Oscar Laurent

Simon Couzinié

Luc S Doucet

Timescales of ultra-high temperature metamorphism and crustal differentiation: Zircon petrochronology from granulite xenoliths of the Variscan French Massif Central

Keywords: UHT metamorphism, partial melting, crustal differentiation, granulite, zircon, Variscan orogeny

come

Introduction

The formation and differentiation of the Earth's continental crust is a two-step process.

New crust produced mainly in magmatic arcs (e.g. [START_REF] Collins | Critical role of water in the formation of continental crust[END_REF][START_REF] Jagoutz | On the importance of crystallization-differentiation for the generation of SiO2-rich melts and the compositional build-up of arc (and continental) crust[END_REF] is reworked during orogenic events through tectonic, metamorphic and/or magmatic processes to produce the continental crust with bulk andesitic composition and layered structure [START_REF] Cawood | The continental record and the generation of continental crust[END_REF][START_REF] Hacker | Continental Lower Crust[END_REF][START_REF] Rudnick | 4.1 -Composition of the Continental Crust[END_REF]. In particular, (ultra-)high-temperature [(U)HT], granulite-facies metamorphism and partial melting of the lower crust, followed by the segregation and ascent of the resulting granitic magmas, is a key process of intracrustal differentiation [START_REF] Harley | A matter of time: The importance of the duration of UHT metamorphism[END_REF]Moyen et al., 2017a;[START_REF] Rudnick | 4.1 -Composition of the Continental Crust[END_REF][START_REF] Zheng | The production of granitic magmas through crustal anatexis at convergent plate boundaries. Lithos, 9th Hutton Symposium on the Origin of Granites and Related Rocks[END_REF]. Mid-to upper crustal granitic plutons and lower crustal granulites would represent the two complementary products of this process i.e., the extracted magma and residual solid respectively [START_REF] Brown | Granite: From genesis to emplacement[END_REF][START_REF] Cawood | The continental record and the generation of continental crust[END_REF][START_REF] Clemens | The granulite -granite connexion[END_REF][START_REF] Clemens | Conditions during the formation of granitic magmas by crustal melting -Hot or cold; drenched, damp or dry?[END_REF][START_REF] Vielzeuf | Granites, granulites, and crustal differentiation[END_REF][START_REF] White | Melt loss and the preservation of granulite facies mineral assemblages[END_REF]. This concept has been validated by phase equilibria modelling [START_REF] White | Calculation of partial melting equilibria in the system Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (NCKFMASH)[END_REF]Yakymchuk and Brown, 2014a), experimental petrology [START_REF] Gao | Experimental melts from crustal rocks: A lithochemical constraint on granite petrogenesis[END_REF] and the investigation of melt inclusions in metamorphic minerals [START_REF] Cesare | Nanogranite" and glassy inclusions: The anatectic melt in migmatites and granulites[END_REF].

Melt segregation and extraction from the lower crust can theoretically happen at the melt connectivity threshold of 5 to 7 vol.% [START_REF] Rabinowicz | Melt segregation under compaction and shear channeling: Application to granitic magma segregation in a continental crust[END_REF][START_REF] Rosenberg | Experimental deformation of partially melted granite revisited: implications for the continental crust[END_REF]. Therefore, melt loss is expected to occur progressively as it is produced along the prograde P-T path of the lower crust [START_REF] Brown | Granite: From genesis to emplacement[END_REF][START_REF] Kelsey | On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings[END_REF][START_REF] White | Melt loss and the preservation of granulite facies mineral assemblages[END_REF]Yakymchuk and Brown, 2014a). Because melt segregation and transport are geologically fast (<100 kyr and down to <1 kyr in duration; [START_REF] Harris | From sediment to granite: timescales of anatexis in the upper crust[END_REF][START_REF] Petford | Granite magma formation, transport and emplacement in the Earth's crust[END_REF][START_REF] Scaillet | Mechanisms and timescales of felsic magma segregation, ascent and emplacement in the Himalaya[END_REF], granite formation should pre-date or at least be synchronous with the lower crustal (U)HT metamorphic peak. However, this hypothesis has not been systematically tested yet, mainly because direct links between granulites and granites are difficult to establish as these represent two different paleo-crustal levels.

