Romain Augier

Flavien Choulet

Michel Faure

Paul Turrillot

A turning-point in the evolution of the Variscan orogen: the ca. 325 Ma regional partial-melting event of the coastal South Armorican domain (South Brittany and Vendée, France)

Keywords: Partial-melting, Strain localisation, Gravitational collapse, U-Th/Pb on monazite, South Armorican domain

published or not. The documents may come

A turning-point in the evolution of the Variscan orogen: the ca. 325 Ma regional partial-melting event of the coastal South Armorican domain (South Brittany and Vendée, France)

Le premier groupe, principalement centré vers 335-330 Ma a été mis en évidence pour des échantillons de restites et pour les domaines de coeur des cristaux de monazite de la plupart de massifs de granite. Sa signification est attribuée à des âges de cristallisation hérités enregistrant probablement un incrément prograde lors de l'augmentation des condi-tions P-T dans un contexte de mise en place de nappes. Le second groupe d'âges, vers 325-320 Ma correspond aux cris-taux de monazite ayant cristallisés à partir de liquides silicatés néoformés. Il est interprété comme l'âge de cristallisation des leucosomes des roches partiellement fondues suite à un événement régional de fusion partielle. Le dernier groupe d'âges, vers 320 Ma, correspondant aux domaines de bordure des cristaux de monazite des massifs de granite " tardifs ". Il est interprété comme l'âge de mise en place de la plupart des grands massifs granitiques et clos ainsi l'épisode de fusion partielle.

INTRODUCTION

Partially molten rocks or migmatites are a common feature of mature orogens [i.e. [START_REF] Sawyer | Melt segregation in the continental crust[END_REF][START_REF] Sawyer | Criteria for the recognition of partial-melting[END_REF][START_REF] Brown | The generation, segregation, ascent and emplacement of granite magma: the migmatite-to-crustally-derived granite connection in thickened orogens[END_REF][START_REF] Brown | -Orogeny, migmatites and leucogranites: a review[END_REF][START_REF] Vanderhaeghe | Partial-melting and flow of orogens[END_REF][START_REF] Vanderhaeghe | Structural development of the Naxos migma-tite dome[END_REF]. Together with strain rates and thermal conditions, partialmelting controls the rheological behaviour of crustal materials by drastically reducing the bulk strength of the crust [i.e. Arzi, 1978;[START_REF] Tikoff | Modification of the regional stress field by magma intrusion and formation of tabu-lar granitic plutons[END_REF][START_REF] Rosenberg | Experimental deformation of partially melted granite revisited: implications for the continen-tal crust[END_REF]2005]. Along with a decrease of tectonic boundary forces, a decrease in the strength of a thickened crust is associated with gravitational collapse of an orogenic belt and the transition from crustal thickening to crustal thinning [i.e. [START_REF] Rey | Gravitational col-lapse of the continental crust: definition, regimes and modes[END_REF][START_REF] Vanderhaeghe | Partial-melting and flow of orogens[END_REF][START_REF] Tirel | -Dynamics and structural development of metamorphic core complexes[END_REF]Rey et al., 2009a;2009b]. Inception of partial-melting in the core of an orogenic belt may thus promote drastic changes in the balance of forces and thus represents a major turning-point in its evolution. Partial-melting in the thickened crust can either be an ulti-mate consequence of the stacking of radiogenic continental crustal slices or is due to a drastic change in heat supply from underneath as in the case of convective removal or slab retreat [START_REF] England | -Some thermal and tectonic models for crustal melting in continental collision zones[END_REF][START_REF] Houseman | The dependence of convection planform on mode of heating[END_REF][START_REF] Vanderhaeghe | Evolution of orogenic wedges and continental plateaux: insights from crustal thermal-mechanical models overlying subducting mantle lithosphere[END_REF].

of typical "migmatite-cored metamorphic core complexes" [START_REF] Brown | Synergistic effects of melting and deformation: an example from the Variscan belt, western France[END_REF]Turrillot et al., 2011a]. Moreover, most of the major shear zones are underlined by syntectonic granite laccoliths, strengthening the link between strain localiza-tion and presence of melt at the scale of the pluton [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF][START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF]. How-ever, to date, time-constraints are scarce and partly conflict-ing, hindering the understanding of this crucial stage in the Variscan belt evolution. This paper provides 29 new, and sometimes pioneer, U-Th/Pb chemical ages on monazite from the main occurrences of migmatites and regional clas-sic plutons exposed along the coast of the South Armorican domain. Together with an accurate separation of inheritance from neoformation, statistics on the dataset ensures internal consistency of the timeconstraints at regional-scale, shedding light on the dynamics of the late-orogenic stages of the West-European Variscanides.

GEOLOGICAL SETTING

The French Armorican massif (fig. 1) is one of the largest pieces of the Variscan belt that extends throughout western and central Europe [i.e. Matte, 1991, 2001]. Together with the Alleghanian belt, further to the West, the Variscan belt Besides, intrusion of magmas in higher structural levels resulted from the convergence of Laurussia and Gondwana or even in the upper brittle crust can locally promote strain that participated in the formation of the supercontinent localisation as shown by the formation of detachments at the Pangaea during the Late Paleozoic [e.g. [START_REF] Ballèvre | Palaeozoic evolution of the Armorican massif: from Gondwana break-up to continental collision. Mechanics of Variscan orogeny: a modern view on orogenic research[END_REF]roof of plutons [i.e. Daniel and Jolivet, 1995;Grasemann and Ballèvre et al., 2009;[START_REF] Nance R | Evolution of the Rheic ocean[END_REF]. The structural [START_REF] Grasemann | Evolution of the Serifos metamorphic core complex[END_REF]Jolivet et al., 2010] or even along volcanic pattern of the Armorican massif is currently dominated by sills or feeders [Famin and Michon, 2009]. The low strength Carboniferous W-E to NW-SE striking, dextral strike-slip of a magmatic body must have, at least temporarily, drastic faults, namely the North Armorican shear zone (NASZ), consequences on strain localisation [START_REF] Hutton | A tectonic model for the emplacement of the Main Donegal granite, NW Ireland[END_REF][START_REF] Neves | Shear zone-controlled magma emplacement or magma assisted nucleation of shear zones? Insights from northeast Brazil[END_REF] and the South Armorican shear zone (SASZ) (fig. 1B [e.g. al., 1996]. It is thus crucial to recognise the timing of both [START_REF] Cogné | Le Massif armoricain[END_REF][START_REF] Watts | Faults rocks as indicators of progressive shear deformation in the Guingamp region, Brittany[END_REF][START_REF] Jégouzo | The South Armorican shear zone[END_REF]; the bulk loss of crustal strength during large-scale par- [START_REF] Matte | Tectonics and plate tectonic model for the Variscan belt of Europe[END_REF]Le Corre et al., 1991;Tartèse et al., 2011a]). tial-melting events and localising events during magmatic in-Furthermore, the SASZ is subdivided into an E-W, and a trusions at more local-scale. In both cases, accurate dating of NW-SE striking northern and southern branches, respecthe crystallisation of melts either trapped at depth in tively. From the paleogeographic point of view, the Armorimigmatite complexes or after upward migration as plutons or can massif is divided into three main distinct parts, namely dykes may put crucial age-constraints on important changes the Léon, North-Central, and South-Armorican domains on mountain belts dynamics.

bounded by major tectonic contacts whose importance is The case of the West-European segment of the Variscan compared to suture zones: the Conquet-Penzé (LCPS), and belt is quite exemplary of this topic. Indeed, one of the most Nort-sur-Erdre (NSES) sutures (fig. 1) [e.g. [START_REF] Cogné | Le Massif armoricain[END_REF] striking features of the belt is the abundance of both [START_REF] Jégouzo | The South Armorican shear zone[END_REF]; Le Corre et al., 1991; Jégouzo and Rossello, migmatitic and plutonic rocks, partly obliterating the initial 1988; [START_REF] Cartier | The Hercynian orogeny in the South Armorican massif (Saint-Georges-sur-Loire unit, Li-gerian domain, France): rifting and welding of continental stri-pes[END_REF][START_REF] Cartier | The Saint-Georges-sur-Loire olistos-trome, a key zone to understand the Gondwana-Armorica boun-dary in the Variscan belt (southern Brittany, France)[END_REF] architecture of the orogen. In the coastal South Armorican [START_REF] Gumiaux | -Tectonic history of the Hercynian Armorican shear belt (Britta-ny, France)[END_REF]Faure et al., 2005]. These domains massif, an almost continuous belt in which migmatites and drastically differ by their metamorphic grade and structural granites currently represent ca. 50% of the outcropping sur-style [e.g. Le Corre et al., 1991;[START_REF] Ballèvre | -Palaeozoic history of the Armorican massif: models for the tectonic evolution of the suture zones[END_REF]; Faure face has long been described [e.g. [START_REF] Cogné | Le Massif armoricain[END_REF]. Besides, a et al., 2008a]. The North-Central Armorican domains expeclose association between crustal partial-melting, magmas rienced only moderate shortening and shearing between two and the development of extensional shear zones (i.e. detach-main suture zones and mainly behaved as rigid, upments) has been recently reemphasised with the description per-crustal continental domains [i.e. Le Corre, 1977;Le Corre et al., 1991]. Conversely, the Léon and the South Armorican domains experienced a succession of tectonometamorphic events related to subduction, collision and finally gravitational collapse phases [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Ballèvre | -Palaeozoic history of the Armorican massif: models for the tectonic evolution of the suture zones[END_REF]. The South Armorican domain con-sists of two parts. Between the Nort-sur-Erdre eo-Variscan suture and the southern branch of the SASZ, the Ligerian domain is an area characterised by Early Paleozoic ophiolites, highpressure mafic and felsic rocks, and Devo-nian migmatites recently dated at ca. 387 ± 8 Ma (fig. 1) [START_REF] Marchand | Ecaillage d'un "mélange tectonique" profond: le complexe cristallophyllien de Champtoceaux (Bretagne méridionale)[END_REF][START_REF] Ballèvre | Eo-Hercynian events in the Armorican massif[END_REF][START_REF] Bosse | Late Devonian subduction and early orogenic exhumation of eclogite-facies rocks from the Champtoceaux complex (Variscan belt, France)[END_REF][START_REF] Cocherie | -Electron-microprobe dating as a tool for determining the closure of Th-U/Pb systems in migmatitic monazi-tes[END_REF]Faure et al., 2008b]. To the south of the southern branch of the SASZ, the coastal South Armori-can domain exhibits metamorphic and granitic rocks [START_REF] Cogné | Le Massif armoricain[END_REF][START_REF] Berthé | Orthogneiss, mylonite and non-coaxial deformation of granites: the example of the South Armorican shear zone[END_REF][START_REF] Jégouzo | The South Armorican shear zone[END_REF].

In the following, only the geology of the coastal South Armorican domain is detailed (fig. 1a). Readers unfamiliar with the geology of the Armorican massif are referred to recent publications [START_REF] Gumiaux | -Tectonic history of the Hercynian Armorican shear belt (Britta-ny, France)[END_REF]Faure et al., 2005Faure et al., , 2008b;;[START_REF] Ballèvre | -Palaeozoic history of the Armorican massif: models for the tectonic evolution of the suture zones[END_REF]. Regionally, the coastal South Armorican domain can be divided in three main groups of units, from top to bottom (fig. 1b) [e.g., [START_REF] Cogné | Quelques observations tectoniques sur les schistes cristallins et les granites de la Cornouaille maritime[END_REF][START_REF] Cogné | Le Massif armoricain[END_REF]Le Corre et al., 1991;[START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]Turrillot et al., 2011a]:

1) the Upper units, formed by metamorphic rocks that have been equilibrated along a ca. 10 o C/km HP/LT subduction gradient. The uppermost units, exposed in the Bois de Céné and Ile de Groix klippes are made of chloritoidmicaschists, lawsonite-blueschists, garnet-blueschists and eclogites derived from metasediments, metabasites and rare felsic layers [START_REF] Bosse | Late Devonian subduction and early orogenic exhumation of eclogite-facies rocks from the Champtoceaux complex (Variscan belt, France)[END_REF][START_REF] El | Geochemical fingerprints of devolatilization reac-tions in the high-pressure rocks of Ile de Groix[END_REF]. The pres-ence of both tholeiitic and alkaline signatures of the metabasites and Mn-rich metasediments argues for an initial oceanic setting or an intracontinental rift [START_REF] Carpenter | Geochemical and isotopic characteristics of blueschist facies rocks from the Ile de Groix, Armorican massif (northwest France)[END_REF][START_REF] Audren | Notice explicative de la carte géologique de la France au 1/50 000, feuille Belle-Ile-en-Mer -Iles Houat et Hoedic[END_REF][START_REF] Audren | Notice explicative de la carte géologique de la France au 1/25 000[END_REF][START_REF] El | Discovery of an albite gneiss from the Ile de Groix (Armorican massif, France): geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith[END_REF]. Metamorphic peak-conditions reached around 18-20 kbar and 450-500 o C [START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican massif, France[END_REF][START_REF] Ballèvre | Lawsonite growth in the epi-dote blueschists from the Ile de Groix (Armorican massif, France): a potential geobarometer[END_REF] and were followed by a limited retro-gression into the greenschist facies conditions [START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican massif, France[END_REF]. The lower ones (i.e. Belle-Ile group) consist of mo-notonous series of metamorphosed volcanics intercalated with black-shales (i.e. the Vendée and Belle-Ile-en-Mer por-phyroids) and graphite-rich micaschists (i.e. Saint-Gilles micaschists). Age of the series was recently settled at ca. 480 Ma in Vendée by dating the emplacement age of the por-phyroids [START_REF] El | Discovery of an albite gneiss from the Ile de Groix (Armorican massif, France): geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith[END_REF]. P-T estimates for the Belle-Ile group rocks yielded HP/LT metamorphic peakcon-ditions of 7-9 kbar and 350-400 o C [Le Hébel et al., 2002a];

