chiraa truncates reactivated D3 shear zones in the Sunyani-Comoé Domain.

  3.6.1.5 D5 WNW-ESE shortening; Reidel shear zonesLate NW-SE, NE-SW and E-W oriented structures, which form conjugate fault sets developed during D5 WNW-ESE shortening. Dextral displacement commonly observed along E-W striking shear zones and sinistral movement long NW-SE striking shear zones. Similar late-stage deformation was documented in neighbouring eastern Ivory Coast

Fig. 3 .

 3 Fig. 3.7: Structural and metamorphic map of the study area displaying S 1 foliation trajectories, major shear zones and the locations of samples mentioned in the text. Major shear zones display most recent fault movement, with the exception of the western boundary of the Chiraa Domain and the central segment of the Kenyase-Yamfo Shear Zone where normal movement and juxtaposition of domains of contrasting metamorphic grade during D 2 are highlighted. S 3 foliation trajectories are extrapolated from both field observations and interpretation of geophysical datasets.

Figures 3 .

 3 Figures 3.7 and 3.8 display the trajectories of the S 1 foliations over the new tectono-metamorphic map of the study area and illustrates the distribution of metamorphic facies, including localised water-fluxed incongruent partial melting along the Hwidiem shear zone within the Sefwi Belt. This

Fig. 3 .

 3 Fig. 3.8. (a) Detailed structural-metamorphic maps of the Chiraa Domain and (b) the intersection of the Hwidiem Shear Zone and the Kenyase Shear Zone (see Fig. 3.6 for locations). Juxtaposition of amphibolite facies and migmatitic rocks with greenschist facies volcano-sedimentary sequences occurs along normal oblique D2 faults, which are reactivated and transposed during D3 deformation.

  .8b) and the NE-SW strike of the major shear zones. The obliquity of this relationship is characteristic of terranes deformed under a transtensional regime. -Sheared S 1 foliation trajectories form S-C fabrics which indicate sinistral movement along multiple D 2 shear zones in the Sunyani-Comoé Domain, often occurring along strike of elongated leucogranitic plutons between the townships of Chiraa and Kawtiego and toward the Ghanaian-Ivorian border (Fig. 3.7).-Rocks of the Kukuom-Juaboso domain are transected by and entrained in the Ketesso highstrain zone during D 3 . Upper amphibolite rocks are documented in the extension of the high strain zone on the Ivorian side.Hirdes, et al. (2007) suggest that the rocks represent a highgrade, deeper crustal equivalent of the Sunyani-Comoé domain metasediments whilst we contended that they form part of the Kukuom-Juaboso domain. E-W shortening during D 3 refolded S 1/2 foliation trajectories within the Kukuom-Juaboso Domain whilst transposing its NNE-striking western boundary to a sub-vertical orientation. Plots of S 3 measurements distal to shear zones indicate a relatively consistent N-S strike attributed to pure shear deformation, whilst simple shear-related deformation localised along major NE-SW-striking shear zones generating dextral S-C 3 fabrics (Fig.3.5).-Foliation trajectories on either side of the NNE-striking Bibiani Shear Zone display drag folds associated with sinistral shearing along the fault. Truncation of the Afema Shear Zone at the southern extent of the Bibiani Shear Zone suggests the latest reactivation post-dates D 3 shearing along the Afema Shear Zone.

Fig. 3 .

 3 Fig. 3.9. a) Block diagram illustrating the interpreted tectonic setting of greenstone belts prior to D 1 , highlighting the close spatial relationship between the Sefwi and Ashanti belts and an unknown distance between the Sunyani-Comoé Domain and the Sefwi Belt prior to D 1 collision; b) D 1 NNW-SSE shortening resulting in collision along the NW margin of the Sefwi belt, associated with folding and low angle thrust faults, crustal thickening and subsequent lower crustal anatexis; c (p. 95)) Initiation of D 2 ENE-WSW sinistral transtension. Figure displays the distribution of major lithological packages following the emplacement of leucogranites (ca. 2902 -2081Ma) parallel to the NW margin of the Sefwi Belt within the Chiraa and the Sunyani-Comoé Domains, generated during D 1 anatexis. D 2 transtension results in the formation of grabens at high angles to major shear zones, or as narrow dilational jogs within shear zones, hosting conglomeratic Tarkwa-like units (yellow unit). Constrictional F 2 fold axis (grey trajectories) and L 2 stretching lineations (black arrows) within the Kukuom-Juaboso Domain display a coaxial relationship pitching shallowly to the WSW or to the ENE in the Hwidiem Shear Zone and Hwidiem Domain; d (p. 95)) Metamorphic block diagram highlighting the differential exhumation of high-grade rocks of the Chiraa and Kukuom-Juaboso domains relative to greenschist facies rocks of the Sunyani-Comoé Domain along west dipping detachments and NE-striking sinistral oblique shear zones. Projected grey shear zones represent NE-SW striking structures transposed or reactivated during subsequent D 3 E-W shortening. D 3 deformation was progressively localised into these shear zones and was associated with a greenschist facies retrograde metamorphic overprint.

  The transition from a compressional to a tensional regime and transcurrent D 2 tectonism is constrained by monazite crystallisation ages hosted in both L 2 kyanite porphyroblasts and the matrix of a biotite-sillimanite magmatic gneiss, which both yielded SHRIMP U-Pb ages of ca. 2073 Ma. This age records the initial stages of cooling along the retrograde path and the initiation of ENE-WSW transtension-related exhumation during D 2 (Fig.3.9c,d). This is consistent with the findings ofJessell, et al. (2012) who concluded that transcurrent tectonism along the NW margin of the Sefwi Belt in the Sunyani-Comoé domain post-dates the emplacement of the Apouasso leucogranite at ca. 2081 Ma.Whilst Jessell, et al. (2012) suggest that the elongation of the leucogranites occurred during dextral shearing, the combined evidence of constriction related structures and the macro-geometries of juxtaposed metamorphic domains (Fig.3.9d), we conclude that D 2 deformation drove oblique sinistral reactivation of major D 1 tectonic boundaries, in agreement with sinistral D 2 deformation interpreted byFeybesse, et al. (2006),and sinistral kinematic indicators observed along strike in southeast Ivory

  feeding into discordant veins and dykes cross-cutting S 1 gneissosity indicate partial melting occurred at mid-to lower crustal depths towards the end of D 1 deformation and the transition to D 2 . The complex foliation and lineation structural pattern associated with D 2 deformation indicates a switch in the dominant strain axes and a change in the tectonic mode. Field observations from throughout high-grade domains of the Sefwi Belt are indicative of a transtensional regime. These include shallowpitching, coaxial F 2 folds and L 2 stretching lineations oriented ~ENE-WSW, indicative of transtensional

  2130 and 2110 Ma, forming a cohesive block. Anatexis and high-grade metamorphism in NW Ghana commence ~40 million years prior to the genesis of collision-related leucogranites along the NW margin of the Sefwi Belt after ca. 2092 Ma, indicating a diachronous metamorphic history for the high-grade terranes exposed in SW Ghana. Furthermore, the exhumation of middle and lower crust documented in both NW Ghana and the study area occur more than 40 million years apart. Such large discrepancies in the timing of deformation and metamorphism between discrete greenstone belts in SW Ghana relative to greenstone-granite-gneiss terranes of NW Ghana and SW Burkina Faso is therefore interpreted as the product of episodic collisional orogenesis between independent juvenile Palaeoproterozoic terranes. The proposed collisional D 1 event associated with NNW-SSE shortening in SW Ghana potentially corresponds with the post-2110 Ma late sinistral shearing along the N-S

Fig. 4

 4 Fig. 4.1. Geological map of theLeo-Man rise (modified after BRGM SIGAfrique, Milési, et al., 2004). Mafic to intermediate volcanic greenstone belts (dark green); volcano-sedimentary, volcaniclastic and intermediate volcanic provinces (light green); Granitoid composition ranges from tonalite-trondhjemite-granodiorite and granite with minor mafic intrusions.Late fluvio-deltaic sediments (yellow) unconformably overlying greenstone belts in narrow, fault bounded basins.