Moreover, even when a field connection between granulites and granites is made possible by tectonic reworking or the presence of lower crustal xenoliths, the comparison is entangled by the complex geochronological record of granulites. Zircon U-Pb dates in granulites commonly span several 10s to 100s of Myr [START_REF] Kelly | An integrated microtextural and chemical approach to zircon geochronology: refining the Archaean history of the Napier Complex, east Antarctica[END_REF][START_REF] Kelsey | On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings[END_REF][START_REF] Harley | A matter of time: The importance of the duration of UHT metamorphism[END_REF][START_REF] Harley | The Archaean geological evolution of Enderby Land, Antarctica[END_REF]Taylor et al., 2016 and references therein).

Interpretations of this protracted zircon record vary from pulsed mineral growth in response to metamorphic reactions along different parts of the P-T path (Baldwin et al., 2007;[START_REF] Hermann | Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust[END_REF][START_REF] Rubatto | Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology[END_REF][START_REF] Vavra | Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study[END_REF]; continuous crystallization through melt Zr saturation upon cooling [START_REF] Guergouz | Zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the Ivrea-Verbano and Strona-Ceneri Zones[END_REF][START_REF] Kelsey | Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites[END_REF]Taylor et al., 2015a); igneous crystallization in the case of mafic granulites [START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: insights from a crustal cross section through a Variscan metamorphic core complex[END_REF]Moyen et al., 2017b); or even post-crystallization disturbance of the U-Pb system [START_REF] Taylor | Post-peak, fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block, southern India[END_REF][START_REF] Kunz | Zircon ages in granulite facies rocks: decoupling from geochemistry above 850° C? Contributions to[END_REF].

In this context, better understanding the processes and timescales of crustal differentiation requires to (i) robustly assess the significance of zircon U-Pb dates in lower crustal granulites; and (ii) directly compare these with the age and origin of co-genetic, mid-/upper crustal granite plutons. For this purpose, we present here the results of zircon petrochronology (coupled U-Pb dating, trace element quantification and Lu-Hf isotopic analyses) from lower crustal felsic and mafic granulite xenoliths of the Variscan eastern French Massif Central (E-FMC). These are interpreted to record (U)HT metamorphism, partial melting and magmatic underplating related to the late stages of the Variscan orogeny [START_REF] Downes | Granulitic xenoliths from the French Massif Centralpetrology, Sr and Nd isotope systematics and model age estimates[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)[END_REF][START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Leyreloup | Catazonal xenoliths in French Neogene volcanic rocks: constitution of the Lower Crust 2. Chemical compostion and consequences of the evolution of the French Massif Central Precambrian crust[END_REF], whose mid-to upper crustal expression is well constrained from the study of exposed migmatites and granites [START_REF] Barbey | Cordierite growth textures and the conditions of genesis and emplacement of crustal granitic magmas: the Velay Granite Complex (Massif Central, France)[END_REF][START_REF] Laurent | Protracted, coeval crust and mantle melting during Variscan lateorogenic evolution: U-Pb dating in the eastern French Massif Central[END_REF][START_REF] Montel | Thermobarometry and granite genesis: the Hercynian low-P, high-T Velay anatectic dome (French Massif Central)[END_REF]Moyen et al., 2017a;[START_REF] Villaros | Plutons and domes: the consequences of anatectic magma extraction-example from the southeastern French Massif Central[END_REF], Williamson et al., 1997). They provide therefore an ideal case study to investigate the nature of the parameters controlling U-Pb age distributions in granulites; and how such distribution relates to melt production in, and extraction from the lower crust to eventually differentiate the crust.