2) the Intermediate units (i.e. the Vilaine Estuary and Sables d'Olonne units) made of metasediments in associa-tion with rare metavolcanics were equilibrated under a MP-MT gradient [START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF] while relics of HP/LT metamorphism have not been recognised. Meta-morphism increases downward from albite greenschist-facies to garnetstaurolite-biotite amphibolite-facies conditions [START_REF] Bossière | Evolutions chimico-minéralogiques du grenat et de la muscovite au voisinage de l'isograde biotite-staurotide dans un métamorphisme prograde de type barrovien: un exemple en Vendée littorale (Massif armoricain)[END_REF][START_REF] Triboulet | Controls on P-T-t deformation path from amphiboles during progressive metamorphism with basic rocks (estuary of the river Vilaine, South Brittany, France)[END_REF][START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF] and locally to partial-melting conditions to the southern Vendée [START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Debrand-Passard | Notice explicative de la carte géologique de la France au 1/50 000, feuille Les Sables d'Olonne[END_REF]. Estimates of the metamorphic peak-conditions yielded 7-9 kbar for 620 o C in the Vilaine Estuary area [Triboulet andAudren, 1985, 1988;[START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF] and 9 kbar for 650-700 o C to the north of the Sables d'Olonne area, where localised partialmelting has been widely described [e.g. [START_REF] Ters | Sur l'extension du Paléozoïque en Vendée littorale. Stratigraphie et structure[END_REF][START_REF] Brillanceau | Guide géologique régional Poitou-Vendée-Charente[END_REF][START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]:

3) the Lower unit consists of high-grade rocks, migmatites and large volumes of granite recognised as the South Brittany Migmatite belt that extends to the south in Vendée [START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF]. Migmatites include restitic rocks with granulites, and kinzigites derived from pre-Carboniferous metasediments and Ordovician orthogneiss protoliths [START_REF] Audren | Mobilisation anatectique et déformation, un exemple: les migmatites du golfe du Morbihan (Bre-tagne méridionale)[END_REF][START_REF] Le Métour | Arguments géochronologiques en faveur de l'existence d'une double ceinture métamorphique d'âge siluro-dévonien en Bretagne méridionale[END_REF][START_REF] Mifdal | Définition géochronologique de la phase bretonne en Bretagne centrale. Etude Rb/Sr de granites du domaine centre armoricain[END_REF][START_REF] Brown | The petrogenesis of some migmatites from the presqu'île de Rhuys, southern Brittany, France[END_REF][START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb): exemples choisis au Groenland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF][START_REF] Audren | Evolution structurale de la Bretagne méridionale au Paléozoïque[END_REF][START_REF] Jones | High-temperature 'clockwise' P-T paths and melting in the development of regional migmatites: an example from southern Brittany, France[END_REF][START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF]. P-T estimates yielded metamorphic peak-conditions at 8-10 kbar and 750-800 o C accompanied and followed by distinctive melt extraction stages down to 4 kbar and 750 o C [START_REF] Brown | The petrogenesis of some migmatites from the presqu'île de Rhuys, southern Brittany, France[END_REF][START_REF] Jones | -Paleozoic continental margin tectonics of southern Armorica[END_REF]Brown, 1989, 1990;[START_REF] Jones | -Paleozoic continental margin tectonics of southern Armorica[END_REF][START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Audren | P-T-t-deformation paths recorded by kinzigites during diapirism in the western Variscan belt (Golfe du Morbihan, southern Brittany, France)[END_REF][START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF][START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF][START_REF] Brown | Quantitative constraints on metamor-phism in the Variscides of southern Brittanya complementary pseudosection approach[END_REF].

Until now, the knowledge of the tectonic evolution of the coastal South Armorican domain remains incomplete. However, an overall two-stepped tectonometamorphic evolution of the domain can be drawn including an initial episode of subduction and crustal-thickening tectonics followed by pervasive extensional tectonics [START_REF]Combined thrusting and wrenching in the Ibero-Armorican arc: a corner effect during continental colli-sion. -Earth Planet[END_REF][START_REF] Bale | Stretching linea-tion and transport direction in the Ibero-Armorican arc during the Siluro-Devonian collision[END_REF][START_REF] Van Den Driessche | -Syn-to post-thickening extension: mode and consequences[END_REF][START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]. Evidence for the subduction stages are quite well preserved in the upper units, particularly in the Ile de Groix and the Bois de Céné klippes where typical subduction-related blueschists and eclogites occur in different tectonics slices [e.g. [START_REF] Barrois C | Sur les amphibolites à glaucophane de l'île de Groix[END_REF][START_REF] Carpenter | Hercynian high pressure/low metamorphism temperature in the Ile de Groix blueschists[END_REF][START_REF] Quinquis | -Intense progressive shear in Ile de Groix blueschists and compatibility with subduction or obduction[END_REF][START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican massif, France[END_REF][START_REF] Ballèvre | Lawsonite growth in the epi-dote blueschists from the Ile de Groix (Armorican massif, France): a potential geobarometer[END_REF]. Architecture of the Ile de Groix has even been interpreted as a remnant of a south-east-verging accretionary wedge related to a northward dip-ping subduction [START_REF] Ballèvre | Geology, petrology and geochronology of the blueschists from the Île-de-Groix[END_REF]. Metamorphic phengite from HP assemblages yielded Late Devonian-Early Carboniferous Rb/Sr and 40 Ar/ 39 Ar ages at 370-360 Ma, while retrogression and final cooling under greenschist-facies conditions occurred at around 355-345 Ma [Le Hébel, 2002;[START_REF] Bosse | -Rb/Sr and 40 Ar/ 39 Ar ages in blueschists from the Île-de-Groix (Armorican Massif, France): Implications for closure mecha-nisms in isotopic systems[END_REF]. These uppermost units are systematically found at the top of the Variscan edifice, overthrusted on lower pressure continental units [START_REF] Bosse | -Rb/Sr and 40 Ar/ 39 Ar ages in blueschists from the Île-de-Groix (Armorican Massif, France): Implications for closure mecha-nisms in isotopic systems[END_REF]. Subsequent accretion of continental slices by thrusting in the orogenic wedge is also locally preserved as exemplified by the tectonic superimposition of the porphy-roids HP/LT units over low-grade metasediments in Vendée [START_REF] Ters | Sur l'extension du Paléozoïque en Vendée littorale. Stratigraphie et structure[END_REF][START_REF] Iglesias | Signification des variations et anoma-lies de la déformation dans un segment la chaîne hercynienne (les séries cristallophylliennes de la Vendée littorale, Massif armoricain)[END_REF][START_REF]Tectonique tangentielle hercynienne en Vendée litto-rale: signification des linéations d'étirement E-W dans les por-phyroïdes à foliation horizontale[END_REF][START_REF] Maillet | Relations des porphyroïdes et schistes de Saint-Gilles avec les formations siluriennes de Brétignolles-Sur-Mer (Vendée Maritime)[END_REF][START_REF] Colchen | Sur l'âge post-tournaisien de la tectonique hercynienne synschisteuse à Brétignoles-sur-Mer, Vendée littorale, Massif armoricain[END_REF][START_REF] Vauchez | Strain and deformation mechanisms in the Variscan nappes of Vendée, South Brittany, France[END_REF][START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Geoffroy | Tectonique tardi-varisque en failles normales ducti-les en Vendée littorale, Massif armoricain[END_REF]. While N-S directions and a preferred top-tothe South sense of shear were proposed [START_REF] Cannat | Linéations N-S en Vendée littorale (Massif armoricain) ; épisodes tangentiels successifs éo-hercy-niens en France occidentale[END_REF], kinematics of thrusting at a regional-scale remain currently not documented [Le Hébel, 2002]. Similarly, the age of final nappe-stacking is only lo-cally settled to the Tournaisian (i.e. 359-347 Ma) as re-corded by syn-thrusting sedimentation [START_REF] Maillet | Relations des porphyroïdes et schistes de Saint-Gilles avec les formations siluriennes de Brétignolles-Sur-Mer (Vendée Maritime)[END_REF][START_REF] Colchen | Sur l'âge post-tournaisien de la tectonique hercynienne synschisteuse à Brétignoles-sur-Mer, Vendée littorale, Massif armoricain[END_REF][START_REF] Rolin | Les cisaillements hercyniens de la Vendée au Limousin[END_REF].

The tectono-metamorphic record of the crustal thicken-ing stages were erased by pervasive partial-melting and extensional tectonics during a subsequent late-orogenic stage [START_REF]Tectonique tangentielle hercynienne en Vendée litto-rale: signification des linéations d'étirement E-W dans les por-phyroïdes à foliation horizontale[END_REF][START_REF]Combined thrusting and wrenching in the Ibero-Armorican arc: a corner effect during continental colli-sion. -Earth Planet[END_REF][START_REF] Cannat | Linéations N-S en Vendée littorale (Massif armoricain) ; épisodes tangentiels successifs éo-hercy-niens en France occidentale[END_REF][START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF][START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF][START_REF] Brown | Synergistic effects of melting and deformation: an example from the Variscan belt, western France[END_REF][START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]Turrillot et al., , 2011a]]. During this stage, the Lower unit was exhumed as a series of domes locally referred to as [START_REF] Gapais | Mountain building and exhumation processes through time: inferences from nature and models[END_REF][START_REF] Bitri | Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/Géofrance 3D Program)[END_REF]. Indicated is the location of the samples used for U-Th/Pb chemical datings. (b) Inset is a sketch map of the French Armorican massif and its main subdivisions. The black rectangle is the location of Golfe du Morbihan area presented in figure 1c. (c) Simplified geological map of the Golfe du Morbihan area [START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF]. The location of samples that provided ages for Sainte-Anne d'Auray and Sarzeau granites is also indicated [START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]. Only the units corresponding to the coastal South Armorican domain have been represented. Note the close association between the migmatite fabrics and the anatectic bodies and particularly the Sainte-Anne d'Auray anatectic granite massif. Conversely, the Carnac, Ploemeur and Quiberon granite intrusions clearly cut across the migmatite fa-brics and the Sainte-Anne d'Auray granite. These plutons locally emplaced within the upper units. migmatite-cored metamorphic core complexes [START_REF] Brown | Synergistic effects of melting and deformation: an example from the Variscan belt, western France[END_REF]Turrillot et al., 2011a] surrounded by the overlying intermediate and Upper units (fig. 1) [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Van Den Driessche | -Syn-to post-thickening extension: mode and consequences[END_REF]Le Hébel, 2002]. At regional-scale, defor-mation associated with the exhumation of the Lower Unit is marked by a regional flat-lying planar fabric, and a domi-nantly W-E to WNW-ESE striking stretching lineation with the exception of the Saint-Nazaire area where the stretching direction strikes almost ca. N-S [Bouchez et al., 1981;[START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF]. Besides, despite strik-ing structural similarities, two contrasted modes of crustal extension were opposed within the coastal South Armorican domain [START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF], and one can distinguish from southeast to northwest: 1) the southeastern end of the domain, in the Sables d'Olonne area, where extensional tec-tonics is mainly accommodated by pervasive vertical thin-ning of the entire metamorphic series [START_REF] Iglesias | Signification des variations et anoma-lies de la déformation dans un segment la chaîne hercynienne (les séries cristallophylliennes de la Vendée littorale, Massif armoricain)[END_REF][START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF] and 2) the Golfe du Morbihan area and to some extent, the Saint-Nazaire area toward the north-west of the domain, where, conversely, most of extension appear concentrated along major extensional shear zones [e.g. [START_REF] Guillet | Structures d'écoule-ment liées à la mise en place du granite de Guérande (Loire-Atlantique, France)[END_REF][START_REF] Guillet | Anisotropy of magne-tic susceptibility and magmatic structures in the Guérande gra-nite massif (France)[END_REF][START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]. Strikingly, most of these extensional shear zones are marked by large tabular two-mica plutons emplaced during shearing within the base of overlying units [START_REF]Gravimétrie et granites armoricains: structure et mise en place des granites hercyniens[END_REF][START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]Gumiaux, 2004;Turrillot et al., 2011b]. Most of these plutons remain undated.

Initially attributed to a Proterozoic basement [START_REF] Ters | Les synclinoriums paléozoïques et le Précambrien sur la façade occidentale du Massif vendéen[END_REF][START_REF] Wyns | Le Précambrien du massif vendéen. Etude détaillée de deux coupes de référence (coupe de l'Evre et coupe de la Divatte) et synthèse des données récentes[END_REF], migmatites of the Inter-mediate and the Lower units yielded two age groups: a De-vonian group including ages at 376 ± 19 Ma and 372 ± 24 Ma in the Golfe du Morbihan area [START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb): exemples choisis au Groenland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF] and at 388 ± 8 Ma in the Sables d'Olonne area [START_REF] Vidal | L'évolution polyorogénique du Massif armoricain: ap-port de la géochronologie et de la géochimie isotopique du stron-tium[END_REF] by whole-rock Rb/Sr, and zircon U/Pb methods and a younger, ca. 320 Ma age group (i.e. 207 Pb/ 206 Pb 323 ± 4 Ma and 315 ± 9 Ma ages on monazite) [START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb): exemples choisis au Groenland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF]. However, these Devonian ages were recently questioned in the Golfe du Morbihan area by monazite in situ U-Th/Pb chemical dating from a foliation-parallel granitic stock closely asso-ciated to the migmatites that yielded a 322 ± 3 Ma age [START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]. This Late Carboniferous age com-plies with the clustering of many isotopic systems around ca. 315-300 Ma both in the Lower unit and the base of the Intermediate unit, suggesting a common final cooling [START_REF] Carpena | Fission track geochronology of the Hercynian platform in France[END_REF][START_REF]Géochronologie des roches métamorphiques (Rb-Sr et U-Pb): exemples choisis au Groenland, en Laponie, dans le Massif armoricain et en Grande Kabylie[END_REF][START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF]Turrillot et al., 2011a]. Besides, geochronological data of synkinematic crystallisations along the Quiberon and the Sarzeau extensional shear zones yielded similar 305-300 Ma ages [START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF]; Ruffet in Le Hébel, 2002;Turrillot et al., 2011a;Tartèse et al., 2011b].