Fig. 4

 4 Fig. 4.2. Regional litho-structural map of the study area (Chapter 4: this study). The Sunyani-Comoé and Kumasi-Afema domains are characterised by isoclinally folded, sub-greenschist to greenschist facies metasedimentary and volcano-sedimentary rocks with rare intercalated intermediate volcanic layers. The Chiraa domain (CD) in the northeast comprises poly-deformed paragneisses and voluminous granite intrusions. Lithologies exposed in the Kukuom-Juaboso domain (KJD) are diverse, consisting of metamorphosed mafic to intermediate volcanic, volcaniclastic and minor sedimentary units and multiple generations of felsic and mafic granitoid intrusions. BD: Bechem domain; SWD: Sefwi-Wiawso domain; HD: Hwidiem domain; HSZ: Hwidiem shear zone. Fault kinematics denoted only where unambiguous,

  1.1 Sample SB398 (garnet-kyanite-rutile migmatitic paragneiss)Sample SB398 is a locally migmatitic paragneiss from the western part of the Chiraa Domain. The outcrop comprised interbedded, gneissic metagreywackes and rare aluminous metapelitic layers.These rocks are intruded by centimetre-to metre-scale granitic dykes. Abundant and variable garnet and kyanite modal proportions in Al-rich layers indicate compositional heterogeneity in the protolith(Fig. 4.3a,b). The primary structures include a penetrative gneissosity (S 1 ) parallel to compositional layering defined predominantly by biotite alternating with quartz-feldspar layers. S 1 is folded around tight, symmetric metre-scale F 2 folds plunging moderately towards west-northwest to southwest. At the outcrop from which sample SB398 was derived (Fig.4.3b), kyanite defines a prominent L 2 mineral stretching lineation, pitching ~30° to the west.

Fig. 4 . 3 .

 43 Fig. 4.3. Outcrop photos of SB398. a) Outcrop photo of SB398 showing refolded pelitic and psammitic layers and parallel biotite schistosity, with parallel to cross-cutting leucosomes and leucocratic veins; b) Aluminium-rich pelitic layer containing high modal proportions of kyanite and garnet with increasing biotite content proximal to stromatic leucomsones.

  aggregates of anhedral plagioclase (Pl1), euhedral rutile, radial chlorite and accessory apatite, zircon and graphite (Fig 4.4b), with abundant, small (<10 μm) quartz inclusions. Chl has a #Mg (Mg/(Mg + Fe)) ranging from 0.29 to 0.53 with a negligible Ti content. Anhedral plagioclase inclusions (Pl1) in the core of garnet (Fig. 4.4a, b) have An content (=100 x (Ca/(Ca + Na)) of 32-40. Assemblage (A), defined as Grt + Chl + Pl1 + Rt + Qz ± Bt , is consistent with greenschist to low-T amphibolite conditions. Pluri-mineral inclusions in the garnet mantle comprise euhedral biotite1 and phengite1 laths and coarse-grained, subhedral to euhedral plagioclase2, with prismatic to elongate, single grain rutile inclusions (Fig 4.4c, d). Biotite1 has #Mg of 50 -52 and Ti vi = 0.11-0.15 a.p.f.u. (based on 11 oxygen).

Fig. 4

 4 Fig. 4.5. Chemical composition profiles of transects across garnet porphyroblasts in samples (a) SB398, (b) SB389, (c) SB344 and (d)SB177. Profiles show location of garnet cores (c) and rims (r).

  Overgrowths of muscovite (Ms) haveMs = 89-91, and lower Si = 3.06-3.08 a.p.f.u. and Al vi = 1.89- 1.91 a.p.f.u. compared to Ph2 in the matrix. Low modal proportion (<1%) of late ilmenite in muscovite cleavage planes (Fig. 4.4f) indicates the rock crossed from the stability field of rutile into the stability field of ilmenite during retrogression. Garnet chemical profiles (Fig. 4.5a) show downward inflection in #Mg, coinciding with a decrease Prp and increased Alm and Sps content (Prp12, Alm78, Sps4)

  Fig. 4.6. Field photos of biotite-sillimanite sample SB389. a) Outcrop showing strongly deformed meta-pelitic and metapsammitic layers, which sheared bedding parallel leucosomes and granititc dykes and leucosomes feeding into high-

Fig. 4

 4 Fig. 4.7. Photomicrographs of migmatitic metapelite sample SB389. (a) Early, rounded, anhedral garnet porphyroblast in contact with muscovite, quartz and plagioclase. Aligned biotite defines penetrative metamorphic fabric. (b) Isolated corroded kyanite porphyroblast with thick overgrowth of muscovite with the square indicating inset (c). (c) A cuspate film of plagioclase pseudomorphing melt between corroded kyanite and rounded, anhedral quartz. Biotite2 visible as inclusion in muscovite2. (d) BSE image of randomly aligned, fine grained, fibrolitic sillimanite in sillimanite nodule. (e)Fibrolitic sillimanite is observed in two morphologies; the first, as coarse-grained knots suggesting it is a retrograde phase, pseudomorphing pre-existing porphyroblasts; the second textural location of fibrolitic sillimanite is sub-parallel to the S 2 foliation. Square in sillimanite nodule shows location of (d), not to scale.

  for high-grade pelites. R4 and R5 are responsible for the formation of the dominant matrix assemblage (C), Sil + Bt + Pl + Qz ± Ms ± Grt. Elongate, fibrolitic sillimanite is also observed sub-parallel to the subhedral, elongate, highly-pleochroic biotite (#Mg 42-45 and Ti vi = 0.10-0.15 a.p.f.u) and muscovite foliation suggesting metamorphism is concurrent with deformation (Fig.4.7d,e), representing the biotite-producing end of R5. Matrix plagioclase has An 27-29. This assemblage is often seen overprinting relict kyanite grains and dark brown, highly corroded biotite grains with round quartz inclusions, representing the early biotite consumption of R5. Despite differences in morphology and microstructural context, the two types of biotite have indistinguishable chemical composition.Late oxidation of the rock is indicated by an overprint of matrix biotite by an intergrowth of muscovite (Ms2) and hematite. The oxidation of biotite and aluminosilicate produces muscovite and hematite, modelled by the following balanced chemical equation: 4Bt + 4Sil + O 2 = 4Ms + 6Hem + 4Qz. Ms2 composition displays lower Si content than from Ms1, with a composition of Ms = 93-96, Si = 3.03-3.05 a.p.f.u. and Al vi = 1.71-1.79 a.p.f.u..

  4.8a). The obliquity between the two foliations suggests garnet growth is inter-tectonic(Fig. 4.8b). Measured garnet composition shows slight zoning from core to rim with Prp6 → Prp7, Grs26 → Grs27, Sps10 → Sps4 and Alm58 → Alm62 (Fig.4.5c). Plagioclase in the matrix has a composition of An 15-21, with one transect revealing a higher An content (An24-30). Matrix amphibole is classified as ferropargasite (Fprg) with #Mg = 39-41, Ca/Na = 2.73-3.11, (Ca+Na) B = 1.79-1.89 a.p.f.u., (Na+K) A = 0.56-0.68 a.p.f.u. and Ti vi = 0.09 a.p.f.u.. The structural formula of ferropargasite was calculated according to the IMA 2012 recommendation, with the use of the excel spreadsheet compiled by[START_REF] Locock | An Excel spreadsheet to classify chemical analyses of amphiboles following the IMA 2012 recommendations[END_REF]. Ilmenite in the matrix is commonly aligned with the foliation and as inclusions in ferropargasite. Minor epidote is observed in the matrix. Assemblage (A) (Grt + Pl + Fprg + Ilm + Ep + Qz) represents the peak assemblage, corresponding with amphibolite facies conditions. Weak retrogression and secondary hydration is indicated by chlorite overgrowth, localised along foliation planes and thin replacement rims on garnet grain boundaries and internal fractures. Incomplete replacement of ilmenite by titanite is confined to S 2 foliation planes, with rounded ilmenite inclusions observed in foliation parallel titanite grains, suggesting a transition from the stability field of ilmenite to titanite (Fig.4.8b). These phases define assemblage (B) Grt + Pl + Fprg + Ep + Chl + Ttn, indicative of high-P greenschist facies conditions.

Fig. 4

 4 Fig. 4.8. Photomicrographs of metamorphosed mafic rocks. (a) Subhedral garnet porphyroblasts with inclusion trails of ilmenite and quartz, defining early metamorphic foliation (S 1 ), hosted in a matrix of aligned, elongate ferropargasite, plagioclase and coarse-grained ilmenite. Strain shadows of garnet are filled with quartz. (b) Retrograde chlorite in along fractures in garnet porphyroblasts and overgrowth of rounded titanite around ilmenite along S 2/3 foliation plane. (c) Polymineralic inclusions of cummingtonite, magnesio-hornblende, quartz, plagioclase and ilmenite in garnet porphyroblasts in amphibolite sample SB164. (d) Early foliation preserved in garnet porphyroblasts at an oblique angle to the penetrative metamorphic foliation (S 2 ) in the matrix defined by elongate hornblende and plagioclase, seen wrapping around garnet porphyroblasts. (e) Pluri-mineralic inclusions in garnet of euhedral plagioclase1, subhedral hornblende1 and ilmenite and isolated inclusions of anhedral clinopyroxene1. (f) Peritectic garnet and clinopyroxene2 hosted in plagioclase-rich leucosome (Pl2) with titanite, ilmenite and clinopyroxene along leucosomes selvedges. A thin rim of hornblende2 and actinolite on the garnet porphyroblast comprise the early retrograde assemblage.