Crustal history and structure of the eastern French Massif Central

The E-FMC is part of the Variscan Belt of Western Europe. The E-FMC crust consists of Ediacaran (605-555 Ma) sedimentary rocks (now paragneisses) deposited in a back-arc basin along the northern Gondwana margin which were intruded by igneous rocks (now orthogneisses) during back-arc inversion in the late Ediacaran-Cambrian (545-525 Ma) and subsequent Ordovician (485-450 Ma) extension [START_REF] Chelle-Michou | Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell[END_REF][START_REF] Couzinié | Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin[END_REF][START_REF] Couzinié | Detrital zircon U-Pb-Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin[END_REF]. These lithologies were reworked in the Devonian-Carboniferous through metamorphism and magmatism, associated with convergence and collision between Laurussia and Gondwana (see review in [START_REF] Vanderhaeghe | Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central[END_REF]. Cenozoic rifting was associated with abundant alkaline volcanism that peaked in Miocene-Pliocene times [START_REF] Michon | The evolution of the Massif Central Rift; spatio-temporal distribution of the volcanism[END_REF].

This study focuses on the area of the Velay migmatite-granite dome in the E-FMC (Fig. 1a). The local metamorphic rocks belong to several nappes (Vanderhaeghe et al., 2020): (i) the allochtonous Upper Gneiss Unit, typified by a bimodal meta-igneous association at its base (the so-called "Leptynite-Amphibolite complex") and a Devonian HP-HT metamorphic evolution; (ii) the Lower Gneis Unit, composed of upper amphibolite-facies, migmatitic orthoand paragneisses devoid of HP record; (iii) the Paurautochtonous Unit, comprising lower-grade metasediments. Migmatitic rocks from the Lower Gneiss Unit define a broad structural dome ~100 km in diameter [START_REF] Ledru | The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution[END_REF], decorated by 340-310 Ma granite massifs/dykes and cored by the 310-300 Ma Velay diatexite and associated granites [START_REF] Montel | Thermobarometry and granite genesis: the Hercynian low-P, high-T Velay anatectic dome (French Massif Central)[END_REF][START_REF] Chelle-Michou | Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell[END_REF][START_REF] Didier | Disturbance versus preservation of U-Th-Pb ages in monazite during fluid-rock interaction: textural, chemical and isotopic in situ study in microgranites (Velay Dome, France)[END_REF][START_REF] Laurent | Protracted, coeval crust and mantle melting during Variscan lateorogenic evolution: U-Pb dating in the eastern French Massif Central[END_REF] (Fig. 1a). Petrological and geochemical data demonstrate that most 340-310 Ma granites were sourced in the lower crust [START_REF] Downes | Sr, Nd and Pb isotopic evidence for the lower crustal origin of Hercynian granodiorites and monzogranites, Massif Central, France[END_REF][START_REF] Villaros | Plutons and domes: the consequences of anatectic magma extraction-example from the southeastern French Massif Central[END_REF]. In contrast, the 310-300 Ma Velay diatexite and granites formed from both the accumulation of lower crustal melts/magmas and anatectic melts formed nearly in situ at mid-crustal depths, largely at the expense of orthogneisses [START_REF] Montel | Thermobarometry and granite genesis: the Hercynian low-P, high-T Velay anatectic dome (French Massif Central)[END_REF][START_REF] Williamson | Geochemical constraints on restite composition and unmixing in the Velay anatectic granite, French Massif Central[END_REF][START_REF] Barbey | Cordierite growth textures and the conditions of genesis and emplacement of crustal granitic magmas: the Velay Granite Complex (Massif Central, France)[END_REF][START_REF] Chelle-Michou | Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell[END_REF][START_REF] Villaros | Plutons and domes: the consequences of anatectic magma extraction-example from the southeastern French Massif Central[END_REF][START_REF] Couzinié | When zircon drowns: Elusive geochronological record of water-fluxed orthogneiss melting in the Velay dome (Massif Central, France)[END_REF].