SAMPLING, METHOD AND PROCEDURE

Sampling strategy and description of the outcrop conditions

Fourteen samples of migmatite were collected from five zones distributed along the coastal South Armorican do-main (fig. 1a), namely, from the NW to the SE, the Hennebont-Quimperlé, the Golfe du Morbihan (fig. 1c), the Vilaine Estuary, the Saint-Nazaire and the Sables d'Olonne areas. Samples were all collected from outcrops showing unambiguous fabrics. Besides, samples sometimes carry evidence of weathering but are systematically devoid of macroscopic alteration. In order to avoid mixing between ageinheritance and melt-related crystallization ages, a par-ticular attention has been paid to separate the juvenile melt fractions, represented by leucosomes either parallel or dis-cordant to the foliation, from cumulative restites (i.e. melanosome) or from remnants of the initial gneiss (i.e. paleosome or mesosome) (fig. 2) [sensu [START_REF] Mehnert | Migmatites and the origin of granitic rocks[END_REF][START_REF] Brown | The definition of metatexis, diatexis and migmatite[END_REF][START_REF] Vanderhaeghe | Migmatites, granites and orogeny: Flow mo-des of partially molten rocks and magmas associated with melt/ solid segregation in orogenic belts[END_REF]. To gather independent age-constraints, this set of samples was completed by 15 granite samples. Among the collection, 5 samples corre-spond to the diffuse-boundary granites often referred to as "anatectic" granites, closely associated with the migmatites [START_REF] De | -Variscan granites from Brittany[END_REF]. The 10 other samples correspond to "late", sharp-boundary granites cutting across the migmatite fabrics either at the scale of the map (fig. 1c) or the scale of the outcrop (fig. 2,3,4). A particular attention was paid on the Golfe du Morbihan area, which is, to date the most stud-ied segment of the coastal South Armorican domain as it of-fers different generations of intrusive rocks of contrasted chemistry and mineralogy (fig. 1c) [START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]. Accordingly, a set of 19 samples was specifically picked from migmatites and different granite generations (i.e. 6 and 13 samples, respectively). Location of all samples is re-ported in figure 1.

The Golfe du Morbihan area

The Golfe du Morbihan area appears as the best exposed occurrence of migmatite of the Lower unit roofed by two major extensional shear zones characterised by opposite kinematics [e.g., [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]. In the core of the Lower unit, migmatites present a N110-120 o E striking subvertical foliation [START_REF] Audren | Evolution structurale de la Bretagne méridionale au Paléozoïque[END_REF][START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]Turrillot et al., , 2011a;;[START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]. Conversely, foliation adopts a gently dipping attitude in the vicinity of shear zones. There, a conspicuous N115 o E stretching lineation is associated with non-coaxial kinematic indicators [e.g. [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]. Several generations of granite were distinguished including anatectic granite bodies that are diffuse within the migmatite and "late" granite plutons and dykes clearly cutting across the migmatite fabrics [e.g. [START_REF] De | -Variscan granites from Brittany[END_REF][START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]. Anatectic granites occur in close association with migmatites as various scale bodies and develops North of Vannes as a 70 km long elon-gated massif, regionally known as the Sainte-Anne d'Auray magmatic complex [START_REF] Cogné | Le Massif armoricain[END_REF][START_REF] De | -Variscan granites from Brittany[END_REF][START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]. Among the "late" intrusive granites, the Carnac magmatic complex is composed of a main pluton and a pervasive dyke array [START_REF] De | -Variscan granites from Brittany[END_REF][START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]. Finally, to the southwest a series of two-mica leucogranite plutons including the Ploemeur, Quiberon and Houat-Hoedic plutons are emplaced within the Intermediate and the Upper units. Intensively sheared during their emplacement, these plutons may have trig-gered, at least locally, strain localisation [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF][START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF].

In the field, depending on the bulk protolith chemistry, on the mechanisms of melt extraction or on subtle differ-ences of peak-metamorphic conditions, migmatites display a wide range of textures and facies (e.g. compare fig. 3a to fig. 3b, fig. 3c and fig. 3d) [START_REF] Brown | The petrogenesis of some migmatites from the presqu'île de Rhuys, southern Brittany, France[END_REF][START_REF] Audren | Evolution structurale de la Bretagne méridionale au Paléozoïque[END_REF][START_REF] Jones | High-temperature 'clockwise' P-T paths and melting in the development of regional migmatites: an example from southern Brittany, France[END_REF][START_REF] Audren | P-T-t-deformation paths recorded by kinzigites during diapirism in the western Variscan belt (Golfe du Morbihan, southern Brittany, France)[END_REF][START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF][START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF]. How-ever, high-grade gneisses, metatexites, diatextites and anatectic granite stocks are unequally distributed and an overall northward increase in the amount of both the in situ melt and the melt collected from the center of the Golfe du Morbihan is classically described [i.e. [START_REF] Audren | Evolution structurale de la Bretagne méridionale au Paléozoïque[END_REF][START_REF] Audren | P-T-t-deformation paths recorded by kinzigites during diapirism in the western Variscan belt (Golfe du Morbihan, southern Brittany, France)[END_REF]. 19 samples were selected throughout the area. 6 samples of migmatite were picked from the northern part of the Ria d'Etel, to the west, to the southern part of Golfe du Morbihan, to the east (fig. 1c). Underlined by the alternation of quartzfeldspar irregular leucosomes and dark biotite (-sillimanitegarnet) melanosomes, the foli-ation is dominantly oriented N110-130 o E and locally isoclinally folded (fig. 3a). Samples 201, 220A and 220B were sampled from leucosome-rich metatexites (fig. 3b). Conversely, samples 220H and 221C were selected from bi-otite-sillimanite-garnet restitic layers (fig. 3c). In order to evaluate the effects of local inheritance on the U-Th/Pb system, samples 220A, 220B and 220H were selected on the same outcrop 10 centimetres apart (fig. 3b). Sample VA1009, located to the north of the Ria d'Etel, is a diatexite that presents a well-developed subhorizontal foliation and a conspicuous N115 o E stretching lineation (fig. 3d). To strengthen the probable genetic link between the migmatites and the anatectic massifs, 3 samples were collected in different petrographic facies throughout the Sainte-Anne d 'Auray massif (i.e. samples 207,211 and 227). Samples 207 and 211 were selected from a porphyritic biotite granite and coarsegrained granite facies (e.g. fig. 3e), while sam-ple 227 was selected from a two-mica fine-grained granite [START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF].

In addition, 13 samples are representative of the "late" intrusive granite massifs (fig. 1c). 3 samples were collected from the Carnac massif to illustrate the petrological vari-ability within the pluton from a biotite (-cordierite) facies (i.e. sample CAP01) to the west, to two-mica granite facies (i.e. samples CAP02 and CAP03). In the field, Carnac gran-ite dykes clearly cut across the migmatite fabrics and the Sainte-Anne d'Auray granite (figs 2 and 3f). 7 samples were collected from the Ploemeur and the Quiberon massifs (i.e. 3 and 4 samples, respectively). The dense sampling in the Ploemeur and the Quiberon two-mica leucogranites reflects the commitment to explore the possible effects of the mylonitic deformation and/or associated fluid circulations on the U-Th/Pb system in monazite. Sample BA122b, collected FIG. 2. -Sampling strategy scheme. For each zone, melt-bearing migmatite samples were picked to gather the opportunity to sample newly-formed material and ultimately newly-formed mo-nazite grains. When possible, associated anatectic, generally foliation-parallel granite bodies were also sampled. Conversely, melanosome layers were col-lected to explore metamorphic record inheritance. Late to post-migmatite granite intrusions that remains poorly dated were also sampled to ensure an independent minimum age for the partial-melting event. Mineral abbreviations are after [START_REF] Whitney | Abbreviations for names of rockforming minerals[END_REF]. . displays evidence of meter-scale top-to-the-West shear zones (fig. 4g) [START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF]. Finally, sample MR102 from the Ploemeur granite and sample QUP03 from the Quiberon granite are typical S-C granite mylonites (fig. 4h) collected along the most deformed zones.

The Hennebont-Quimperlé area

The Hennebont-Quimperlé area is characterised by high-grade paragneisses and migmatites closely associated with anatectic granite bodies (fig. 1). Migmatites present N110-120 o E striking subvertical foliation [START_REF] Cogné | Notice explica-tive de la carte géologique de la France au 1/50 000[END_REF]. To the south, the migmatites present a rather gradual transition to the overlying micaschists units without evi-dence of important detachment zones [START_REF] Cogné | Notice explica-tive de la carte géologique de la France au 1/50 000[END_REF]Béchennec et al., 2012].

In the field, metatexites exhibit a rhythmic alternation of biotite ( sillimanite melanosomes and scarce well-developed quartz-feldspar leucosomes or small-scale anatectic granite stocks (fig. 4a). Laterally, the migmatites evolve to larger-scale lensoid stocks of anatectic granite bodies with 10-100 m typical dimension. Sample VA0609 was picked from a metatexite (fig. 4a). Sample VA0709 was sampled 5 km to the SE along the strike of the foliation from a ca. 2 km thick anatectic granite stock [START_REF] Cogné | Notice explica-tive de la carte géologique de la France au 1/50 000[END_REF]Béchennec et al., 2012].

The Vilaine Estuary area

The Vilaine Estuary area is characterised by a 2 km large band exposing migmatites, limited to the north by the Sainte-Anne d'Auray anatectic granite through a diffuse transition [START_REF] Plaine | Notice explicative de la carte géologique de la France au 1/50 000[END_REF]. To the south, the migmatites are separated from the Intermediate units by a ca. 100-150 m thick gently-dipping mylonite zone belonging to the Sarzeau extensional shear zone [START_REF] Plaine | Notice explicative de la carte géologique de la France au 1/50 000[END_REF][START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]Turrillot et al., , 2011a;;[START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]. While the N120 o E strike of foliation remains consistent regionally, the dip of the metamorphic layers evolves from subvertical in the vicinity of the Sainte-Anne d'Auray granite to gentle (20-40 o ) toward the south and the Sarzeau shear zone [START_REF] Plaine | Notice explicative de la carte géologique de la France au 1/50 000[END_REF][START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF].

At the outcrop-scale, most rocks present conspicuous evidence of partial-melting and the increase in the amount of the in situ melt toward the north, where a ca. 500 m large diffuse zone marks the southern boundary of the Sainte-Anne d'Auray granite massif (fig. 1) [START_REF] Plaine | Notice explicative de la carte géologique de la France au 1/50 000[END_REF]. Two samples, (VA1309 and VA1409) were picked from leucosomerich metatexites at ca. 500-700 m to the south of the Sainte-Anne d'Auray granite massif (fig. 4b).

The Saint-Nazaire area

The Saint-Nazaire area appears as a large outcrop of the Lower unit (fig. 1). The overall structure of the Lower unit defines a structural dome, namely the St-Nazaire dome [START_REF] Cogné | Le Massif armoricain[END_REF], with a dominant gently north-dipping folia-tion to the northwest (i.e. La Baule area) and a gently south-dipping foliation to the south (i.e. Saint Brévin area) [Ters et al., 1978;[START_REF] Ters | Les synclinoriums paléozoïques et le Précambrien sur la façade occidentale du Massif vendéen[END_REF]. On both flanks, the foliation bears a N-S striking lineation [START_REF] Guillet | Structures d'écoule-ment liées à la mise en place du granite de Guérande (Loire-Atlantique, France)[END_REF][START_REF] Guillet | Anisotropy of magne-tic susceptibility and magmatic structures in the Guérande gra-nite massif (France)[END_REF]. To the south, the dome is roofed by an extensional shear zone [e.g. [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF][START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF][START_REF] Bitri | Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/Géofrance 3D Program)[END_REF]. To the north, the contact with the Intermediate unit is underlined by a large-scale syn-kine-matic gently north-dipping laccolith called the Guérande pluton [START_REF]Gravimétrie et granites armoricains: structure et mise en place des granites hercyniens[END_REF][START_REF] Guillet | Anisotropy of magne-tic susceptibility and magmatic structures in the Guérande gra-nite massif (France)[END_REF]).

In the field (mostly along the coast), the migmatites consist of coarse-grained alternations of quartz and feldspar or even pegmatitic layers and of dark biotite, sillimanite and garnet layers. Locally, diatexites and anatectic granite stocks crop out in alternation with massive restitic layers or boudins embedded in a heterogeneous granite (fig. 4c). Sample GnIP03 corresponds to a migmatitic gneiss. Sample VA1909 was collected from a large leucosome stock within typical metatexites (fig. 4d), while sample VA1809 was sampled from a large restitic body (fig. 4e).