5

  km east of Chiraa (Fig.4.2). The outcrop SB177 contains large blocks of megacrystic metamorphic rocks preserving high-grade garnet-clinopyroxene-bearing metamorphic assemblages. The tectonic fabric strikes east-west, dipping steeply to the south with a gently west-plunging mineral stretching lineation. Leucosomes occur as discrete, disconnected patches, often pooling in fold hinges suggesting syn-tectonic partial melting and minimal melt segregation and loss. Peritectic garnet porphyroblasts are subhedral with irregular, embayed grain boundaries, often occurring in contact with peritectic clinopyroxene porphyroblasts. Garnets are heavily fractured. Three metamorphic assemblages have been deduced from petrographic relationships and mineral compositions. Chemical profiles of garnets show strong zonation in grossular and almandine content(Fig. 4.5d). Changes in garnet composition from core (Grt1) to rim (Grt2) are Grs44-46→ Grs 39-43, Alm49-50 → Alm51-54, Prp3-4 → Prp 5-6 and Sps1-2 → Sps0-1(Fig. 4.5d). Pluri-mineral aggregates and singular inclusions of titanite and ilmenite, and plagioclase and hornblende constrain an early, amphibolite facies metamorphic assemblage (Fig. 4.8e), defined by (A) = Grt1 + Hbl1 + Pl1 + Ilm + Ttn + Mag ± Cpx ± Ep. Pl1 has An41-43 and Hbl1 has #Mg = 28-34, Ca/Na = 4-6, (Ca+Na) B = 1.89-1.93 a.p.f.u. and (Na+K) A = 0.51-0.61 a.p.f.u. Clinopyroxene in all textural locations as classified as Ca-Mg-Mg (Quad) clinopyroxenes. Clinopyroxene inclusions in garnet display an intermediate diopside-hedenburgite composition with #Mg = 0.44-0.49, Ca/(Ca + Na) = 0.97-0.99, high wollastonite content of Wo(100 *Ca/(Ca+Mg+Fe)) = 47-49, enstatite (En) (100*Mg/ (Mg+Ca+Fe)) = 22-26 and ferrosillite (Fs) (100*Fe T /(Fe T +Ca+Mg)) = 24-28, after Morimoto et al. (1988). Metamorphic assemblage (B) records the transition to suprasolidus conditions under the upper amphibolite to granulite conditions. It comprises Grt2 + Cpx + Pl2 + Qz + Ttn + Ilm ± Czo + melt (L) (Fig. 4.8f). Garnet and clinopyroxene porphyroblasts are hosted in granoblastic plagioclase-rich leucosomes, where grain boundaries have visible 120 o angles. Plagioclase in leucosomes is classified as Pl2, which has higher An content (62-71) than Pl1 inclusions in garnet. Clinopyroxene porphyroblasts hosted in leucocratic domains have #Mg 50-55, Ca/(Ca + Na) = 0.97-0.99, Wo= 49-51, En = 26-27 and Fs = 22-25. Clinopyroxene selvedges grow in equilibrium with both titanite and ilmenite on leucosomes margins (Fig. 4.8f). Early garnet and clinopyroxene growth are interpreted as the product of progressive dehydration of hornblende defined by the following reaction (R5), typical of transitional amphibolite metamorphism:

  Fig. 4.9. (a) Leucosome with hornblende selvedges in migmatitic amphibolite gneiss located along the Hwidiem shear

  SB398 (garnet -kyanite -bearing paragneiss) P-T pseudosections for paragneiss sample SB398 (Fig.4.10) show stability fields of observed assemblages and mineral stability fields derived from measured mineral compositions. Observed pluri-mineralic inclusions of anhedral plagioclase, radial chlorite and euhedral rutile in garnet porphyroblasts cores potentially partially record an early prograde assemblage (A) at temperatures below the calculated Chl-out-Grt-in line at 590 °C (Fig.4.10a) and pressures above 10 kbar as indicated by the inclusion of rutile. Subsolidus high-pressure amphibolite conditions of 9.9-11.5 kbar, 590-640 °C are deduced from measured garnet composition (Prp13-15, Alm75-77, Grs5-6) and biotite inclusion composition (Bt1 Mg# = 50-52), corresponding to the calculated stability field of Grt + Bt + Pg + Ms + Rt + Qz. This assemblage partially resembles that of assemblage (B), Grt + Ph1 + Bt1 + Pl2 + Rt + Qz ± L. The inclusions of euhedral plagioclase crystals in the mantle of garnet porphyroblasts potentially suggesting the assemblage is metastable as it first crossed the solidus. Paragonite (Pg) was not preserved as inclusions in garnet, nor in the matrix, potenitially indicating a preservation issue. The transition from assemblage (B) to assemblage (C) records heating concomitant with < 2 kbar of exhumation indicating suprasolidus upper amphibolite facies conditions of 690 °C at 9.5 kbar within the stability field of kyanite. The calculated stability field associated with these conditions has the assemblage Grt + Bt + Ky + Pl + Ms + L ± Rt, corresponding well with the observed assemblage (C) Grt + Bt2 + Ky + Pl2 + Qz + Rt ± Ms ± L. At the calculated suprasolidus conditions, calculated plagioclase composition is consistent with that of measured euhedral garnet inclusions of plagioclase (Pl2) and matrix plagioclase (Pl3) have anorthite content(30)(31)(32)(33)(34)(35)(36), corresponding also to 4-6 % volume of kyanite (Fig.4.10c).Calculated conditions and observations are consistent with estimations for partial melting of similar pelitic lithologies in the Caledonides by[START_REF] Kalsbeek | Geochemistry and petrogenesis of S-type granites in the East Greenland Caledonides[END_REF] of ~ 750 °C and 10-12 kbar. Monazite is observed as inclusions in matrix biotite and in kyanite porphyroblasts (Fig.4.4e, f) and is interpreted to have crystallised during peak temperature metamorphism.

Fig. 4 .

 4 Fig. 4.10. MnNCKFMASHTi P-T pseudosections for phase equilibria for metapelite sample SB398. (a) Calculated stable assemblages. (b) Calculated garnet isopleths for pyrope (dotted lines and values in circles) and almandine (dashed lines and values in squares). Colour spectrum represents garnet volume. (c) The colour spectrum shows modal proportions of aluminosilicates with anorthite content of plagioclase in dashed lines with values in circles. (d) Isopleths of Bt #Mg in dashed line with value in squares.

Fig. 4 .

 4 Fig. 4.11. MnNCKFMASHTi P-T pseudosections for phase equilibria for metapelite sample SB389. (a) Calculated stable assemblages. (b) Calculated garnet isopleths for pyrope (dotted lines and values in circles) and grossular (dashed lines and values in squares). Colour spectrum represents garnet volume. (c) The colour spectrum shows modal proportions of aluminosilicates with anorthite content of plagioclase in dashed lines with values in circles. (d) Isopleths of Bt #Mg in dashed line with value in squares presented with Ti content of biotite (grey dotted line and values in circles).

Fig. 4 .

 4 Fig. 4.13. Results of U-Pb dating of samples SB398 and SB389. (a, b, d) Back-scattered electron images of monazites showing textural context. (a) Deformed monazite in kyanite porphyroblast in sample SB398. (b) SHRIMP analysis points

  4.13d). Back-scattered electron images show rare patchy compositional zoning and large fractures. Monazites in sample SB389 show two distinct morphologies. One population is subhedral to euhedral coarse grains (up to 200 µm along the c-axis). These occur in either the matrix or biotite porphyroblasts. The other population are elongate, sub-rounded grains, often with embayed grain boundaries. They are often hosted within large fibrolitic sillimanite pseudomorphs (Fig. 4.13e, 4.13f). Six U-Th-Pb analyses were performed on five grains, representative of the two morphologies, yielding an upper intercept age of 2073 ± 6 Ma (MSWD = 0.18, P = 0.95) and a weighted mean 207 Pb/ 206 Pb age of 2074 ± 3 Ma (MSWD = 0.18, P = 0.97) (Table

6 .

 6 Overprinting M2 metamorphism, M3 metamorphism is characterised by parallel retrograde P-T paths of samples located in the Chiraa high-grade domain (SB398 and SB389), which reach average amphibolite facies conditions of 5.5-7.0 kbar and 560-620 °C (SB398: Fig. 4.10c, d; SB389: Fig. 4.11c, d). This is interpreted as exhumation of the entire domain during M3. Neither P-T paths nor petrographic relationships in these samples indicate isothermal decompression, instead, they follow a moderately steep gradient with decreasing pressures and temperatures. Preservation of these assemblages suggests minimal fluid interaction in the later stages of the metamorphic history.

Fig. 4 .