The studied xenolith suite originates from the Bournac site (Fig. 1a) where they are hosted in a phreatomagmatic breccia pipe of Upper Miocene age [START_REF] Couthures | Step-by-step eruptive dynamism in the rift of Velay: the Bournac volcano (Mezenc massif, Massif Central, France)[END_REF]. The basaltic breccia contains numerous enclaves of amphibolite-to granulite-facies crustal rocks [START_REF] Leyreloup | Catazonal xenoliths in French Neogene volcanic rocks: constitution of the Lower Crust 2. Chemical compostion and consequences of the evolution of the French Massif Central Precambrian crust[END_REF]. The granulite-facies xenoliths reached peak metamorphic conditions of up to 6-15 kbar and 800-1000°C (Fig. 1b, [START_REF] Dostal | Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene rocks of the Massif Central, France -Implications for the lower crust[END_REF][START_REF] Downes | Granulitic xenoliths from the French Massif Centralpetrology, Sr and Nd isotope systematics and model age estimates[END_REF][START_REF] Leyreloup | La croûte métamorphique du Sud de la France (Massif Central, Languedoc). Géologie de surface et des enclaves remontées par les volcans cénozoïques : le rôle des intrusions mafiques basicrustales dans la croûte inférieure[END_REF] and were thus interpreted as sampling the lower crust [START_REF] Leyreloup | La croûte métamorphique du Sud de la France (Massif Central, Languedoc). Géologie de surface et des enclaves remontées par les volcans cénozoïques : le rôle des intrusions mafiques basicrustales dans la croûte inférieure[END_REF]. Among these, two distinct lithologies were identified by previous studies: (i) mafic granulites, of igneous origin (47%); (ii) felsic granulites corresponding to meta-sedimentary (42%) or meta-igneous rocks (11%) [START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Dostal | Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene rocks of the Massif Central, France -Implications for the lower crust[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)[END_REF][START_REF] Downes | Granulitic xenoliths from the French Massif Centralpetrology, Sr and Nd isotope systematics and model age estimates[END_REF][START_REF] Leyreloup | La croûte métamorphique du Sud de la France (Massif Central, Languedoc). Géologie de surface et des enclaves remontées par les volcans cénozoïques : le rôle des intrusions mafiques basicrustales dans la croûte inférieure[END_REF][START_REF] Leyreloup | Catazonal xenoliths in French Neogene volcanic rocks: constitution of the Lower Crust 2. Chemical compostion and consequences of the evolution of the French Massif Central Precambrian crust[END_REF].

Whole-rock Nd model ages of the granulite xenoliths range from 1.0 to 1.8 Ga [START_REF] Downes | Granulitic xenoliths from the French Massif Centralpetrology, Sr and Nd isotope systematics and model age estimates[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)[END_REF], similar to those of exposed Variscan rocks in the FMC [START_REF] Downes | Sr, Nd and Pb isotopic evidence for the lower crustal origin of Hercynian granodiorites and monzogranites, Massif Central, France[END_REF]. Published geochronology includes thermal ionization mass spectrometry (TIMS) U-Pb dating of monazite and zircon that yielded a dominant 320-280 Ma range in both meta-sedimentary and mafic meta-igneous xenoliths, interpreted as reflecting granulite-facies metamorphism and coeval crystallization of underplated basaltic magmas, respectively [START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)[END_REF]. Further secondary ion mass spectrometry (SIMS) zircon U-Pb dating in a meta-sedimentary granulite revealed two populations of Neoproterozoic to early Paleozoic, detrital grains (630-430 Ma) and Carboniferous to Permian (360-250 Ma) metamorphic grains [START_REF] Rossi | Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France[END_REF].

The Bournac xenoliths would sample the lower crust formed during the Variscan orogeny and underwent minimal disturbance since then [START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Downes | Granulitic xenoliths from the French Massif Centralpetrology, Sr and Nd isotope systematics and model age estimates[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)[END_REF], notably as this crust segment remained less affected by Tethyan oceanization than other Variscan domains like Corsica [START_REF] Rossi | Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France[END_REF] or the Ivrea Zone [START_REF] Guergouz | Zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the Ivrea-Verbano and Strona-Ceneri Zones[END_REF]. The felsic granulites would represent lower crustal residues after extraction of magmas that rose and accumulated in mid-crustal migmatite units and upper crustal, peraluminous granite bodies (Fig. 1b) [START_REF] Vanderhaeghe | Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central[END_REF][START_REF] Villaros | Plutons and domes: the consequences of anatectic magma extraction-example from the southeastern French Massif Central[END_REF]. The mafic meta-igneous rocks are generally interpreted as under/intraplated mantle-derived magmas [START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)[END_REF] or alternatively as a constituent lithology of the local pre-Variscan crust [START_REF] Dostal | Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene rocks of the Massif Central, France -Implications for the lower crust[END_REF][START_REF] Downes | Granulitic xenoliths from the French Massif Centralpetrology, Sr and Nd isotope systematics and model age estimates[END_REF].