The Sables d'Olonne area

The regional-classic coastal section of the Sables d'Olonne area shows a complete Barrovian metamorphic sequence, from low-grade metasediments to the north, to migmatites to the south in the Sable d'Olonne bay [START_REF] Ters | Sur l'extension du Paléozoïque en Vendée littorale. Stratigraphie et structure[END_REF][START_REF] Iglesias | Signification des variations et anoma-lies de la déformation dans un segment la chaîne hercynienne (les séries cristallophylliennes de la Vendée littorale, Massif armoricain)[END_REF][START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Debrand-Passard | Notice explicative de la carte géologique de la France au 1/50 000, feuille Les Sables d'Olonne[END_REF] without evi-dence of major detachment zones [START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]]. E-W-striking and N-dipping in the northern part of the area, and WNW-ESEstriking and NE-dipping in the eastern part (fig. 4f), the overall attitude of the foliation is consistent with the dome-shaped structure [START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF]. Along with meta-morphic grade, the intensity of the foliation strongly in-creases toward the north [START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Debrand-Passard | Notice explicative de la carte géologique de la France au 1/50 000, feuille Les Sables d'Olonne[END_REF][START_REF] Rolin | Les cisaillements hercyniens de la Vendée au Limousin[END_REF]. The foliation carries a clear E-W to WNW-ESE striking stretching lineation [START_REF] Iglesias | Signification des variations et anoma-lies de la déformation dans un segment la chaîne hercynienne (les séries cristallophylliennes de la Vendée littorale, Massif armoricain)[END_REF], while a dominant top-to-the-west sense of shear is observed [START_REF]Combined thrusting and wrenching in the Ibero-Armorican arc: a corner effect during continental colli-sion. -Earth Planet[END_REF][START_REF] Cannat | Linéations N-S en Vendée littorale (Massif armoricain) ; épisodes tangentiels successifs éo-hercy-niens en France occidentale[END_REF][START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF][START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF].

In the field, conspicuous traces of partial-melting are only restricted to the southernmost parts of the section (fig. 1a). There, the N-dipping foliation is locally outlined by cm to dmsized leucosomes. A dense array of variably deformed dykes cuts across the migmatite fabrics (figs. 4f and4g). Most of the dykes display conspicuous internal (i.e. foliation and lineation) or external strain (e.g. folds, boudins; fig. 4g) consistent with the strain axes defined from the migmatites [START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]. Interestingly, some leucosome layers appear connected to the dykes indi-cating a clear synkinematic melt extraction. Samples SP0710 and SP07bis10 were sampled few meters apart from leucosome-poor metatexites of probably contrasted meta-sedimentary protoliths. Rich in biotite and sillimanite, sam-ple SP0710 rather corresponds to a metapelitic protolith while SP07bis10, rich in quartz to an impure mica-rich metaquartzite. Conversely, sample SP0610 was sampled within a granite dyke transposed in the regional foliation (fig. 4g).

Sample preparation, analytical procedure and data processing

Depending on the nature of the rock, preparation followed two different strategies. Because of the intrinsic complexity of migmatite in which neocrystallisation and inheritance are common, migmatite samples were studied on polished thinsections to perform in situ chemical datings on monazite (fig. 2). In situ analysis allows for combining geo-chronology and rock-texture (i.e. phases equilibria and ulti-mately P-T conditions) to make inferences on the crystallization conditions of monazite [e.g. [START_REF] Pyle | Four generations of accessory-phase growth in low-pressure migmatites from SW New Hampshire[END_REF][START_REF] Goncalves | Petrology and in situ U-Th/Pb monazite geochronology of ultrahigh-temperature metamorphism from the Andriamena mafic unit, north-central Madagascar. Significance of a petrographical P-T path in a polymetamorphic context[END_REF], particularly relevant for dat-structural plane. Conversely, granites were mostly studied ing metamorphic rocks and migmatites. To ensure an accu-by mineral separation that provides the largest monazite rate textural control, thin sections were cut in the XZ grains likely to contain useful internal information on the growth and dissolution history of the grains [i.e. [START_REF] Kelsey | Thermobarometric model-ling of zircon and monazite growth in melt-bearing systems: Examples using model metapelitic and metapsammitic granuli-tes[END_REF]. Rocks were first finely crushed using disk grind-ers. Obtained powders were sieved and only the fraction size below 0.5 mm was retained. Then, mineral density sep-aration using heavy liquids, followed by magnetic separa-tion using a Frantz separator were applied to sort the monazite from other accessory heavy minerals. A final opti-cal control based on the morphological characteristics of phosphates was made. The separated monazite grains were hand-picked and mounted in epoxy as plots, with further fine polishing. Mineral separation has been made at BRGM (French Geological Survey). Reader unfamiliar with the de-tailed procedure is referred to recent publications [Be Mezeme, 2005;[START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF].

The same analytical procedure was applied to all sam-ples. Monazite grains have been observed under JSM-6400 JEOL Scanning Electron Microscope (SEM) hosted at Institut des Sciences de la Terre d'Orléans (ISTO). Thin sections and polished plots were first coated by a thin car-bon layer. Acceleration voltage and beam-current intensity were 20 kV and 10 nA, respectively. For thin-sections, SEM observations provided for the distinction of monazite from other accessory minerals, using Energy-Dispersive X-ray spectrometer (EDS) and IdFix Software package for data processing. Then, backscattered electrons (BSE) im-aging mode was used for evaluating intra-grain composition variations within monazite, whereas the textures were ex-amined with secondary electrons (SE) imaging mode. Only BSE images are here presented. Monazite grains were fur-ther analysed with the Cameca SX50 electron probe mi-cro-analyser (EPMA) cooperated by BRGM and ISTO, equipped with four Wavelength Dispersion Spectrometers (WDS). For this study, we have followed the procedure de-scribed by [START_REF] Cocherie | Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total Pb determination: implications for lead behaviour in monazite. -Geochim[END_REF]. The accelerating-voltage and beam-current intensity conditions were routinely fixed to 20 kV and 100 nA providing absolute error of 150 ppm for U, Th and Pb [Cocherie and Albarède, 2001]. Beam cur-rent intensity was increased to 200 nA, for samples SP0710 and SP07bis10, providing lower absolute errors for U, Th and Pb of 105 ppm, 130 ppm and 110 ppm, respectively. As the respective amount of U and Th may significantly vary in natural monazite [START_REF] Linthout | Tripartite division of the system 2REEPO4-CaTh (PO4)2-2ThSiO4, discreditation of brabantite, and recognition of cheralite as the name for members dominated by CaTh (PO4)2. -The Can[END_REF], relative errors for U, Th and Pb can be very low especially when large amount of U and Th are incorporated in the monazite structure, and this may hide the errors due to instrumentation and background measurements. Therefore a minimum value of 2% is as-sumed for the relative errors. Natural and synthetic silicates, phosphates, oxides, and sulfides were used as standards. Total duration of background and peak counts is 240 s for Pb, 200 s for U, and 40 s for other elements (Ca, Ce, Gd, La, Nd, P, Pr, Si, Sm, Th, Y). The EPMA resolution of 2 µm, combined with the high-resolution SEM images al-lows for avoiding the altered domains of the grains with possible significant Pb loss. Structural formulae of monazite (A4B4O16) were also calculated to reveal the composition variations due to chemical substitutions between and within samples. Raw or treated data and chemical tables that can-not be presented here for space limitations are available upon request from the first author. Only the substitution be-tween monazite and cheralite (Lnl 3+ + Si 4+ ↔ Ca 2+ + P 5+ ) is presented here.

Data sorting was further performed using the "EPMA dating" Excel® package [START_REF] Pommier | EPMA dating user's manual: age calculation from electron probe microanalyser measurements of U-Th/Pb[END_REF], to calculate individual U-Th-Pb age. Several filters (15% < Ce < 35%; 9% < P < 17%; Pb > 0,015%; 97% < Total 103%) were then applied to reject anomalous punctual results. Using ISOPLOT Excel(r) package [START_REF] Ludwig | User's manual for Isoplot 3[END_REF], the mean age of each data population was calculated according to two methods:

weighted average (mean) age calculation based on the regression line that intercepts the individual ages, which are weighted by the inverse of their related error. The aberrant values are thus easily rejected and the scattering of individ-ual ages with respect to the average age can be evaluated by calculating the Mean Square Weighted Deviation (MSWD);

isochron U-Th/Pb age following the procedure of Cocherie and Albarède [2001]. As monazite may present a strong variability of the Th/U ratio, Th/Pb versus U/Pb plots can be used to represent the individual data and their respective error (represented by ellipse of confidence). The isochron U-Th/Pb age of the sample was computed from different graphical parameters such as the slope with error of the best-fit line, the value of the Th/Pb and U/Pb inter-cept ages, and the coordinates of the centroid of the best-fit line.

RESULTS

All U-Th-Pb age-results calculated either as isochron U-Th/Pb age or as weighted average age, are presented in figures 5 to 10. A representative BSE image of the internal texture of monazite is given for each sample. For the sake of clarity, results are displayed according to sampling zones discriminated in figure 1.

The Golfe du Morbihan area

Migmatite samples

Sample VA1009, collected in the Ria d'Etel (western part of the Golfe du Morbihan) contains monazites of typical size below 50 µm. Monazite occurs as numerous sub-euhedral grains (fig. 5a) at the boundary between quartz and feldspar and frequently presents patchy zoning due to sectorial variations of U and Th contents. Individual data are scattered along a line in the Th/Pb versus U/Pb plot and a robust best-fit line can be drawn. The theoretical isochron is not strictly parallel to the best-fit line but both are similar within error and an U-Th/Pb isochron age at 325 ± 6 Ma has been calculated (fig. 5a).

Sample 201B contains sub-rounded monazite grains of small size (below 50 µm) set at the boundary between quartz and feldspar or as inclusions in biotite. No compositional zoning has been observed by SEM imaging and the narrow range of Th/Pb and U/Pb ratios do not provide a well-constrained best-fit line. Nevertheless, the best-fit line is simi-lar within error to the theoretical isochron and an isochron U-Th/Pb age at 326 ± 5 Ma was calculated for this migmatite leucosome (fig. 5b).

Samples 220A, 220B and 220H, collected from the same outcrop include monazite of size ranging from 40 µm to 100 µm. In the leucosome (samples 220A and 220B), monazite occurs as sub-euhedral grains at the boundary be-tween quartz and feldspar and rarely as inclusion within bi-otite. Conversely, in the melanosome (sample 220H), monazite occurs as slightly corroded euhedral grains en-sample 220B, the wide range of Th/Pb and U/Pb ratios closed in biotite or garnet. In these samples, no textural and gives a good constraint for the best-fit line, which coincides chemical zoning has been observed or measured. For sam-within error with the theoretical isochron to yield an ple 220A, the best-fit line presents a large error envelop due isochron U-Th/Pb age at 320 ± 5 Ma for this leucosome to close values of the Th/Pb and U/Pb ratios. But the best-fit (fig. 5d). For sample 220H, the values of the Th/Pb and line is similar within error to the theoretical isochron, pro-U/Pb ratios slightly vary and the best-fit line displays a viding an isochron U-Th/Pb age at 323 ± 6 Ma (fig. 5c). For large error envelope that incorporates the theoretical FIG. 5. -Geochronological results from migmatite samples collected in the Golfe du Morbihan area. For Th/Pb vs. U/Pb isochron diagrams, all standard deviations are quoted at 95% confidence level and error ellipses are plotted at 2. Thick (thin) solid line corresponds to best-fit line (envelop error); dotted line is the theoretical isochron for the calculated isochron age and should be comprised in the error envelope. SEM images were obtained using back-scat-tered electrons (BSE) imaging mode. Chemical transect presents the spatial variations of Ca + P and LnL + Si, according to monazite-cheralite substitu-tion. To illustrate internal composition zoning, average UO2 and ThO2 contents are also presented for each grain domain. Mineral abbreviations are after [START_REF] Whitney | Abbreviations for names of rockforming minerals[END_REF]. T refers to as transects across monazite grains.

. isochron. The isochron U-Th/Pb age at 332 ± 6 Ma has been calculated for the melanosome (fig. 5e). Sample 221C (melanosome) contains small monazite grains ranging from 20 µm to 50 µm. The grains are subrounded and occur as inclusions in the biotite and in the garnet. SEM imaging does not reveal significant compositional zoning. The narrow range of Th/Pb and U/Pb ratios do not provide a well-constrained best-fit line, but the error enve-lope includes the theoretical isochron. The isochron U-Th/Pb age at 338 ± 8 Ma has been calculated (fig. 5f).

Anatectic granite samples (Sainte-Anne d'Auray massif)

Sample 207 contains very large sub-euhedral crystals of monazite (up to 200 µm) set at the boundary between quartz and feldspar or enclosed in biotite. Due to variable U and Th contents, a slight compositional zoning was observed (fig. 6a) and a darker core may be distinguished from a lighter rim. This mantle-core texture was additionally re-vealed by high-Ca and P content in the rim and high-REE, Y and Si content in the core (fig. 6b). However, for the anal-yses from the grain core, the range of Th/Pb and U/Pb ratios is very narrow; this leads to extremely large error envelope for the best fit line and high value of MSDW above validity criteria [START_REF] Wendt | The statistical distribution of the mean squared weighted deviation[END_REF]. The Th/Pb versus U/Pb plots cannot be drawn and no isochron U-Th/Pb age has been calculated for this sample. Weighted average (or Mean) ages have been determined instead, yielding ages of 318 ± 3 Ma and 327 ± 9 Ma for the rim and the core, re-spectively (figs. 6c and 6d).

Sample 211 also presents very large monazite grains (up to 200 µm), which are associated with zircon and apa-tite. Grains are euhedral (fig. 6e) or rounded and they do not present any internal variation of composition. The Th/Pb and U/Pb ratios strongly vary and give a robust con-straint to the best-fit line that is parallel to the theoretical isochron. An isochron U-Th/Pb age at 317 ± 6 Ma was thus calculated (fig. 6e).