 4 Fig. 4.14. a) Metamorphic map of the Sefwi Greenstone belt and adjacent metasedimentary provinces with calculated P-T paths. The migmatitic Chiraa domain is located in the northeast, separated from the amphibolite facies to locally migmatitic domains of the central portions of the Sefwi belt by the reactivated Kenyase-Yamfo fault. The greenschist facies volcanoplutonic Sefwi-Wiawso domain represents the southern portion of the belt, tectonically bounded by the reactivated Ketesso shear zone to the north and the Afema shear zone to the south. Fault kinematics indicate latest movement with the exception of the sinistral strike-slip component of D 2 inferred from structural and metamorphic relationships; b, c, d) Synthesis of metamorphic paths of high-metamorphic rocks exposed in SW Ghana with geothermal gradients and the aluminosilicate triple junction as a point of reference, revealing an initial apparent geothermal gradient of 15 -17 °C km -1 recorded by samples in multiple tectono-metamorphic domains. Subsequent partial melting is associated with sustained high pressures as the rocks crossed over the solidus. Note the distinct difference between the geothermal gradients recorded by retrograde paths between the anatectic samples (SB389; SB398) and the garnet amphibolite (SB344).

7.

  Southwest of the Chiraa domain in the Kukuom-Juaboso domain, the garnet amphibolite sample SB344 of point 2 above shows a different metamorphic history after the highpressure metamorphism of M1. It does not record an increase in temperatures after peak temperatures. It instead records cooling during exhumation, associated with an overprinting high-pressure greenschist facies metamorphism (6.5-9 kbar, 450-500°C) (Fig.4.12c). This is interpreted as a divergence in the metamorphic histories of the two terranes, occurring at the M2/M3 transition.8. The latest recognisable metamorphic overprint is interpreted as M4. Continued retrogression to greenschist facies conditions is localized along tectonically-reactivated high-strain zones and domain boundaries(Fig. 4.8f; Fig. 4.14). On the southern margin of the Chiraa domain, the peak assemblage of mafic granulite sample SB177 is overprinted by an assemblage typical of lower greenschist facies (Chl + Ab + Cal + Ep + Qz).

Figure 4 .

 4 Figure4.14 presents a synthesis of the P-T paths and P-T conditions recorded by high-grade rocks in SW Ghana. In summary, bringing together all the available metamorphic data derived from these samples, we suggest that the history of metamorphism for the belt is: an initial crustal thickening event (D 1 ), responsible for the burial of supracrustal rocks and the M1e and M1 assemblages; subsequent heating during D 2 is only recorded in the Chiraa high-grade domain with western samples indicating heating during exhumation and eastern samples indicating heating during burial, demonstrated by the M2 phase assemblages; exhumation during cooling occurs in both the Chiraa and the Kukuom-Juaboso domain during D 3 , however, this occurs under contrasting apparent geothermal gradients between the domains suggesting heterogeneous metamorphic histories exist within the study area. The tectonic juxtaposition of the Chiraa and Kukuom-Juaboso domain with the adjacent, greenschist-facies, chlorite-white mica schists of the Sunyani-Comoé domain to the west is interpreted as the product of exhumation during D 3 .

  (locations in Fig. 4.12), along strike of the Chiraa domain. The timing of peak to post-peak metamorphism in the Chiraa high-grade domain, as constrained by U-Pb monazite ages, suggests temperatures in the Chiraa domain remained above monazite closure temperatures until ca. 2073 Ma. As the Chiraa domain and the Sunyani-Comoé domain are now exposed at the same crustal level, it is reasoned that tectonic juxtaposition by differential exhumation post-dates both monazite growth in the Chiraa domain and the along-strike emplacement of leucogranites in low-grade metasediments of the Sunyani-Comoé domain.

Fig. 4 .

 4 Fig. 4.15. a) Metamorphic map with foliations trajectories of the Chiraa high-grade domain showing rotation of the S1 and S2 foliations within the domain, and subsequent overprinting and transposition

  vertical shear zones along domain boundaries are attributed D 3 E-W bulk shortening. Pure shearrelated structures forming under E-W shortening are evident in anisotropic units striking obliquely to the major NE-oriented crustal architecture. A penetrative, N-S striking, crenulation cleavage and upright, open F 3 folds overprint S 1 and S 2, decreasing in intensity towards the south. Retrograde greenschist facies metamorphism is localised along shear zones, defined by chlorite-white mica S-C or S-C' fabrics.

  petrological and geochemical analysis of representative magmatic suites from throughout the Sefwi Greenstone Belt and the Sunyani-Comoé Domain. Major and trace element data analysis investigates potential magma sources and petrogenetic processes responsible for the Palaeoproterozoic crust formation. Zircon U-Pb and Lu-Hf analyses constrain the timing of magmatism, as well as providing us with information on the timing of mantle extraction, crustal residence times, and the nature of the proto-crust. The timing of magmatism and εHf (t) values are compared to other areas in the West African Craton in order to investigate potential isotopic boundaries within the craton, providing crucial information on the craton architecture. Finally, we consider the broader geodynamic setting responsible for the formation and preservation of the West African Craton relative to Archaean and Phanerozoic geodynamic models.

Fig. 5

 5 Fig. 5.1. Simplified map of the West African Craton showing the Archaean Kénéma-Man domain and the Palaeoproterozoic Baoulé-Mossi domain, comprising volcanic greenstone belts (dark green), meta-sedimentary basins (light green) and large granitoid domains (orange). Yellow areas represent late fluvio-deltaic sequences of the Tarkwa Group (modified after BRGM SIGAfrique map,Milési, et al., 2004).

Fig. 5

 5 Fig. 5.2: Litho-structural map of the study area showing the location of samples discussed in this study and existing published geochronological data. CD = Chiraa Domain; HD = Hwidiem Domain; KJD = Kukuom-Juaboso Domain; SWD = Sefwi-Wiawso Domain; SZ = shear zone. Zircon age: (1)Hirdes, et al. (2007); (2):Adadey, et al. (2009);(3) Hirdes, et al. (1992); (4): Petersson, et al. (2017); (5)Hirdes and Davis (1998);[START_REF]= 0.01-0.22), associated with moderately negative Rb, weakly positive Ba and mixed Sr anomalies (Sr = 94-460 ppm). Furthermore, only pyroxenites and gabbros samples display positive V and Sc anomalies with[END_REF]: Petersson, et al. (2016); (7): Chapter 4this study; (8): Agyei Duodu, et al. (2009).

Fig. 5 . 3 .

 53 Fig. 5.3. Representative samples: a) Medium-grained foliated biotite-hornblende trondhjemite; b) Typical trondhjemite assemblage including zoned, euhedral plagioclase (Pl) phenocrysts, aligned biotite (Bt), quartz (Qz) and magmatic, subhedral epidote (Ep); c) Outcrop of trondhjemite in Sefwi-Wiawso domain with mafic enclave containing retrograde chlorite and actinolite; d) Hand sample of mesocratic quartz diorite; e) Outcrop of coarse-grained potassic biotitehornblende monzonite; f) Petrographic assemblage of potassic biotite-hornblende monazite with euhedral plagioclase displaying sericite alteration, interstitial microcline (Mc) and isolated, small hornblende (Hbl) and biotite grains; g)

  , distinguishing tonalite, trondhjemite, granodiorite and granite rocks based on normative CIPW norms, specifically normative feldspar composition. Six samples plot in the trondhjemite field, six in the tonalite field and three in the granite field (Fig. 5.5b). Interestingly, no samples plot within the granodiorite field. Quartz-diorites, diorites and most granites plot along the calc-alkaline differentiation trend in the Na-K-Ca ternary diagram. The more sodic trondhjemite and tonalite samples, as well as the quartz monzonite, plot in the Archaean trondhjemite field (Fig. 5.5c). In the Harker diagrams (Fig. 5.5a), the SiO 2 vs. K 2 O plot shows that the trondhjemite plutons display both tholeiitic (TH) and calc-alkaline (CA) affinities, whilst quartz-diorites and diorites plot primarily in the CA series, with one sample falling in the high-potassium calc-alkaline (HKCA) series. The quartz monzonite and granite samples all plot in the HKCA series. Overall, oxide and silica relationships in the Harker diagrams show approximately linear differentiation trends, with increasing SiO 2 associated with decreasing MgO, CaO, TiO 2 and FeO. Poor correlation, however, is observed between Al 2 O 3 , Na 2 O and K 2 O in relation to increasing SiO 2 , consistent with sericite alteration.