Materials and methods

We have investigated two meta-sedimentary, felsic granulites (HS15-BOU01 and HS15-BOU17) and one meta-igneous, mafic granulite (HS15-BOU07). These were selected on petrological and geochemical grounds as to be best representative of the two dominant lithological groups identified among the lower crustal xenoliths throughout the French Massif Central (see sections 2 and 4.1). We have worked on zircon separates instead of directly on thin sections as to provide a statistically meaningful dataset for each sample. For this purpose, we targeted samples among the largest xenoliths (spheroids 10-15 cm in diameter), which allowed to analyze 80 to 142 grains per sample (in comparison, thin sections contain about 10 useable grains). After sawing out the outermost 2-3 cm-thick edge of the samples, affected by interactions with the host [START_REF] Leyreloup | La croûte métamorphique du Sud de la France (Massif Central, Languedoc). Géologie de surface et des enclaves remontées par les volcans cénozoïques : le rôle des intrusions mafiques basicrustales dans la croûte inférieure[END_REF], the innermost part was selected for thin section preparation and ≳ 500 g powdered down to <80 µm for bulk rock elemental analysis. The remaining material was crushed and sieved to <500 µm for heavy mineral separation. Zircon grains were concentrated using standard techniques (panning, magnetic and heavy liquids separation, handpicking), cast in epoxy resin and polished down to a sub-equatorial section.

Bulk rock major and trace element compositions were determined at the Service d'Analyse des Roches et des Minéraux (CNRS-CRPG, Nancy, France). Major element compositions of minerals were measured in thin sections by electron microprobe analysis using a CAMECA SX100 instrument at Service Microsonde Sud (Université Montpellier 2, France). Thin section cathodoluminescence imaging was performed using a "cold-cathode" Technosyn 8200 Mk2 at CEREGE (Marseille, France) and a "hot-cathode" Lumic HC6-LM at GET (Toulouse, France). Zircon cathodoluminescence imaging was performed at the Laboratoire Magmas et Volcans (Clermont-Ferrand, France) using a Jeol JSM-5910 scanning electron microscope. Simultaneous zircon U-Pb dating and trace element analyses were carried out at ETH Zürich (ETHZ), Switzerland, by laser ablationinductively coupled plasma mass spectrometry (LA-ICP-MS) using a RESOlution S-155 (ASI, Australia) 193 nm (ArF) excimer laser system coupled to an Element XR (Thermo Scientific, Germany) sector-field ICP-MS. Analyses of Lu-Hf isotopes in zircon were performed by by laser ablationmulticollectioninductively coupled plasmamass spectrometry (LA-MC-ICP-MS) at ETHZ using the same laser ablation system as for U-Pb dating and trace elements, attached to a Nu Plasma 2 (Nu Instruments, UK) MC-ICP-MS. Garnet and rutile trace element data were obtained by LA-ICP-MS at the Service ICP-MS of Observatoire Midi-Pyrénées in Toulouse (France), using a NWRfemto (ESI, USA) solid-state laser ablation system coupled to an Element XR sector-field ICP-MS. Phase equilibrium modelling was undertaken using the software Perple_X (version 6.9. 1, Nov. 2021[START_REF] Connolly | The geodynamic equation of state: What and how[END_REF] in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2(-O2), i.e. NCKFMASHT(O).

Details about the analytical and modelling procedures are provided in the Supplementary Material. The data on samples and reference materials are provided in the Data Tables.

Results

Petrography and geochemistry

Felsic granulites HS15-BOU-01 and HS15-BOU-17 (Fig. 2a,b) are composed of a seriate interlobate to polygonal assemblage of quartz (15-25 vol.%), plagioclase (An30-38Ab52-60Or9-16; 20-28 vol.%), slightly perthitic alkali feldspar (An1-5Ab24-33Or63-74; 25-32 vol.%), chemically unzoned garnet (alm51-56pyr39-45grs4sps0-1; 22-27 vol.%) (Supplementary Figures S1 andS2) and prismatic sillimanite, which occurs associated with plagioclase in HS15-BOU-17 (making up 5-6 vol.% of the rock) and only as inclusions within garnet in HS15-BOU-01.