Sample 227 contains large sub-euhedral crystals of monazite (ca. 100 µm) set at the boundary between quartz, feldspar and biotite or enclosed in biotite. SEM imaging does not reveal any particular internal texture of the grains, as they present homogeneous content in U and Th (fig. 6f). However, based on other elements incorporated in the monazite composition, a rim rich in Ca and P can be distin-guished from a core rich in REE, Y and Si (fig. 6g). The in-dividual analyses that correspond to the rim present weak variations of the Th/Pb and U/Pb ratios, inducing a signifi-cant error for the best-fit line. This latter one is similar within error to the theoretical isochron, though, yielding an isochron U-Th/Pb age at 317 ± 3 Ma (fig. 6h). Conversely, individual data from the core well constrain the best-fit line that parallels the theoretical isochron and an isochron U-Th/Pb age at 335 ± 6 Ma has been calculated (fig. 6i).

Carnac massif samples

Sample CAP01 contains monazite, whose size ranges be-tween 50 µm and 200 µm. The grains are often euhedral and SEM observation has revealed various types of inter-nal zoning, such as patchy zoning, mantle-core zoning or combination of both (fig. 7a). Zoning is caused by signifi-cant variations of the U and Th contents in the monazite.

Variations of the other elements are also observed and two populations, namely a core and a rim can be distinguished. The two calculated isochron U-Th/Pb ages are similar within error (320 ± 4 Ma for the rim and 323 ± 6 Ma for the core) and an average isochron U-Th/Pb age has been com-puted. The Th/Pb and U/Pb ratios are well distributed in the diagram providing very good constraints for the best-fit line that parallels the theoretical isochron. Hence, an isochron U-Th/Pb age at 321 ± 3 Ma was calculated (fig. 7a).

Sample CAP02 presents very large euhedral monazite grains (up to 300 µm) showing growth and patchy zoning. This is due to differences of the U and Th contents, and variations of other elements are also observed. Neverthe-less, it is difficult to discriminate two populations, as ob-served growth zoning reveals successive chemical stages during crystallization. Despite significant variations of the U and Th contents, the range of the Th/Pb and U/Pb ratios is narrow and individual data are grouped, providing poor constraints for the best-fit line. This latter one is similar to the theoretical isochron, though, yielding an isochron U-Th/Pb age at 319 ± 5 Ma (fig. 7b).

In sample CAP03, small to medium-sized (below 100 µm) euhedral grains of monazite do not display evi-dence of zoning. The large variations of the Th/Pb and U/Pb ratios give robust constraints for the best-fit line that paral-lels the theoretical isochron. An isochron U-Th/Pb age has been calculated at 323 ± 5 Ma (fig. 7c).

Ploemeur-Quiberon leucogranite samples

Sample MR100 from the Ploemeur massif contains me-diumsized (80 µm to 150 µm), generally sub-euhedral monazite grains that do not present evidence of zoning. The range of the Th/Pb and U/Pb ratios is wide and well con-strains the best-fit line, which parallels the theoretical isochron. An isochron U-Th/Pb age at 335 ± 6 Ma has been calculated for sample MR100 (fig. 7d). Sample MR101, presents medium size (80 µm to 150 µm) sub-rounded monazite grains. SEM observations did not reveal internal zoning (fig. 7e). Although there are no significant varia-tions of the U and Th contents, the composition of the monazite is heterogeneous. A horseshoe-shaped core with low Ca and P contents and high REE, Y and Si contents can be distinguished from a rim with high Ca and P contents and low REE, Y and Si contents (fig. 7f). Hence two diachronous populations have been distinguished. For the rim, the distribution of the Th/Pb and U/Pb ratios provides a good constrain for the best-fit line and its error envelope that encompasses the theoretical isochron. Analyses from the rim yielded an isochron U-Th/Pb age at 320 ± 8 Ma (fig. 7g). For the core, although the Th/Pb and U/Pb ratios are distributed in two separate groups, the best-fit line is similar within error to the theoretical isochron, yielding an isochron U-Th/Pb age at 333 ± 8 Ma (fig. 7h). Sample MR102 presents large, up to 200 µm euhedral monazite grains. They do not present significant element variations within grains, and no internal texture has been observed un-der SEM. Despite a distribution of the Th/Pb and U/Pb ra-tios into two separate groups, the best-fit line presents a narrow error envelope and parallels the theoretical isochron. An isochron U-Th/Pb age at 316 ± 4 Ma has been calculated for sample MR102 (fig. 7i). Sample BA122b from the Quiberon massif contains medium-sized (30 µm to 80 µm) sub-euhedral monazite grains, set between quartz and feldspar. The grains do not display any textural or compositional zoning, but the range of the Th/Pb and U/Pb ratios is wide, constraining the position of the best-fit line accurately. This latter one is parallel to the theoretical isochron, and an isochron U-Th/Pb age at 320 ± 3 Ma has been calculated for sample BA122b (fig. 8a). Sample QUP01 generally presents large (up to 300 µm) euhedral monazite grains. SEM observations have revealed mantle-core texture in accordance with the significant dif-

Migmatite samples from the Vilaine Estuary area ferences of Th content between

the core and the rim Sample VA1409 presents monazite, whose size ranges be-(fig. 8b). In addition, the core presents low Ca and P contents and high REE, Y and Si contents, whereas the rim is tween 50 µm and 100 µm. The grains are often interstitial relatively rich in Ca and P and relatively poor in REE, Y and set between chloritised biotite, quartz and feldspar. and Si (fig. 8c). Hence, two Th/Pb versus U/Pb plots have Growth, patch and mantle-core zoning (fig. 9f) have been been produced. For the rim, the Th/Pb and U/Pb ratios are observed, but no significant variations of the element congrouped, providing a significant error for the best-fit line.

tents were observed. Conversely, the range of the Th/Pb and This latter one is parallel to the theoretical isochron, U/Pb ratios is wide and the well-constrained best-fit line though, and we have calculated an isochron U-Th/Pb age at parallels the theoretical isochron, yielding an isochron 323 ± 6 Ma (fig. 8d). Conversely, individual Th/Pb and U-Th/Pb age at 318 ± 2 Ma (fig. 9f). Sample VA1309 con-U/Pb ratios from the core display a linear dispersion, which tains large monazite crystals (up to 150 µm). Monazite ocprovides robust constraints for the best-fit line. The best-fit curs as sub-rounded grains, without prominent internal line parallels the theoretical isochron, yielding an isochron zoning and mostly included in biotite, but also as set be-U-Th/Pb age at 330 ± 5 Ma (fig. 8e). Sample QUP03 also tween quartz and feldspar. The relatively large range of the contains large euhedral monazite grains (up to 300 µm), but Th/Pb and U/Pb ratios provides a good constraint for the no textural or composition zoning has been found. Al-best-fit line that is parallel to the theoretical isochron. An though the Th/Pb and U/Pb ratios are divided into two dif-isochron U-Th/Pb age has been calculated at 326 ± 5 Ma ferent groups, the best-fit line presents a relatively narrow (fig. 9g). error envelope and parallels the theoretical isochron. Hence an isochron U-Th/Pb age at 335 ± 6 Ma has been calculated (fig. 8f). Sample QUP04 contains large monazite grains (up to 300 µm) that are often euhedral and present mantle-core texture superimposed to growth zoning (fig. 8g). The rim is slightly lighter than the core because of higher U content. In addition, other elements incorporated in the monazite composition argue for a two-stage grain crystallization, with a rim rich in Ca and P and a core rich in REE, Y and Si (fig. 8h). For the rim, the Th/Pb and U/Pb ratios are sepa-rated into two different groups, but the best-fit line is well constrained. This latter one is similar within error to the theoretical isochron and we have calculated an isochron U-Th/Pb age at 320 ± 4 Ma (fig. 8i). Conversely, the range of the Th/Pb and U/Pb ratios is wide and a larger error for the best-fit line is observed. The best-fit line parallels the theoretical isochron, though. Hence, an isochron U-Th/Pb age at 328 ± 6 Ma has been calculated (fig. 9j).

The Hennebont-Quimperlé area

Migmatite samples from the St-Nazaire area

Sample GnIP03 contains various-sized sub-euhedral monazite grains (30 µm to 200 µm) set between quartz and feldspar or included in biotite. It is worth to notice that each grain presents evidence of incipient alteration (corrosion and dissolution) along crystal cleavage, possibly due to late weathering. Monazite is associated with apatite and zircon (fig. 10a). In Th/Pb versus U/Pb plot, individual data are well scattered and constrain a best-fit line parallel to the theoretical isochron. We have calculated an isochron U-Th/Pb age at 328 ± 4 Ma (fig. 10a). Sample VA1909 presents two morphological types of monazite. The first type is characterised by very large crystals of sub-rounded monazite (up to 300 µm) sometimes with dissolution evi-dence at the quartz, feldspar and biotite grain boundaries (fig. 10b). Conversely, the second type is characterised by rare, medium-sized euhedral monazite (80 µm) included in biotite is rarely observed (fig. 10c). Beyond morphological differences, each population presents distinct composition.

Sample VA0609 contains monazite grains ranging from

The large monazite grains present significant internal ele-60 µm to 100 µm. The grains are included in biotite flakes or ment variations within a grain, with low contents in Ca and set at the boundary between quartz and feldspar. They dis-P and high contents in LREE, Y and Si. Conversely, the play an apparent rim (darker in SEM images), which may small monazite grains present regular element compositions correspond to post-crystallization alteration due to iron (hy-characterized by high Ca and P contents and low LREE, Y dro-) oxides mixtures (fig. 9a). Individual microprobe analy-and Si contents (fig. 10d). The population composed by the ses from these rim domains have been excluded from the large monazite present weak variations of the Th/Pb and isochron U-Th/Pb age calculation. The Th/Pb and U/Pb ra-U/Pb ratios, but the number of individual data (122) well tios present important variations giving a good constraint for constrains the position of the best-fit line and its error. the best-fit line that is very close to the theoretical isochron.

Hence, the best-fit line is similar within error to the theoret-An isochron U-Th/Pb age has been calculated at 319 ± 5 Ma ical isochron, yielding an isochron U-Th/Pb age at 331 ± (fig. 9a). Conversely, sample VA0709 contains small 3 Ma (fig. 10e). For the population of small monazite monazite grains (i.e. less than 60 µm) that were repeatedly grains, Th/Pb and U/Pb ratios slightly vary and the best-fit observed as biotite inclusions. SEM imaging does not reveal line presents a large envelope error, which encompasses the internal zoning, as U and Th contents are relatively constant theoretical isochron. An isochron U-Th/Pb age has been (fig. 9b). But variations of others elements are significant calculated at 319 ± 4 Ma (fig. 10f). Sample VA1809 is a and a Ca and P-poor core was distinguished from a Y and restite that contains medium-sized sub-euhedral monazite Si-rich rim (fig. 9c). The separation of the two inferred popu-grains (ca. 100 µm) set at the boundary between quartz and lations provides two different isochron U-Th/Pb ages, which feldspar or included in quartz, feldspar and chloritised biomeets the validity criteria despite a narrow range for Th/Pb tite. The range of the Th/Pb and U/Pb ratios is wide and the and U/Pb ratios. For the rim, an isochron U-Th/Pb age has best-fit line is well constrained. This latter one is similar been calculated at 318 ± 4 Ma (fig. 9d), whereas the isochron within error to the theoretical isochron, providing an U-Th/Pb age of the core is 328 ± 6 Ma (fig. 9e).

isochron U-Th/Pb age at 352 ± 10 Ma (fig. 10g).

The Sables d'Olonne area

Samples SP0710 and SP07bis10 contain euhedral to subrounded monazite grains, with variable size (up to 200 µm in sample SP0710 and less than 60 µm in sample SP07bis10). In sample SP0710, the grains are set between quartz, feldspar and biotite, whereas in sample SP07bis10, they may also be located as inclusions in biotite. Although both samples present different ranges and distributions of the Th/Pb and U/Pb ratios, the best-fit lines are robust and parallel the respective theoretical isochron. Hence we have calculated an isochron U-Th/Pb age at 320 ± 5 Ma for sam-ple SP0710 (fig. 10h) and an isochron U-Th/Pb age at 323 ± 6 Ma for sample SP07bis10 (fig. 10i). The spatially (and probably genetically) associated granite sample (sample SP0610) contains small sub-rounded monazite grains (less than 50 µm), which often occur at the boundary between quartz, feldspar and muscovite, or rarely trapped as inclu-sion in the muscovite. The Th/Pb and U/Pb ratios are well distributed and provide excellent constrains for the best-fit line. This latter one is similar within error to the theoretical isochron, yielding an isochron U-Th/Pb age at 328 ± 4 Ma for sample SP0610 (fig. 10j).

INTERPRETATIONS : RELIABILITY AND SIGNIFICANCE OF THE AGE DATA

EPMA U-Th/Pb chemical dating of monazite has become an increasingly popular, non-destructive and in situ method providing high-resolution texturally-controlled ages. This method was successfully applied to various types of mag-matic and metamorphic rocks, including migmatites [e.g. Cocherie et al., 2005;Be Mezeme 2005;Be Mezeme et al., 2006;Faure et al., 2008a[START_REF] Faure | Middle Carboniferous crustal melting in the Variscan belt: New insights from U-Th-Pb tot. monazite and U-Pb zircon ages of the Montagne Noire Axial zone (southern French Massif Cen-tral)[END_REF][START_REF] Gagné | -Texture, composition, and age variations in monazite from the lower amphibolite to the granulite facies, Longstaff Bluff Formation, Baffin Island, Canada. -The Can[END_REF][START_REF] Majka | Multiple monazite growth in the Åreskutan migmatite: evidence for a polymetamorphic Caledonian evolution of the Seve nappe complex in west-central Jämtland, Sweden[END_REF]. While new geochronological constraints were accumulated and data reduction methods were refined, the significance of the ages was questioned, as conditions of monazite crystallization remain not clearly understood, es-pecially for metamorphic monazite [e.g. [START_REF] Vavra | Multiple zircon growth and recrystallization during polyphase Late Car-boniferous to Triassic metamorphism in granulites of the Ivrea zone (southern Alps): an ion microprobe (SHRIMP) study. -Contrib[END_REF]Forster et al., 2000Forster et al., , 2002;;Forster and Parrish, 2003;[START_REF] Pyle | Four generations of accessory-phase growth in low-pressure migmatites from SW New Hampshire[END_REF][START_REF] Goncalves | Petrology and in situ U-Th/Pb monazite geochronology of ultrahigh-temperature metamorphism from the Andriamena mafic unit, north-central Madagascar. Significance of a petrographical P-T path in a polymetamorphic context[END_REF][START_REF] Pyle | Temperature-time paths from phosphate accessory phase paragenesis in the Honey Brook Upland and associated cover sequence[END_REF][START_REF] Kelsey | Thermobarometric model-ling of zircon and monazite growth in melt-bearing systems: Examples using model metapelitic and metapsammitic granuli-tes[END_REF][START_REF] Janots | Metamorphic rates in collisional orogeny from in situ allanite and monazite dating[END_REF].