Fig. 5

 5 Fig. 5.5. Major element geochemistry of plutonic rocks of the study area, presented in (a) Harker diagrams (excluding

  dacitic samples plot close to the calc-alkaline field boundary, indicating more Fe-rich compositions. Meta-basalt sample SB164 displays positive Pb and Sr anomalies, a negative Zr anomaly, and a relatively flat trace element spectra. Both meta-basalt and basaltic andesite samples have similar REE patterns characterised by moderate LREE (La N = 16.88-42.19) and HREE-Y (Yb N = 10.87-11.49) contents, and low REE fractionation (La N /Yb N = 1.47 -3.88). The basalt-andesite displays weak LILE enrichment with weakly positive Ba, U and La anomalies, as well as a positive Pb anomaly. Overall the group shows very weak LILE enrichment (Rb N = 0.21-1.48; Rb N /La N = 0.01-0.04) and neither sample displays a Eu anomaly (Eu N /Eu* = 1.02 -1.11). Dacites display more fractionated REE patterns, with moderate to high LREE content (La N = 37.55-110.97) and moderate to high HREE -Y content (Yb N = 2.98 -26.58), and La N /Yb N = 2.86-30.15. Dacites display LILE enrichment (Rb N = 17.34 -53.91; Rb N /La N = 0.22-1.02), strong negative HFSE anomalies (Nb, Ta, Ti) and no Sr anomalies, with the exception of a plagioclase-phyric dacite sample (SB464B), which shows a strong positive Sr anomaly. Foliated dacitic volcanoclastic sample SB090 shows unique trace element patterns, displaying a distinct positive Ce anomaly and negative Y anomaly. Eu anomalies in dacite samples are slightly negative to neutral (Eu N /Eu* = 0.88 -1.05). Rhyolitic samples display heterogeneous trace element patterns in multi-element diagrams, although each have negative Nb, Tb and Ti anomalies and negative Eu anomalies (Eu N /Eu* = 0.52-0.85). REE spectra for each of the rhyolites show similarly moderate REE fractionation patterns (La N /Yb N = 2.91-7.94), however, quartz-feldspar-phyric rhyolite sample (SB076) contrasts all other felsic lavas. It displays extreme LILE enrichment (Rb N = 35.22; Rb N /La N = 5.56) with strong positive Pb, Ba, K and Sr (1090 ppm) anomalies, depletion in MREE and a weakly negative Eu anomalies (EuN/Eu* = 0.85) and a REE pattern less than an order of magnitude greater than chondrite values (Yb N = 2.17).

Fig. 5

 5 Fig. 5.7. (a) TAS diagram[START_REF] Le Maitre | A classification of igneous rocks and glossary of terms: Recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks[END_REF] displaying volcanic samples from across the field area, ranging frombasalt (circle) to basaltic andesite (diamond) to dacite (triangle) and rhyolite (square). (P-B) Picro-basalt; (B-A) Basaltic andesite; (A) Andesite; (T-A) Trachyandesite; (B-T-A) Basaltic trachyandesite; (T-B) Trachybasalt; (T/B) Tephrite/ Basanite; (P-T) Phonotephrite; (T-P) Tephriphonolite. (b) AFM ternary diagram[START_REF] Irvine | A guide to the chemical classification of the common volcanic rocks[END_REF][START_REF] Kuno | Basalts: The Poldervaart treatise on rocks of basaltic composition[END_REF]

  Fig. 5.8. Cathodoluminescence (CL) images of representative zircon grains from analysed samples showing SHRIMP spot location (red dashed circles).

Fig. 5

 5 Fig. 5.9. Weighted average 207 Pb/ 206 Pb ages and Tera-Wassaberg concordia diagrams displaying SHRIMP U-Pb zircon spot data for samples SB248 (a,b), SB137B (c,d) and SB092 (e,f), displaying error bars of ±1σ. Accepted analyses shown in blue and rejected analyses shown in red.

  8 (mean εHf (t) = 4.0 ± 1.1), corresponding to a range of T

Fig. 5 .

 5 Fig. 5.10. Results of combined U-Pb and Hf zircon analyses for samples from the study area in southwest Ghana, presented as 207 Pb/ 206 Pb age vs two-stage Hf model ages in billion years using the measure 167 Lu/ 177 Lu (first stage), a value of 0.0115

  1), which corresponds to a two-stage model age (2.43 and 2.61 Ga, clustering in two groups at ~2.45 and ~2.56Ga (Fig. 5.10c), suggesting two distinct mantle extraction events.5.5.3.4 Biotite-hornblende potassic quartz monzonite (SB023)Sample SB023 zircon morphology is commonly elongate, prismatic and clear to yellow colouring. Zircons are inclusion free and varied in size from 100 to 250 µm along the c-axis. CL imaging revealed common xenocrystic inherited cores with both dull and bright luminescence, commonly with broad irregular zonation (Fig. 5.8d). Zonation in xenocrystic cores is often truncated along the rim with rare evidence of metamictization on core rims. Oscillatory zoning is common within thick (30 -60μm) growth rings. Elongate grains without cores show pyramidal grain termination and regular oscillatory zoning. Large grains show more extensive cracking and fractures. Thirty-six analyses were performed on twenty-six grains. Of the thirty-six analyses, four were performed on xenocrystic cores, two of which were rejected for common Pb above 1% and for 232 Th/ 238 U ratio <0.1. Two xenocrystic core analyses yielded 207 Pb/ 206 Pb ages of 2250 ± 28 Ma and 2270 ± 21 Ma. The remaining twenty-six analyses were performed on growth rims and homogeneous zircons. Eight of these analyses were rejected: one analysis had common Pb above 1%, two analyses yielded 1σ > 50Ma and five grains had discordance > 15%. One population was identified from remaining 18 analyses yielding a weighted average 207 Pb/ 206 Pb age of 2136 ± 7 Ma (MSWD = 1.4, n = 18), which is interpreted to be the crystallization age of the rock (Fig. 5.10a). The Concordia diagram reveals an upper intercept age of 2138 ± 15 (MSWD = 0.82, n = 18) (Fig. 5.10b)), with the plotted from both concordant and

  references therein), Figure 5.12a indicates a common low-K mafic source for both TTG groups, with the exception of two samples, with one indicating a high-K mafic source, and the second high-Al 2 O 3 sample indicating remelting of pre-existing TTGs. Although both groups of TTGs have low potassium values, normalised LREE/HREE ratios (Fig. 5.12b) indicate the hornblende-bearing granitoids show similar ratios to modern day basalt-andesite-dacite-rhyolite (BADR) arc sequences,

Fig. 5

 5 Fig. 5.12. a) Ternary diagram for major element of granitoid samples analysed in this study used to discriminate between potential source rocks, with fields after Laurent, et al. (2014, and references therein) based on experimental melt compositions. Binary diagrams (b-f) used to discriminate: (b) interaction and role of fluids or enriched TTG magmas in mantle-derived magmas (fields based onMartin, et al., 2009, and references therein); (c, d, e) depth of melt generation and residual phase assemblage, comparing late Archaean TTGs and post-2.5Ga TTG and arc magmatism (Drummond

  with the 2158 ± 6 Ma syn-depositional dacitic volcanism from the Sunyani-Comoé Domain (this study) indicates diachronous basin development with sediment deposition commencing between 10 and 30 Myrs earlier in the Sunyani-Comoé Domain. This finding would support the timing of extension event between ca. 2145 and 2135 Ma, hypothesised by Hirdes, et al. (2007), potentially responsible for the original basin opening of the Kumasi-Afema Domain. The original spatial relationships between the Sefwi Belt and the Sunyani-Comoé Domain is largely obscured by deformation during the Eburnean Orogeny, however, we proposed that the tectonic contacts between the two domains represents a narrow collisional front or suture, with evidence of collision between the two discrete crustal fragments, represented by southern Ghana and central Ghana/Ivory Coast, recorded by the emplacement of syn-collisional granites (Fig. 5.12f) between ca. 2092 and 2081 Ma, and high-P, intermediate-T metamorphism and clockwise P-T metamorphic paths (Ch. 4).

  . The range of both εHf values and T for magmatism at ca. 2155 Ma, with greater ranges of εHf values and T H D f M model ages in the ca. 2136 Ma quartz monzonite suggesting increasingly heterogeneous sources late in the magmatic evolution, in agreement with geochemical analysis. Two-stage Hf model ages for all samples indicate the juvenile crust was extracted from the mantle between ca. 2260 Ma and 2650 Ma (Fig. 5.13), indicating a Neoarchaean maximal age of a reworked proto-crust of 2650 Ma, from which the Palaeoproterozoic crust was derived. When considering both inherited zircon cores and crystallisation ages of magmatic rocks in the study area, crustal residence times vary significantly between 100 and 400 million years.