In the following, the reli-ability of the calculated ages and their significance are dis-cussed in term of geological events, in the light of the present knowledge of monazite chemical system.

Reliability of ages

Reliability of the U-Th/Pb chemical ages is based on two main assumptions related to the intrinsic mineral structure of monazite [START_REF] Suzuki | Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by the chemical Th-Utotal Pb isochron ages of monazite, zircon and xenotime[END_REF] including that 1) common Pb is negligible compared to the amount of thorogenic and uranogenic Pb [START_REF] Parrish R | U-Pb dating of monazite and its application to geological problems[END_REF][START_REF] Cocherie | Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total Pb determination: implications for lead behaviour in monazite. -Geochim[END_REF]] and 2) no radiogenic Pb loss has occurred since the closure of the system [e.g. [START_REF] Smith | Lead diffusion in monazite. -Geo-chim[END_REF][START_REF] Cherniak | -Pb diffusion in monazite: a combine RBS/SIMS study[END_REF][START_REF] Paquette J.-L | -Experimental resetting of the U-Th/Pb systems in monazite[END_REF]Gardés et al., 2006]. These hypotheses were verified on nat-ural samples by numerous studies, where results obtained by the chemical method were compared to isotopic tech-niques like TIMS, SIMS and LA-ICPMS [i.e. [START_REF] Cocherie | Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total Pb determination: implications for lead behaviour in monazite. -Geochim[END_REF]2005;[START_REF] Avignant | -X-ray diffraction study of brabantite-monazite solid solutions[END_REF].

The isochron method [Cocherie and Albarède, 2001;[START_REF] Cocherie | -Electron-microprobe dating as a tool for determining the closure of Th-U/Pb systems in migmatitic monazi-tes[END_REF] was used to represent the individual ages and to compute an isochron age for each sample. This method requires a variation of the Th/U ratio to provide a large data span in the Th/Pb vs. U/Pb diagram, constraining the error envelop of the best-fit line. According to [START_REF] Cocherie | -Electron-microprobe dating as a tool for determining the closure of Th-U/Pb systems in migmatitic monazi-tes[END_REF] the computed regression must satisfy 3 criteria: 1) the theoretical isochron age must be comprised in the error envelop of the best-fit line and must crosscut each individual data within the ellipse error; 2) the calculated MSWD must be lower than the statistical param-eter [START_REF] Wendt | The statistical distribution of the mean squared weighted deviation[END_REF] and 3) the two intercept ages (i.e. Th/Pb and U/Pb ages) must be similar within error. These conditions were satisfied for all samples except for sample 207 that presents very low variations of the Th/U ra-tio. In this case, the weighted mean age method was used as an alternative age calculation procedure. Similarly to the isochron method, the calculated age is statistically accept-able, since, for a given population, the calculated MSWD value falls below the deviation parameter (table I) [START_REF] Wendt | The statistical distribution of the mean squared weighted deviation[END_REF]. This method assumes that age errors are symmetric, producing ages, which are usually younger (ca. 3 to 5 Ma) than the age obtained from other methods [Be Mezeme, 2005].

Classification of the monazite grains from the dataset

Validation of all criteria of the U-Th/Pb chemical dating method allows interpreting each calculated age (weighted mean and isochron age) as related to a single geological event. However, monazite grains present morphological and chemical differences and provide a variety of ages for monazite (re-) crystallisation (table I). The occurrence of growth zoning among numerous analyzed grains suggests a more intricate crystallisation evolution for monazite crys-tals, which cannot be portrayed with the time resolution of present geochronological tools. Depending on crystal habits and composition together with phase-relationships, five types of monazite (A1 and A2 types for migmatite, B1, B2 and B3 types for granite) have been recognized as following (fig. 2).

-A1-type monazite grains of various sizes that occur either included in biotite flakes or set at the boundary be-tween quartz and feldspar in the leucosome (migmatite samples VA0609, VA1009, 201B, 220A, 220B, SP0710, SP07bis10, VA1409, VA1309 and GnIP03). Although growth, patchy and mantle-core zoning have been some-times observed among the samples, only one population has been distinguished and a single age for each sample has been calculated. The ages range from 326 ± 5 Ma to 318 ± 6 Ma (table I). For sample GnIP03 (migmatitic gneiss), we cannot ascertain the significance of the age (328 ± 4 Ma), as both melanosome and leucosome parts were collected to-gether. Despite a partial overlap of ages within error brack-ets, this rather old age could show mixing between two age populations of monazite, without significant composition variations.

-A2-type contains small to medium-sized monazites that occur either at grain boundary or enclosed in biotite and garnet in the restite (samples 220H, 221C and VA1809). No internal zoning has been revealed. The calcu-lated ages range between 352 ± 10 and 332 ± 6 Ma and (ta-ble I) that is significantly older than the ages provided by the monazite of the leucosomes.

-Sample VA1909 presents two populations of monazite (table I). Euhedral small grains of medium-sized monazite the age of other leucosome samples. The old age (331 ± 3 Ma) is very similar to the age of restite samples. sharing similarities with A1-type monazite are included in bi--Sub-euhedral to euhedral B1-type monazite up to otite, whereas large crystals of sub-rounded monazite similar 300 µm was identified in granite samples BA122b, MR102, to A2-type monazite are set at grain boundaries and present CAP01, CAP02, CAP03, and 211. The grains that display evidence of corrosion. Each population presents distinct con-growth zoning occur at the boundary between quartz, feldtents in Ca, P, LREE, Y and Si. The age of the population spar and muscovite. The calculated ages range from 316 ± marked by the small grains (319 ± 4 Ma) is consistent with 4 Ma to 323 ± 5 Ma (table I). -B2-type monazite, with a magmatic rim and an inher-ited core was identified in granite samples QUP01, QUP04, MR101, 207, 227 and VA0709. The grains are usually euhedral, medium to large in size. They occur as inclusions in biotite or at the grain boundary between quartz, feldspar growth zoning. The isochron U-Th/Pb ages are bracketed between 338 ± 9 Ma and 328 ± 4 Ma (table I).

Geological significance of the calculated ages and mica. Due to repeatedly observed low variations of U Monazite has been proposed to resist dissolution at temperand Th contents, mantle-core textures were not systematiatures up to 900 o C [START_REF] Braun | Electron microprobe da-ting of monazite from high-grade gneisses and pegmatites of the Kerala Khondalite belt, southern India[END_REF]Cherniak et al., cally observed on BSE images. However, obvious variations 2004], ensuring, at least partially its preservation in the of the Ca, P, Si, REE and Y contents and significant differstudied rocks [START_REF] Brown | The petrogenesis of some migmatites from the presqu'île de Rhuys, southern Brittany, France[END_REF][START_REF] Jones | -Paleozoic continental margin tectonics of southern Armorica[END_REF]Brown, 1989, 1990; ences in the degree of chemical substitution help to distin-Goujou, 1992; Brown and Dallmeyer, 1996; Marchildon guish a core from a rim. The core presents a sub-rounded [START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF][START_REF] Brown | Quantitative constraints on metamor-phism in the Variscides of southern Brittanya complementary pseudosection approach[END_REF]. A2-type shape, showing embayment-like textures. It is always low in monazites from restite present ages ranging between Ca and P and rich in REE, Y and Si. Conversely, the rim is 352 Ma and 332 Ma. This age group may represent inhermarked by euhedral shape and internal growth zoning has ited pre-partial-melting stages, either during the prograde been observed for some grains. Rim and core populations evolution of the rocks, or during an early metamorphic often present different ranges of value for the U/Th ratio.

event. Melt extraction and formation of the leucosomes may The age of the rim vary between 323 ± 6 Ma and 317 ± also provoke encapsulating of these old monazite grains. In 3 Ma, whereas the age of the core is bracketed between addition, monazite solubility remains generally limited in 335 ± 6 Ma and 327 ± 9 Ma (table I).

peraluminous melts [START_REF]Experimental determination of the solubility of Cemonazite in SiO2-Al2O3-K2O-Na2O melts at 800 o C, 2 kbar, under H2O-saturated conditions[END_REF]1987;1993] allowing -B3-type monazite was identified in granite samples the persistence of round-shaped old monazite grains in the QUP03, MR100, and SP0610. The grains are generally leucosome. In this case of partial dissolution, new grains of small (except for sample QUP03) and often occur included monazite may crystallise, while melt cools to form the in biotite or rarely at grain boundary between quartz and leucosome [START_REF] Hawkins | U-Pb monazite, xenotime and titanite geochronological constraints on the prograde to post-peak metamorphic thermal history of Paleoproterozoic migmatites the Grand Canyon, Arizona. -Contrib[END_REF]Rubatto et al., feldspar. The grains do not present internal zoning, such as 2001; [START_REF] Kohn | Five generations of monazite in Langtang gneisses: implications for chronology of the Himalayan metamorphic core[END_REF]. The 326-318 Ma ages of A1-type MSWD is the calculated mean square weighted deviation; DP is theoretical deviation parameter [START_REF] Wendt | The statistical distribution of the mean squared weighted deviation[END_REF]. All ages are isochron ages, except for sample 207 (spotted by *) for which a weighted average age has been calculated. monazite, collected from leucosomes are interpreted as melt crystallisation ages. As monazite corrosion by the melt is limited to the grain boundary [START_REF] Braun | Electron microprobe da-ting of monazite from high-grade gneisses and pegmatites of the Kerala Khondalite belt, southern India[END_REF], mixture of A1-type and A2-type monazites may occur in the leucosome as for sample VA1909.

The occurrence of monazite in granitic rocks depends mainly on the availability of REE and actinides content of the original magma [START_REF] Förster | The chemical composition of REEY-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany, Part I: The monazite (Ce)-brabantite solid solution series[END_REF]. Because of their com-position, peraluminous granites usually host a significant fraction of monazite, which often crystallised at near-soli-dus conditions. B1-type monazite grains occur at grain boundary and repeatedly display growth zoning that reveals successive chemical stages during crystallisation, and inter-action between monazite growth and the melt composition evolution. Their ages (323-316 Ma) can be therefore inter-preted as the crystallization age. As the emplacement of granite pluton is characterized by a cooling period shorter than the error bars of the calculated age, it is generally as-sumed that the age of monazite crystallisation represents the age of granite intrusion [START_REF] Parrish R | U-Pb dating of monazite and its application to geological problems[END_REF]. For several granite intrusions, B2-type monazite grains showing man-tle-core texture and the obvious differences of chemical composition between the rim and the core support crystallisation during two separate stages. Age of the rim (323-317 Ma) is consistent with the age of B1-type monazite, whereas the ages of grain core are significantly older (335-327 Ma). While core ages may represent an in-herited magmatic or metamorphic event, rim ages are inter-preted as the late crystallisation episode, coeval with melt cooling and pluton emplacement. Pre-anatectic monazite may therefore be preserved and coexists with grains formed during late magmatic events. If the melt contains sufficient P and REE, a new generation of monazite can crystallise through nucleation on inherited grains. If the melt is poor in P and REE, no monazite will form during magma crystallisation and only inherited monazite such as B3-type monazite will be found. Their age (338 ± 9 Ma to 328 ± 4 Ma) is very similar to the core age of B2-type monazite grains; hence it may correspond to inherited, magmatic or metamorphic events.

In the lights of these results, it is proposed that sub-solidus ductile deformation and the possible associated fluid circulations did not significantly affect the U-Th/Pb system in monazite, as variously deformed samples of Ploemeur and Quiberon granites present similar ages. This observa-tion is in agreement with geochronological data from other peraluminous plutons associated to ductile shear zone that provided same U-Th/Pb isochron ages on monazite for the undeformed granite and for the mylonite [Choulet et al., 2012]. Conversely, the lack of age reset due to deformation does not comply with other U-Th/Pb isotopic ages from monazite from southern Brittany granites [Tartèse et al., 2011b], whose younger ages can be explained rather by per-vasive fluid circulations along permeable conduits than by methodological bias.

AGE OF PARTIAL-MELTING AND TECTONIC IMPLICATIONS

Among the main results of this study, textural/chemical analyses allow recognising five monazite types. However, conspicuous similarities between different types and results of the U-Th/Pb datings permit to distinguish only two genuinely different generations of monazite in both migmatites and granite rocks: i) inherited grains occur in mesosomes and melanosomes in migmatite (A2 type), as inclusions in biotite (B3 type) or in the core of younger crystals in the granite rocks (B2 type) that yielded ca. 352-330 Ma ages and ii) newly formed grains are found in leucosomes (A1 type) and in (B1 type) crystals and rim around old grains (B2 type) crystals settled at ca. 325-315 Ma. Implications of these ages will be now discussed in the regional and the geodynamical context.

Age-constraints for the evolution of the Lower unit

New U-Th/Pb chemical age-results provide absolute timeconstraints on the evolution of the Lower unit for the whole coastal South Armorican domain (fig. 11). In the Golfe du Morbihan area, ca. 326-318 Ma ages were obtained for A1 monazites from small-scale leucosomes within 4 migmatite samples (fig. 11), interpreted as monazite crystallisation ages during melt cooling. These results are consistent with the parallel dating at ca. 318 ± 3 Ma of B1 monazites from the closely associated large-scale anatectic granite massif (i.e. Sainte-Anne d'Auray). Besides, samples from 4 other migmatite sampling zones yielded crystallization ages for A1 monazites from leucosomes, clustering at ca. 326-318 Ma. Furthermore, these ages are in agreement with the scarce previous time constraints [e.g. [START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF] and support a strikingly synchronous partial-melting event over the whole coastal part of South Armorican domain. Interest-ingly, the solidus was intersected during burial along a typi-cal ca. 30 o C/Km Barrovian gradient (fig. 12) [START_REF] Brown | The petrogenesis of some migmatites from the presqu'île de Rhuys, southern Brittany, France[END_REF][START_REF] Jones | -Paleozoic continental margin tectonics of southern Armorica[END_REF]Brown, 1989, 1990;[START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF][START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF][START_REF] Brown | Quantitative constraints on metamor-phism in the Variscides of southern Brittanya complementary pseudosection approach[END_REF]. Effective melt generation was subse-quently initiated at peak-P metamorphic conditions at ca. 8-9 kbar for 750-800 o C at variance with the examples of the Canadian Cordillera and the Aegean Sea, where partial-melting occurs during decompression some 20 Ma after peak-P conditions [START_REF] Vanderhaeghe | Structural and geochronological constraints on the role of partial-melting during the formation of the Shuswap metamorphic core complex at the latitude of the Thor-Odin dome[END_REF][START_REF] Hinchey | -Paleo-cene-Eocene high-grade metamorphism, anatexis, and deforma-tion in the Thor-Odin dome, Monashee complex, southeastern British Columbia. -Can[END_REF][START_REF] Duchêne | Pressure-tempera-turetime evolution of metamorphic rocks from Naxos (Cycla-des, Greece): Constraints from thermobarometry and Rb/Sr dating. -Geodin[END_REF][START_REF] Jolivet | -Cenozoic geodynamic evolution of the Aegean[END_REF]. Ages of late granite intrusions, such as Carnac, Quiberon and Ploemeur granites also cluster at ca. 320 Ma, at the end of the partial-melting event at conditions of ca. 4-5 kbar for 700-750 o C [Johnson and [START_REF] Brown | Quantitative constraints on metamor-phism in the Variscides of southern Brittanya complementary pseudosection approach[END_REF]. Despite clear cross-cutting relationships at mapscale (fig. 1) and limited single-age error brackets the ability to geochronologically distinguish the different late intrusive granite complexes has not been possible.

Melanosome samples (i.e. A2 monazite type) mostly yielded a subordinate age cluster at ca. 352-332 Ma, which is similar to the inherited ages, repeatedly observed in the B2 monazites from intrusive granites. Apart the ca. 352 Ma age that may have recorded an earlier event, this older age group that significantly differs from the age of partial-melt-ing is relatively new for the South-Armorican domain. Based on microtextural and compositional criteria, this age group is believed to reflect a regional metamorphic event, related to the crossing of monazite producing reactions (fig. 12). Despite a complex behaviour of monazite during the metamorphic evolution, peak-crystallisation conditions mainly occur during increasing P-T conditions from the up-per-greenschist facies and in the amphibolite facies through . monazite-producing reactions in the range of ca. 400 to 650 o C [e.g. [START_REF] Smith | Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists. -Contrib[END_REF][START_REF] Pyle | -Monazite-xenotime-garnet equilibrium in metapelites and a new monazite-garnet thermometer[END_REF][START_REF] Pyle | Temperature-time paths from phosphate accessory phase paragenesis in the Honey Brook Upland and associated cover sequence[END_REF]Forster et al., 2002;[START_REF] Janots | Metamorphic rates in collisional orogeny from in situ allanite and monazite dating[END_REF][START_REF] Spear | Monazite-allanite phase relations in metapelites[END_REF]. This assumption is strengthened by a ca. 338 ± 7 Ma metamorphic age recently retrieved from a non-melted micaschist sample from the overlying Vilaine Estuary unit [START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]. The ca. 335-330 Ma ages therefore probably record monazite crystallisation ages related to the crossing of a monazite producing reaction during increasing P-T conditions in an overall nappe-stacking context [i.e. Goujou, 1992;[START_REF] Brown | Quantitative constraints on metamor-phism in the Variscides of southern Brittanya complementary pseudosection approach[END_REF][START_REF] Gloaguen | Apports d'une étude intégrée sur les relations entre granites et minéralisations filoniennes (Au et Sn-W) en contexte tardi-orogénique (Chaîne hercynienne, Galice centrale, Espagne)[END_REF][START_REF] Janots | Metamorphic rates in collisional orogeny from in situ allanite and monazite dating[END_REF][START_REF] Spear | Monazite-allanite phase relations in metapelites[END_REF].

The ca. 325-320 Ma turning-point in the evolution of the coastal South Armorican domain

Gravitational collapse corresponds to the decay of lateral contrast in the gravitational potential energy stored within orogenic belts by gravity-driven flow and ultimately triggers crustal thinning of the orogenic crust [e.g. [START_REF] Rey | Gravitational col-lapse of the continental crust: definition, regimes and modes[END_REF][START_REF] Vanderhaeghe | Partial-melting and flow of orogens[END_REF]. Gravita-tional collapse appears controlled by three main mechanisms including 1) a decrease or even an inversion in the tectonic forces applied at the boundary of the orogenic system, 2) an of the potential energy caused by convective removal, delamination of the lithospheric mantle root or slab break-off or 3) a decrease in the strength of the thickened orogenic crust due to thermal weakening thereby emphasising the role of partial-melting [e.g. [START_REF] Vanderhaeghe | Partial-melting and flow of orogens[END_REF]. continental crustal units, rich in radiogenic elements, led to an increase of temperature within the orogenic wedge pro-voking partial-melting, the resulting drop in the crustal strength inducing lateral flow of a low-viscosity partially At variance with the examples of the Canadian Cordil-molten layer, like in the recent evolution of the Tibetan plalera or Aegean domain where 1) precise geodetic or teau [START_REF] Nelson | Partially molten middle crust beneath southern Tibet: synthesis of project INDEPTH results[END_REF], or (2) a drastic change of the geophysical-constraints on plate-kinematics, 2) the behav-boundary conditions such as convective removal of the iour of the mantle or 3) the geometry of the continents in-lithospheric mantle, or inception of slab retreat, slab detachvolved in the collision are reasonably well-constrained, the ment or tearing has induced hot asthenospheric upwelling case of the West-European Variscan orogenic segment is which in turn led to coeval extension and partial-melting. quite complex as none of these parameters is accurately At a more local scale, interrelations between partialconstrained. Conversely, one of the most important advan-melting or magmatism in a broad sense, and deformation tages of the coastal South Armorican domain is to offer an were observed from the scale of large-scale plutons (fig. 11) opportunity to study deep-orogenic processes over series of to the scale of the outcrop (fig. 4g) [START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]; crustal-scale sections and more particularly, to understand [START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]. Strain thus benefited of the lowthe role of partial-melting for the late-evolution of the strength of the magma prior to complete crystallisation pro-Variscan orogenic belt.

moting intense shearing of large-scale laccoliths often re-An unexpected major result of this study is the recogni-ferred to as synkinematic plutons [START_REF] Jégouzo | The South Armorican shear zone[END_REF] Gapais et tion of a common, regional-scale probable signature of al., 1993;Tartèse et al., 2011a]. The Ploemeur-Quiberon prograde metamorphism dated at ca. 335-330 Ma and as-leucogranite set of plutons is particularly exemplary of that cribed to crystallisation ages during monazite-forming reac-discussion as it concentrated large amount of strain over the tion(s) before partial-melting [e.g. [START_REF] Gloaguen | Apports d'une étude intégrée sur les relations entre granites et minéralisations filoniennes (Au et Sn-W) en contexte tardi-orogénique (Chaîne hercynienne, Galice centrale, Espagne)[END_REF]Janots km-thick top-to-the-WNW mylonitic Quiberon extensional et al., 2009]. A single, common ca. 326-318 Ma age-cluster shear zone [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]. By accurately constraining was also retrieved for the melt crystallisation age following the emplacement-age of these plutons within non-melted a major partial-melting event throughout the coastal South overriding units, this study retrieved an early increment of Armorican domain. Partial-melting thus occurred at re-the extensional deformation at ca. 321 ± 3 Ma. Cooling and gional-scale some ca. 35-40 m.y. after the youngest age of further strain localisation at the roof of leucogranite subduction-relation HP metamorphism recognised within laccoliths was subsequently recorded by ca. 310-300 Ma the uppermost units [Le Hébel et al., 2002a[START_REF] Hébel | Les « porphyroïdes » de Bretagne méridionale: une unité de HP-BT dans la chaîne hercynienne[END_REF]Bosse Ar/Ar datings (fig. 12) [Ruffet in Le Hébel, 2002;Turrillot, et al., 2005] and ca. 20-25 m.y. after the integration of the 2010]. By comparison with the French Massif Central, the first continental-derived units in the orogenic wedge during Brâme leucogranite laccolith [START_REF] Gébelin | Structure of late Variscan Millevaches leucogranite massif in the French Massif Central: AMS and gravity modelling results[END_REF][START_REF] Gébelin | Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France[END_REF]] collision [START_REF] Maillet | Relations des porphyroïdes et schistes de Saint-Gilles avec les formations siluriennes de Brétignolles-Sur-Mer (Vendée Maritime)[END_REF][START_REF] Colchen | Sur l'âge post-tournaisien de la tectonique hercynienne synschisteuse à Brétignoles-sur-Mer, Vendée littorale, Massif armoricain[END_REF][START_REF] Rolin | Les cisaillements hercyniens de la Vendée au Limousin[END_REF] bounded by the top-to-the-WNW Nantiat mylonitic and [START_REF] Rolin | Les cisaillements hercyniens de la Vendée au Limousin[END_REF]. Traces of initial stacking-structures are extensional shear zone [START_REF] Mollier | Structuration magmatique du complexe granitique de Brâme-St Sylvestre-St Goussaud (Li-mousin, Massif central français)[END_REF]; Faure, however extremely rare and heavily reworked by subse-1989[START_REF] Faure | Late Carboniferous extension in the Variscan French Massif Central[END_REF][START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Wesphalian leucogranite in the Variscan Belt of the northwest Massif Central, France[END_REF], was emplaced quent extensional deformation except at the top of the struc-syntectonically at ca. 324 ± 4 Ma [START_REF] Holliger | -Age carboni-fère de l'unité de Brâme du complexe granitique peralumineux de St Sylvestre (NW du Massif central) défini par les données isotopiques U-Pb sur zircon et monazite[END_REF]. tural pile [e.g. [START_REF] Iglesias | Signification des variations et anoma-lies de la déformation dans un segment la chaîne hercynienne (les séries cristallophylliennes de la Vendée littorale, Massif armoricain)[END_REF][START_REF] Maillet | Relations des porphyroïdes et schistes de Saint-Gilles avec les formations siluriennes de Brétignolles-Sur-Mer (Vendée Maritime)[END_REF] Active exhumation, strain localization and cooling occur [START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF][START_REF] Geoffroy | Tectonique tardi-varisque en failles normales ducti-les en Vendée littorale, Massif armoricain[END_REF]Le Hébel et al., 2002a, from ca. 320 to 305 Ma including a rapid exhumation stage 2002b; [START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican massif, France[END_REF][START_REF] Bosse | -Rb/Sr and 40 Ar/ 39 Ar ages in blueschists from the Île-de-Groix (Armorican Massif, France): Implications for closure mecha-nisms in isotopic systems[END_REF]. The currently outcropping (1.5 mm/yr) at ca. 305 Ma [Scaillet et al., 1996a]. These rearchitecture of the coastal South Armorican Domain thus sults reinforce the recognition of a large-scale WNW-ESE displays large-scale portions of Variscan lower crust heavily strain continuum without significant kinematic change duraffected by partial-melting and pluton intrusions that were ing the exhumation of metamorphic domes from the end of exhumed in several migmatite-cored metamorphic domes the partial-melting event in the upper amphibolite facies at roofed by extensional shear zones [START_REF] Brown | Synergistic effects of melting and deformation: an example from the Variscan belt, western France[END_REF]Turrillot ca. 320 Ma and continued to near surface brittle faulting unet al., 2009; 2011a]). til ca. 302-298 Ma controlling the sedimentation in As inferred from existing P-T constraints of the studied intramontane coal-bearing basins [e.g. Pin and Vielzeuf, migmatite belt [START_REF] Brown | The petrogenesis of some migmatites from the presqu'île de Rhuys, southern Brittany, France[END_REF][START_REF] Jones | -Paleozoic continental margin tectonics of southern Armorica[END_REF]Brown, 1989, 1990;1983;[START_REF] Gébelin | -Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: structural and 40 Ar/ 39 Ar investigations[END_REF]. Similarly, strain localisation Goujou, 1992; [START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF][START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF] over series of major extensional shear zones seem greatly [START_REF] Marchildon | Spatial distribution of melt-bearing structures in anatectic rocks from southern Brittany, France: implications for melt-transfer at grain-scale to orogen-scale[END_REF][START_REF] Brown | Quantitative constraints on metamor-phism in the Variscides of southern Brittanya complementary pseudosection approach[END_REF], the solidus was in-influenced by the emplacement of thin leucogranite tersected during burial along a typical Barrovian collisional laccoliths from ca. 320 Ma [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]Gébelin et gradient (fig. 13) [e.g. Gardien et al., 1997]. Accordingly, in-al., 2006Accordingly, in-al., , 2007Accordingly, in-al., , 2009;;Turrillot et al., 2011]. ception of partial-melting at peak-P conditions therefore co-Geometry, and in some extent, kinematics and timing of incides with a major change in the tectonic regime recorded exhumation of migmatite-cored metamorphic domes of the at regional-scale. The inception and drastic generalisation of coastal South Armorican domain or even of the western partial-melting within the coastal South Armorican orogenic parts French Massif Central are therefore quite comparable segment was coeval with a fast transition from net crustal [START_REF] Caen-Vachette M | -Age radiométrique du granite syntectonique du Gouffre d'Enfer et du métamorphisme hercynien dans la série de basse pression du Pilat (Massif central français)[END_REF][START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF] Burg and thickening to net crustal thinning. Accordingly, partial-melt -Vanderhaeghe, 1993;[START_REF] Van Den Driessche | -Syn-to post-thickening extension: mode and consequences[END_REF][START_REF] Scaillet | Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: II. Thermal modelling and implications for the mechanisms of U-mineralization[END_REF]; ing that must have lowered the overall strength of the [START_REF] Ledru | -The Velay dome (French Massif Central): melt generation and granite emplacement du-ring orogenic evolution[END_REF][START_REF] Vanderhaeghe | Partial-melting and flow of orogens[END_REF]; Cagnard et orogenic crust would be an important factor facilitating the [START_REF] Vanderhaeghe | Structural development of the Naxos migma-tite dome[END_REF][START_REF] Gébelin | Structure of late Variscan Millevaches leucogranite massif in the French Massif Central: AMS and gravity modelling results[END_REF][START_REF] Gébelin | -Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: structural and 40 Ar/ 39 Ar investigations[END_REF][START_REF] Gébelin | Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France[END_REF][START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF]; late-orogenic gravitational collapse of the Armorican Turrillot et al., 2011a]. Besides, as constrained by emplaceorogenic segment or the Variscan belt. However, transition ment ages of synkinematic plutons and both cooling and defrom compression to extension may also imply a contempo-formation ages (see synthesis on figure 12) [START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF]; raneous change of kinematic boundary conditions. Two sce- [START_REF] Augier R | Notice explicative de la carte géologique de la France au 1/50 000, feuille Vannes-Saint-Gildas de Rhuys[END_REF]Tartèse et al., 2011a;2011b, 2012], narios can then be envisaged: (1) continuous stacking of dextral displacement along the South Armorican shear zone FIG.