Fig. 5

 5 Fig. 5.13. εHf values versus Pb-Pb age diagram for individual zircon grains from the study area. Average continental crust evolution is modelled with 176 Lu/ 177 Hf = 0.0115(Condie, et al., 2005) and a mafic crust evolution with 176 Lu/ 177 Hf = 0.0210

  for the composition of the depleted mantle display similar results for Hafnium model ages, represented by stars at the intersection of depleted mantle and crustal evolution trends. Two conclusions may be drawn from the combined geochemical and isotopic characteristics of igneous rocks in the Sefwi Belt. Firstly, the high-pressure TTG group, represented by ca. 2155 Ma trondhjemite sample (SB248), was generated through the partial melting of a subducting slab, extracted from the depleted mantle between approximately ca. 2300 and 2260 Ma. Secondly, diorites, quartz diorites and younger quartz monzonites have a wider range of εHf values, consistent with continued contribution of radiogenic material from the depleted mantle at ~ 2250 Ma combined with recycled juvenile proto-crust with mantle extraction ages spanning from ~2650 to 2300 Ma. Within the Baoulé-Mossi domain of the WAC, inherited zircon ages span from the Mesoarchaean to Palaeoproterozoic, with approximately 10 grains in southern Mali presenting inherited ages between ca. 3600 and 2700 Ma proximal to the Archaean Kénéma-Man Domain (Parra

Fig. 5 .

 5 Fig. 5.15. εHf (t) values yielded by Palaeoproterozoic rocks of the West African Craton (Data sources: Block, et al.,

  highlighting the isotopic boundary (blue rectangle), represented by the Sefwi Belt in southwest Ghana, and significant reworked Archaean components in southeast Ghana, southern Mali and Guinea. Simplified greyscale map of the southern portion of the West African Craton from WAXI3.

Fig. 5 .

 5 Fig. 5.16. Zircon εHf values yielded by this study presented relatively to the global detrital zircon database (Roberts & Spencer, 2015, and references therein), indicating the consistently radiogenic Hf signatures of magmatism in the Sefwi Belt, compared to the broader Hf array of the global record.

Fig. A2SI .

 A2SI Fig. A2SI. Bouguer gravity anomaly grid highlighting the dominant northeast trend in the regional architectures, with gravity highs associated with the central Sefwi Greenstone Belt and the Ashanti Greenstone Belt in the southeast. The Kumasi-Afema Domain between the two belts is associated with a 40km-wde intermediate gravity anomaly, and intermediate to low-gravity anomalies associated with the Sunyani-Comoé Domain.

Fig. A3SI .

 A3SI Fig. A3SI. Negative reduced to the equator (RTE) of residual magnetic intensity (RMI) grid in colour using to interpret litho-tectonic domains and some lithology distribution with greyscale first vertical derivative (1VD) overlay to highlight magnetic fabrics and major lineaments.

Fig. A4SI .

 A4SI Fig. A4SI. Composite image displaying a greyscale scale digital elevation model with 90m resolution (Shuttle Radar TopographicMission, SRTM, 2000) with an overlay of a ternary image of gamma-ray signal from three channels, K, eTh, and eU, highlighting the partial coverage of radiometric data of the study area.

  1,2 , Block, S. 2 , Aillères, L. 1 , Betts, P. 1 Ganne, J. 2 ,Baratoux, L. 2,3 Jessell, M. 4

	Affiliations:
	1 School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, VIC 3800,
	Australia, helen.mcfarlane@monash.edu
	2 GET, Observatoire Midi Pyrénées, IRD, 14 ave E. Belin, Toulouse 31400 France
	3 IFAN, Cheikh Anta Diop, Dakar, Senegal
	4 CET, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009 Australia

  (http://www.perplex.ethz.ch/). Calculations were conducted in the Na 2 O-K 2 O-CaO-MgO-MnO-FeO-Al 2 O 3 -TiO 2 -SiO 2 -H 2 O chemical system for metapelitic samples and in the Na 2 O-CaO-MgO-MnO-FeO-Al 2 O 3 -TiO 2 -SiO 2 -H 2 O for mafic samples.

  The secondary beam was energy filtered by ~70% to minimise scattered ions and interference on the common Pb isotope204 Pb and to reduce the Th peaks. Analyses of the unknowns measured 6 scans through the mass stations203 CeO 2 , 204 Pb. 204.1 (background), 206 Pb, 207 Pb, 208 Pb, 232 Th, 238 U, 248 Th and 254

Table 4 .

 4

1. Peak metamorphic mineral assemblage and location of the investigated samples.

Table 4.2. Whole rock bulk compositions (oxide wt %) of samples used for calculation of P-T pseudosections.

Acknowledgements

Acknowledgements

We wish to gratefully acknowledge AMIRA International and the industry sponsors, including AusAid and the ARC Linkage Project LP110100667, for their support of the WAXI project (P934A).

Field expenses were funded by the School of Earth, Atmosphere and Environment, Monash Research Initiative Fund -Dr L. Aillères. We acknowledge the facilities and technical staff at the Monash Centre for Electron Microscopy. Special thanks are extended to the Geological Survey Department of Ghana for logistical support in the field and additional data, including chauffeurs K. Duah.

Acknowledgements

We wish to gratefully acknowledge AMIRA International and the industry sponsors, including AusAid and the ARC Linkage Project LP110100667, for their support of the WAXI project (P934A), including EMPA analysis performed as part of this body of research, conducted at the Centre de Micro-Caractérisation Raimond Castaing, Toulouse. Field expenses were funded by the School of Earth, Atmosphere and Environment, Monash Research Initiative Fund -Dr L. Ailleres. SHRIMP dating was funded in part by School of Earth, Atmosphere and Environment, Monash Research Initiatives Fund -Dr Robin Armit and Prof. Peter Betts. R. Armstrong is warmly thanked for his expertise and assistance concerning SHRIMP dating at the Research School of Earth Sciences, of ANU. We acknowledge the facilities and technical staff at the Monash Centre for Electron Microscopy.

We thank P. de Parseval and S. Gouy for assistance in microprobe data acquisition and T. Aigouy for his assistance in SEM imaging at the Géosciences Environnement Toulouse (GET) laboratory. Special thanks are extended to the Geological Survey Department of Ghana for logistical support in the field and additional data, including chauffeurs K. Duah and Mr Brown. Prof. Roberto Weinberg is warmly thanked for his thorough and thought-provoking editorial contribution.

Acknowledgements

We gratefully acknowledge AMIRA International and industry sponsors, including AusAid and the ARC Linkage Project LP110100667 for their support of the WAXI project (P934A), which funded the zircon SHRIMP analyses. Zircon analyses were carried out on the Sensitive High Resolution Ion Micro Probe mass spectrometer (SHRIMP II) at the John de Laeter Centre, Curtin University, with the financial support of the Australian Research Council and Auscope NCRIS. Field expenses were funded by the School of Earth, Atmosphere and Environment, Monash Research Initiative Fund -Dr L. Ailleres. Geochemical analyses were funded by the Institut de Recherche pour le Développement (IRD), provided by Dr. J. Ganne. We are deeply thankful to Chris Spencer for the zircon Lu-Hf analyses, which greatly improved and expanded our findings. We thank Massimo Raveggi, Rachelle Pierson and Junnel Alegado for their support in the sample preparation and mineral separation procedures. We thanks the Geological Survey Department of Ghana for their logistical support and data resources, as well as drivers from the GSD including Mr Kwasi Duah and Mr Brown.

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West African Craton. The integrated use of field observations, structural mapping and interpretation of regional airborne geophysical data provides new information on the architecture, deformation sequence, overprinting relationships and tectonic evolution of the sparsely exposed region during the Palaeoproterozoic Eburnean Orogeny (2150( -2070 Ma) Ma). This study reveals tectonically juxtaposed domains of contrasting metamorphic grade comprising high-grade paragneisses and meta-volcanic rocks and low-grade volcano-sedimentary packages, both of which are extensively intruded by multiple generations of granitoids. The earliest deformation is characterised by a ubiquitous, bedding-parallel metamorphic foliation (S 1 ) preserved throughout the study area, axial planar to poorly preserved isoclinal folds. D 1 NNW-SSE shortening is associated with high-grade metamorphism with partial melting occurring in its later stage. It is interpreted as a crustal thickening event, generating twomica and muscovite granites emplaced between 2092 and 2081 Ma, providing a minimum age for D 1 deformation. D 2 structures are characterized by a moderate to steeply dipping, WSW-ENE to NE-SW striking, penetrative S 2 mineral cleavage. The transtensional nature of the deformation is indicated by the sub-parallel to coaxial nature of F 2 fold axes and L 2 stretching lineations, pitching shallowly ENE-WSW to NE-SW and striking obliquely to the major shear zones. Under a transtensional regime, late-orogenic differential exhumation of the middle and lower crust occurs along NNE-striking extensional detachments, coincident with oblique sinistral displacement along NE-SW regional-scale shear zones. The timing of exhumation is constrained by metamorphic monazite U-Pb ages at ca. 2073

Ma. Overprinting by subsequent E-W shortening is attributed to a change in boundary conditions and is associated with dextral reactivation of regional NE-SW striking shear zones and refolding of earlier structures distal to major shear zones. We propose that the north-western margin of the Sefwi Belt represents the collision of southern Ghana and central and NW Ghana/Ivory Coast which display contrasting tectonic histories. Furthermore, we suggest the accretion of the West African Craton is the product of episodic collisional orogenesis, providing new insights into the evolution of orogenic processes during the Palaeoproterozoic. Affilliations:

Keywords

1 School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, VIC 3800, Australia, helen.mcfarlane@monash.edu 2 GET, Observatoire Midi-Pyrénées, IRD, 14 ave E. Belin, Toulouse 31400 France

Abstract

New petrological and geochronological data are presented for migmatitic paragneisses and high-grade metavolcanic rocks exposed in southwest Ghana in the south-eastern portion of the Palaeoproterozoic (2300-2070 Ma) West African Craton. The study focuses on tectono-metamorphic domains within and adjacent to the NE-striking Sefwi Greenstone Belt, formed during the relatively understudied Eburnean Orogeny (2150-2070 Ma). These domains are bounded or transected by steeply dipping NE-to ENE-striking reactivated regional ductile shear zones.

North of the Sefwi Belt, a newly interpreted high-grade terrane comprises heterogeneous, migmatitic paragneisses with voluminous biotite-muscovite granite intrusions and is interpreted to be part of the Sefwi Belt due to its metamorphic and structural similarities. Paragneiss and garnet amphibolite samples sporadically reveal initial high-pressure amphibolite facies metamorphic conditions (10.0-11.5 kbar, 580-650 °C), corresponding to a cool apparent geothermal gradient of 15-17 °Ckm -1 . Migmatites record peak temperature conditions at the amphibolite-granulite facies transition (7.5-9.5 kbar, 650-700 °C), establishing a new apparent geothermal gradient of 20-25 °C km -1 . Variable prograde paths in the migmatites indicate contrasting prograde paths showing heating during decompression, or, conversely, concomitant heating and burial. Subsequent coeval exhumation is suggested by parallel clockwise P-T-t paths, simultaneous decreases in pressure and temperature and a regional amphibolite facies metamorphic overprint. In-situ monazite U-Pb SHRIMP ages of ca. 2073 Ma constrain the timing of the initial stages of cooling and exhumation. In contrast to the migmatites, a garnet amphibolite sample records cooling during exhumation immediately following initial HP-MT conditions. We interpret the clockwise P-T-t paths of two of the three samples as evidence of convergence-related crustal thickening and collision along the northwest margin of the Sefwi greenstone belt. Exhumation of the lower crust along NE-striking shear zones, and west-dipping normal detachment post-dates leucogranite emplacement in lower and upper crustal slices at ca. 2092 -2081 Ma.

High-pressure metamorphic assemblages and low apparent geothermal gradients distinguish SW Ghana from similar Precambrian granite-greenstone provinces elsewhere. Furthermore, diverse thermal histories and the tectonic juxtaposition of crustal slices from different depths do not conform with the homogeneous metamorphism and strain patterns attributed to the "hot orogen" model proposed for Precambrian orogens. Furthermore, such young metamorphic ages have not been previously documented in the south-eastern portion of the West African Craton. These findings instead suggest episodic collisional orogenesis occurred during craton assembly resulting in a diachronous metamorphic history. The metamorphic record of the West African Craton supports the notion of a Palaeoproterozoic transitional tectonic regime, prior to the appearance of the hallmark ultra-highpressure (UHP) metamorphism attributed to modern subduction-related plate tectonics. first of which suggests that concurrent cooling and decompression held the rocks at conditions above that of the stability field of sillimanite, preserving the kyanite porphyroblasts. The second explanation suggests problematic crystallisation due to decreasing temperature or fluid-absent conditions during retrogression.

Overall, P-T modelling reveals a clockwise P-T path for sample SB398. Garnet porphyroblasts record burial along a cool geothermal gradient (~15° C/km) crystallising under high-pressure amphibolite conditions near the amphibolite-granulite boundary (Vernon & Clarke, 2008). Following peak pressure, sample SB398 records a near isobaric increase in temperature to amphibolite facies suprasolidus conditions crossing geotherms and recording a transient apparent geothermal gradient of 20-25 °C/km. The retrograde path records a 3 kbar decrease in pressure and a drop of 100° C. Ma, revealing contrasting sources. Diorite magmas are produced thorough the partial melting of the incompatible element-enriched metasomatised mantle wedge, whilst the biotite-bearing TTGs were likely produced through the partial melting of young, hot subducting oceanic crust at depth. The hornblende TTGs are attributed to the shallow, partial melting of mafic crust based on low Sr and high HREE concentrations, weak REE fractionation and major element ratios compared to experimental melts. The emplacement of high-K quartz monzonites at ca. 2136 Ma reveals mixing of mantlederived magmas and partial melts derived from existing TTGs in the crustal pile. The subsequent emplacement of two-mica granites and leucogranites between ca. 2092-2081 Ma is interpreted to be the product of crustal anatexis during an arc-arc collisional event, with a proposed suture parallel to the NW margin of the Sefwi Belt. In-situ zircon Lu-Hf analysis reveals consistently positive εHf (t) values between +4.9 and +8.0 for TTGs, +2.2 and +5.5 for diorites and +2.7 to +5.8 for dacitic lavas, highlighting a distinctly radiogenic source during crustal growth, potentially representing a juvenile isotopic boundary within the craton. Subsequent emplacement of quartz monzonites reveals a wider range of εHf (t) values between +1.2 and +6.5, suggesting growing heterogeneity of magma sources. We hypothesise that the formation and accretion of the West African Craton reflects the rapid accumulation and amalgamation of individual island arc terranes generated by two coeval intra-oceanic subduction zones, representing an ancient expression of the Wilson cycle. al., 1992) and(d, f, h) chondrite-normalised REE (McDonough &[START_REF] Mcdonough | The composition of the Earth[END_REF] spidergrams.

Sample SB389 (biotite-sillimanite gneiss)

P-T pseudosections for sample SB389 are presented in

Conversely, sample SB044 and SB244 are enriched in most incompatible elements and have negative Sr (63-83 ppm) and Eu (Eu N /Eu* = 0.51-0.56), with sample SB044 displaying extreme enrichment in both HREE and LREE (La N = 282; Yb N = 61.55).

Geochronology

Results of zircon U-Pb and Lu-Hf analysis

In-situ zircon SHRIMP U-Pb analyses were performed on four samples, including three plutonic Twenty Hf analyses in dated grains yielded 176 Lu/ 177 Hf of < 0.0007, 176 Yb/ 177 Hf of < 0.02 and 176 Hf/ 177 Hf range from 0.281543 to 0.281630 ± 0.000034-0.000067 (2SE). Individual Pb-Pb ages and initial 176 Lu/ 177 Hf ratios for each zircon reveal radiogenic εHf (t) values between +4.5 to +7.4 (mean εHf (t) = 6.0 ± 0.9), corresponding to a relatively tightly grouped range of two-stage T H D f M model ages between 2.25 and 2.45 Ga (Fig. 5.10a).

Foliated dacitic volcanoclasite (SB173B)

Sample SB173B zircon morphology is commonly euhedral and prismatic, and rarely pyramidal, with ratios of 1:2 to 1:4, 80-150 µm in length and 50-75 µm in width, with larger zircons reaching 200 µm in length. Smaller prismatic grains are pale coloured and relatively free of fractures whilst larger grains show brown colouration and more extensive fracture networks. Cathodoluminescence images reveals fine, low-luminescent, oscillatory zoning in some grains with no clear evidence of xenocrystic cores, whilst others show broad zoning with low luminescence (Fig. 5.8b). BSE images show small mineral inclusions in a number of zircons and extensive fracturing in some smaller grains (likely to be the product of the mounting process).

discordant accepted age indicating slight modern lead loss. A total of twenty-two Hf analyses were performed on dated cores, rims and unzoned zircons from sample SB023, yielding 176 Lu/ 177 Hf of < 0.003, 176 Yb/ 177 Hf between 0.01 and 0.1, and 176 Hf/ 177 Hf range from 0.281447 to 0.281600 (± 0.000032 -0.000068 2SE). Analyses revealed a varied, radiogenic Hf signature, with suprachondritic εHf (t) values between +1.4 and +7.2 (mean εHf = 4.1 ± 0.6). The two inherited cores yielded strongly radiogenic Hf signatures, with values of εHf (2250Ma) = +6.7 and εHf (2270Ma) = +4.1, within range of the εHf values for homogeneous zircons and zircon rims in the same sample, for which there was no discernible systematic variation. The significant range of six epsilon units for all zircons analysed in sample SB023 is consistent with either heterogeneous magma sources or crustal contamination and corresponds to a wide range of T H D f M values, from 2.34 to 2.63 Ga.