12. -Correlations between Armorican massif and the French Massif Central. Particularly highlighted are correlations of wrench and extensional shear zo-nes, (leuco-) granite massifs and metamorphic domes (inspired by [START_REF] Gébelin | -Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: structural and 40 Ar/ 39 Ar investigations[END_REF]; data are from Autran and [START_REF] Lameyre | Conclusions générales[END_REF]; [START_REF] Faure | L'amincissement crustal dans la chaîne varisque à par-tir de la déformation ductile des leucogranites du Limousin[END_REF][START_REF] Faure | Late Carboniferous extension in the Variscan French Massif Central[END_REF]; [START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Wesphalian leucogranite in the Variscan Belt of the northwest Massif Central, France[END_REF]; [START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF]; [START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF]; [START_REF] Gébelin | Structure of late Variscan Millevaches leucogranite massif in the French Massif Central: AMS and gravity modelling results[END_REF][START_REF] Gébelin | -Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: structural and 40 Ar/ 39 Ar investigations[END_REF][START_REF] Gébelin | Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France[END_REF]; Turrillot et al. [2011aTurrillot et al. [ , 2011b] ] and unpublished data from our team). Qu.S. appears coeval with extensional tectonics. The common WNW-ESE stretching and the dominant top-to-the-WNW kinematics of the extensional deformation thus imply an overall post-thickening northwestward non-rigid motion of the internal zones with respects to the non-thickened Cen-tral Armorican domain controlled by the South Armorican shear zone. It is therefore tempting to propose that the dextral motion over the South Armorican shear zone would be as a major orogeny-parallel transfer zone as initially pro-posed by [START_REF] Gapais | La zone de cisaille-ment de Quiberon: témoin d'extension de la chaîne Varisque en Bretagne méridionale au Carbonifère[END_REF]. Furthermore, non-rigid tec-tonic escape does imply a free-boundary to the northwest of the orogenic system that may have played a major role in the formation, or at least to the amplification of the Ibero-Armorican arc. Constraints on the full spatial extent of ca. 320-300 Ma extensional tectonics as well as better, understanding of the orocline formation, still remain to be achieved and requiring integration at the scale of the Variscan belt.

FIG. 1

 1 FIG. 1. -Geological maps of the studied area. (a) Simplified geological map of the coastal South Armorican domain [modified after[START_REF] Gapais | Mountain building and exhumation processes through time: inferences from nature and models[END_REF][START_REF] Bitri | Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/Géofrance 3D Program)[END_REF]. Indicated is the location of the samples used for U-Th/Pb chemical datings. (b) Inset is a sketch map of the French Armorican massif and its main subdivisions. The black rectangle is the location of Golfe du Morbihan area presented in figure1c. (c) Simplified geological map of the Golfe du Morbihan area[START_REF] Turrillot | Fusion crustale et évolution tardi-orogénique du Domaine sud armoricain[END_REF]. The location of samples that provided ages for Sainte-Anne d'Auray and Sarzeau granites is also indicated[START_REF] Turrillot | The top-to-the-east Sar-zeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica[END_REF]. Only the units corresponding to the coastal South Armorican domain have been represented. Note the close association between the migmatite fabrics and the anatectic bodies and particularly the Sainte-Anne d'Auray anatectic granite massif. Conversely, the Carnac, Ploemeur and Quiberon granite intrusions clearly cut across the migmatite fa-brics and the Sainte-Anne d'Auray granite. These plutons locally emplaced within the upper units.

  FIG. 3. -Representative pictures of the outcrop conditions of the migmatites and the granites in the Golfe du Morbihan area. (a, b and c) Different outcrops of the Golfe du Morbihan showing the general aspects of the migmatites. Most outcrops are characterized by metatexites (a and b) while some consists in restitic layers (c). (d) Typical outcrop of the Etel area where migmatites present a subhorizontal attitude. (e) Detailed view of the heterogeneous porphyritic biotite granite facies of the Sainte-Anne d'Auray granite, few tens of meters north of the contact with the migmatites. (f) Cross-cutting relationships between the Carnac granite and the migmatites. Note that granite dykes clearly cut across the migmatite fabrics. (g) Deformed facies of the Ploemeur granite. Foliation is heterogeneously developed and most of the deformation concentrates along low-angle top-to-the-W extensional shear zones. (h) S/C mylonites developed at the expense of the Quiberon granite within the Quiberon extensional shear zone.

  FIG. 4. -Representative pictures of the outcrop conditions of the migmatites and the granites in the other sampling areas. (a) Interlayering of paleosome and finely alternating leucosome and melanosome in the Hennebont-Quimperlé area. (b) Leucosome-rich migmatites expo-sed in the Vilaine Estuary area. Note that the paleosome is still locally preserved. (c, d and e) Representative pictures of the migmatites exposed on the coastal outcrop on the Saint Nazaire area. Picture (c) and (d) present different sizes of melt accumulation as foliation-concordant large leucosome (c) or as genuine small-scale anatectic granite stock (d). (f and g) General aspects of the migmatites and the granite dyke array in the Sable d'Olonne area. Note that leucosomes appear sometimes connected to the dykes indicating that melt extraction is synkinematic.

  FIG. 6. -Geochronological results from Sainte-Anne d'Auray granite samples collected in the Golfe du Morbihan area. Details may be found in the caption of figure 5. For sample 207, weighted average ages are presented for the rim (C) and the core (D), as the weak chemical variations preclude isochron age calculation. Thick line corresponds to the mean age. Error bars are plotted at 2s. Mineral abbreviations are after Whitney and Evans [2010].

  FIG. 7. -U-Th/Pb geochronological results from Carnac (A to C) and Ploemeur (D to I) granites collected in the Golfe du Morbihan area. Details are given in the caption of figure 5.

  FIG. 9. -U-Th/Pb geochronological results from migmatite and granite samples collected in the Hennebont-Quimperlé (A to E) and the Vilaine Estuary areas. Details are given in the caption of figure 5.

  FIG. 10. -U-Th/Pb geochronological results from migmatite samples collected in the Saint-Nazaire (A to G) and in the Sables d'Olonne areas (H to J). Details are given in the caption of figure 5.

  FIG. 11. -Synoptic chart of the U-Th/Pb geochronological results across the coastal South Armorican domain. Despite local partial age overlapping, 3 different stages can be distinguished. Old inherited ages remain rare (e.g. the 352 Ma age), whereas a pre-anatectic, prograde episode probably associated to nappe-stacking is recorded throughout the study area at ca. 335-330 Ma. Partial-melting that probably starts at ca. 325 Ma, and became widespread during the 325-320 Ma period, accompanied by the accumulation of large volume of anatectic melts as anatectic granites (e.g. Sainte-Anne d'Auray granite). The ca. 320-318 Ma age is ascribed to late or even post-migmatitic granites (e.g. Carnac, Ploemeur and Quiberon granites).

  FIG. 12. -Correlations between Armorican massif and the French Massif Central. Particularly highlighted are correlations of wrench and extensional shear zo-nes, (leuco-) granite massifs and metamorphic domes (inspired by[START_REF] Gébelin | -Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: structural and 40 Ar/ 39 Ar investigations[END_REF]; data are from Autran and[START_REF] Lameyre | Conclusions générales[END_REF];[START_REF] Faure | L'amincissement crustal dans la chaîne varisque à par-tir de la déformation ductile des leucogranites du Limousin[END_REF][START_REF] Faure | Late Carboniferous extension in the Variscan French Massif Central[END_REF];[START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Wesphalian leucogranite in the Variscan Belt of the northwest Massif Central, France[END_REF];[START_REF]Analyse pétro-structurale dans un avant-pays métamorphique: influence du plutonisme tardi-orogénique varisque sur l'encaissant épi à mésozonal de Vendée[END_REF];[START_REF] Cagnard | Late pervasive crustal-scale extension in the south Armorican Hercynian belt (Vendée, France)[END_REF];[START_REF] Gébelin | Structure of late Variscan Millevaches leucogranite massif in the French Massif Central: AMS and gravity modelling results[END_REF][START_REF] Gébelin | -Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: structural and 40 Ar/ 39 Ar investigations[END_REF][START_REF] Gébelin | Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France[END_REF];Turrillot et al. [2011aTurrillot et al. [ , 2011b] ] and unpublished data from our team). Qu.S.Z.: Quiberon shear zone; Sa.S.Z.: Sarzeau shear zone; N.A.S.Z.: North Armorican shear zone; S.A.S.Z.: South Armorican shear zone; Ch.S.Z.: Cholet shear zone; B.-M.S.Z.: Bussière-Madelène shear zone; Ou.S.Z.: Ouzilly shear zone; Marche shear zone; Am.S.Z.: Ambrugeat shear zone; S.H.F.Z.: Sillon Houiller fault zone. (b) Deformation ages and inferred T-t evolution of the South Armorican shear zone and the associated synkinematic gra-nites. U/Pb ages on zircon (zrn) and monazite (Mnz) are from Tartèse et al. [2011a, 2011b, 2012]; 40 Ar/ 39 Ar time-constraints on muscovite, phengite (Ph) and biotite (Bt) from Ruffet in Le Hébel [2002], Tartèse et al. [2011a, 2011b], Turrillot [2010] and Augier et al. [2011]. (c) P-T-t evolution and inferred T-t evolu-tion of the migmatites of the Golfe du Morbihan area from prograde to final cooling stages. Quantitative P-T path where obtained by MnNCKFMASH pseudo-section modelling [Johnson and Brown, 2004]. Metamorphic facies are after Spear [1993]. 207 Pb/ 206 Pb ages on monazite (Mnz) are from Peucat [1983]; 40 Ar/ 39 Ar time-constraints on hornblende (Hbl),phengite (Ph) and biotite (Bt) from[START_REF] Brown | Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France[END_REF] andTurrillot et al. [2011a]; Rb/Sr on biotite (Bt) data from Peucat[1983]; apatite (Ap) fissiontrack (FT) ages from[START_REF] Carpena | Fission track geochronology of the Hercynian platform in France[END_REF]. Available geochronological time-constraints on the Sarzeau ex-tensional shear zone mylonites are given for comparison (see details inTurrillot et al. [2011a]). Mineral abbreviations are after[START_REF] Whitney | Abbreviations for names of rockforming minerals[END_REF].

TABLE I .

 I -Results of U-Th/Pb geochronology on monazite.

Acknowledgements. -This paper is a contribution in the scientific context of the ERC Rheolith Project and Labex Voltaire (ANR-10-LABX-100-01) projects. We would like to acknowledge particularly Laurent Jolivet, co-PI of the ERC project for discussions at various stages of this work. Financial support for this study was provided by the national 1: 50,000 mapping program of France, by the ISTO-BGRM co-operation in the course of the Paul Turrillot's PhD and by the ERC Rheolith Project. We are indebted to the editor O. Lacombe and the invited editor P. Barbey together with M. Ballèvre and O. Vanderhaeghe for their constructive reviews that have greatly helped to improve the manuscript. The authors are grateful to P. Jézequel (BRGM) for the mineral separation, to S. Janiec and J.G. Badin (ISTO) for the preparation of thin sections of variably weathered samples and to O. Rouer and I. Di Carlo (ISTO) for their assistance during the nu-merous electron microprobe sessions.

SGF guest editor : Pierre Barbey.