Discussion

Petrogenesis of magmatic suites

The observed major and trace element patterns of magmatic rocks are the product of source composition, petrogenetic mechanisms, magma mixing and fractionation crystallisation (e.g. [START_REF] Brown | The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources[END_REF]Pearce, et al., 1984a), as well as enrichment of magma sources and the residual phase assembly of partial melting, indicative of the pressure of melt production (e.g. Defant & Kepezhinskas, Burkina Faso (Parra-Avila, et al., 2017), and 2253 ± 9 Ma, yielded by a granodiorite gneiss in northeast Burkina Faso (Tshibubudze, et al., 2013). With the exception of the Gondo granite gneiss, these ages are comparable to inherited ages yielded by the quartz monzonite in this study. The lack of rocks with ages between ca. 2650 and 2300 Ma preserved in the Baoulé-Mossi Domain suggests that the juvenile proto-crust from which the domain was derived was completely recycled prior to the oldest crystallisation age in the Sefwi Belt of 2222 ± 32 Ma (Feybesse, et al., 2006). Alternatively, crust older than ca. 2222 Ma may be as yet undocumented or extensively eroded, as suggested by the presence of older zircons in stream sediments in southern Mali.

Implications for the crustal architecture of the West African Craton

A database compilation of Hf-isotope data for igneous and detrital zircons sourced from Palaeoproterozoic rocks within the West African Craton is illustrated in Figure 5.14. Figure 5.14 contains all εHf (t) values currently available for WAC (pale grey diamonds) (data sourced from : Block, et al., 2016a;Eglinger, et al., 2017;Parra-Avila, et al., 2016;Petersson, et al., 2017;Petersson, et al., 2016), demonstrates the spectra of values within and between discrete samples, highlights a 150 M.y. (91500, GJ-1, Mudtank, Plešovice, R33 and MUN 1,3,4) were conducted throughout the session to verify accuracy and ensure reproducibility. These ratios are within the range of TIMS derived values reported for 91500 by Wiedenbeck, et al. (1995) (0.282290 ± 0.000014), and LA-ICP-MS values for GJ-1 (0.282000 ± 0.000005) (Morel, et al., 2008), Mudtank (0.282504 ± 0.000044) (Woodhead & Hergt, 2005), Plešovice (0.282482 ± 0.000013) (Sláma, et al., 2008), R33 (0.282764 ± 0.000014) (Fisher, et al., 2014) and doped synthetic zircon standards MUN 1, MUN3 and MUN 4 (0.282140±0.000006, 0.282135 ±0.000005, 0.282141±0.000006, respectively ) (Fisher, et al., 2011).

Chapter VI

Discussion and conclusion

dipping mineral stretching lineations; an asymmetric crustal architecture including structurallycontrolled crust-derived peraluminous granites, orogen-parallel metamorphic gradients and sharp, orogen-oblique lateral metamorphic breaks; and, the presence of vast, low-grade metasedimentary domains either side of the belt. Given the volume and significance of the data that disagrees with the sagduction model, we suggest that it is unlikely to have played a role in the tectonic evolution of the Sefwi Belt.

For the third end-member model, we emphasise that the low apparent geothermal gradients and narrow (<80km) orogenic front documented in this thesis are inconsistent with the hot-orogen model for Precambrian provinces in general (Cagnard, et al., 2011;Cagnard, et al., 2006;Cagnard, et al., 2007;Chardon, et al., 2009;Rey & Houseman, 2006;Rey, et al., 2003), and the West African Craton in particular (Chardon, et al., 2009;Vidal, et al., 2009). The findings of this thesis, therefore, support a plate tectonic regime, within which a nascent form of intra-oceanic subduction generated both arclike magmatism and TTG-like magmatism, as well as sufficient lateral deformation for collisional orogenesis during arc accretion. Furthermore, in the case of the broader geodynamic setting of the formation and amalgamation of the West African Craton, as discussed in Chapter 5, it is possible that there were multiple subduction zones operating simultaneously, likely in association with or adjacent to a Palaeoproterozoic oceanic plateau.

The new geochronological constraints and deformation history presented in this study highlight the diachronous nature of the tectonic evolution of the West African Craton, warning against craton-scale correlation of deformation events, especially on the basis of interpreted orientation of the strain regime. The transition from peak metamorphic temperatures to retrogression and exhumation is constrained by in-situ monazite ages at ca. 2073 Ma, hosted within or aligned with D2 mineral assemblages. We speculate that the preceding terminal collisional event between modern-day SW Ghana and central Ghana/Ivory Coast occurred at ~2100 Ma, which led to the emplacement of syn-tectonic leucogranites along the NW margin of the Sefwi Belt between ca. 2092 and 2081 Ma (Agyei Duodu, et al., 2009;Hirdes, et al., 1992;Hirdes, et al., 2007). This significantly later than the hypothesised collisional event between NW Ghana and SW Burkina Faso at ca. 2130 Ma (Block, et al. 2016). The final collision of the Palaeoproterozoic Baoulé-Mossi and Archaean Kénéma-Man domains occurred between ca. 2050 and 2030 Ma (Kouamelan, et al., 1997). These ages indicate lateral variation in the timing of deformation and metamorphism, consistent with progressive accretion and episodic collision of discrete crustal fragments during the Palaeoproterozoic Eburnean Orogeny. This which we interpret to be a lateral continuation of the D2 deformation and metamorphism recorded on the northwest margin of the Sefwi Belt. This suggests that the Sefwi Belt and the Ile de Cayenne were contiguous during the Palaeoproterozoic Eburnean-Transamazonian accretion/collision orogeny. This hypothesis is supported also by the formation of granulitic domains during late-orogenic crustal extension in the Ile de Cayenne (Delor, et al., 2001), potentially representing the spatial and temporal continuation of the constrictional deformation event documented in the Sefwi Belt in this study.

Implications for Palaeoproterozoic geodynamics and orogenesis

Controversy surrounding the geodynamic setting of the formation of juvenile crust of the West African Craton is borne of the relatively limited geochronological constraints on the timing of deformation and metamorphism, as well as ambiguous or often obscured, field relationships between greenstone belts, granite-gneiss domains, and sedimentary basins, a problem also frequently encountered in Archaean terranes (Condie, 1981;de Wit, 1998). We conclude that the magmatic record of the West African Craton represents an ancient example of the subduction-collision tectonics, with a crustal evolution reminiscent of a number of Neoarchaean provinces formed at the advent of "modernsubduction" processes (Laurent, et al., 2014;Martin & Moyen, 2002). It represents the amalgamation of arc terranes forming in an intra-oceanic setting. We emphasise that the original distribution of the terranes may have been thousands of kilometres apart, with their amalgamation requiring the closure of multiple oceans over the ~ 200 Myr evolution of the Eburnean-Transamazonian Orogeny, potentially analogous to the Miocene geodynamics of the Maramuni arc of Papua New Guinea, and the South Pacific [START_REF] Holm | Post 8Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting[END_REF][START_REF] Holm | Continental collision, orogenesis and arc magmatism of the Miocene Maramuni arc, Papua New Guinea[END_REF][START_REF] Schellart | A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: tectonics controlled by subduction and slab rollback processes[END_REF].

The Neoarchaean to Palaeoproterozoic period coincides with a number of significant changes in the Earth, including changes in the tectonic style, secular evolution recorded in the metamorphic record and evolving chemistry of magmatic rocks (e.g. Brown, 2006;Brown, 2007;Dhuime, et al., 2012;Keller & Schoene, 2012;Laurent, et al., 2014). This is a unique period prior to the Neoproterozoic appearance of blueschist facies or ultra-high-pressure (UHP) metamorphism and tectonic indicators, such as ophiolite sequences, attributed to the modern plate tectonic regime (Stern, 2005;Stern, 2007). It is widely accepted that the ambient upper-mantle temperatures were hotter in the past (e.g. Herzberg, et al., 2010;Labrosse & Jaupart, 2007), which likely affected the tectonic style and the rheology of the lithosphere (Burov & Yamato, 2008;Burov, 2011;Sizova, et al., 2010;van Hunen & van den Berg, 2008). Rey and Coltice (2008) propose that the secular cooling of the Earth over the

Appendices

The following information was used to constrain the new litho-tectonic maps for southwest Ghana. Fig. A1SI. Map of displaying survey coverage of airborne magnetic surveys. The residual magnetic intensity (RMI) was reduced to the pole (RTP) to remove any asymmetry of magnetic anomalies caused by the inclination of the Earth's magnetic field. The RTP images highlight the N-S artefacts commonly produced by the RTP transformation when performed in areas of low latitude (Li, 2008;MacLeod, et al., 1993). Table A1SI. Individual airborne magnetic survey details. (91500, GJ-1, Mudtank, Plešovice, R33 and MUN 1,3,4) were conducted throughout the session to verify accuracy and ensure reproducibility. These ratios are within the range of TIMS derived values reported for 91500 by Wiedenbeck, et al. (1995) (0.282290 ± 0.000014), and LA-ICP-MS values for GJ-1 (0.282000 ±

Appendix B -Magnetic susceptibility histograms