Both hand specimens exhibit mm-scale banding with alternating plagioclase vs. alkali feldspar-quartz layers (Fig. 2). Accessory minerals are rutile, apatite, and zircon. Zircon is mainly located at grain boundaries and rarely as inclusion in garnet (Fig. 2). Whole-rock compositions match those of meta-sedimentary granulite xenoliths from the E-FMC (Fig. 3a,b).

The felsic granulites are selectively enriched in Fe, Mg, Ti and depleted in alkalis at a given SiO2 content with respect to lower-grade metasediments of the local crust (Fig. 3a,b), which likely results from melt loss during anatexis [START_REF] Leyreloup | Catazonal xenoliths in French Neogene volcanic rocks: constitution of the Lower Crust 2. Chemical compostion and consequences of the evolution of the French Massif Central Precambrian crust[END_REF]. The same hypothesis was proposed to explain similar chemical differences between granulite-facies ("stronalites", to which our samples are akin, Fig. 3) and amphibolite-facies ("kinzigites") meta-sediments from the deep crustal section of the Ivrea-Verbano Zone, NW Italy [START_REF] Redler | Migmatites in the Ivrea Zone (NW Italy): Constraints on partial melting and melt loss in metasedimentary rocks from Val Strona di Omegna[END_REF][START_REF] Schnetger | Partial melting during the evolution of the amphibolite-to granulite-facies gneisses of the Ivrea Zone, northern Italy[END_REF]. From a trace element perspective, the felsic granulites are chemically complementary to peraluminous granites from the E-FMC (Fig. 3c).

Mafic granulite sample HS15-BOU-07 consists of a granoblastic assemblage of plagioclase (~60 vol.%), orthopyroxene (~22 vol.%), clinopyroxene (~14 vol.%), ilmenite (~2 vol.%) with accessory apatite and zircon, the latter being either included in major minerals (most often plagioclase) or located along grain boundaries. Both ortho-and clinopyroxene grains appear corroded by a microcrystalline, dark-coloured (in plane-polarized light), Fe-rich but alkali-free alteration rim, possibly representing a former melt film that infiltrated the xenolith.

Whole-rock geochemical data indicate that the xenolith has a quartz-normative, basic (51.7 wt.% SiO2), Al-rich (17.9 wt.% Al2O3) and K2O-poor (0.3 wt.%) composition, lacks typical features of igneous cumulates (e.g. positive Eu anomaly) and shows an affinity to calc-alkaline suites (Fig. 3c). Such composition is consistent with that of a high-Al basaltic liquid [START_REF] Crawford | The origin of island arc high-alumina basalts[END_REF]. These characteristics make HS15-BOU-07 representative of the most common group (~70%) of mafic granulites in the E-FMC [START_REF] Dostal | Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene rocks of the Massif Central, France -Implications for the lower crust[END_REF][START_REF] Downes | Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)[END_REF].

The lower crustal origin of the three xenoliths is supported by their high Vp values of 6.5-6.9 km.s -1 (calculation procedure in the Supplementary Material), matching those of the lowermost crust (6.5-6.8 km.s -1 at 20-28 km depth) in the E-FMC based on seismic data [START_REF] Perrier | Structure profonde du Massif Central français[END_REF].

Thermobarometry and phase equilibrium modelling

The results of pseudosection modelling indicate that the dry (biotite-free), mineralogy of felsic granulite samples entails temperatures >840°C, while the coexistence of garnet, sillimanite and rutile constrains pressures between 6.5 and 10-13 kbar (Fig. 1d and Supplementary Figure S3). It is worth noting that solidus temperatures are about 200 °C higher than the wet solidus of fertile meta-sedimentary rocks. This observation and the preservation of dry mineral assemblages imply that the rocks have experienced significant melt loss [START_REF] Redler | Migmatites in the Ivrea Zone (NW Italy): Constraints on partial melting and melt loss in metasedimentary rocks from Val Strona di Omegna[END_REF][START_REF] White | Melt loss and the preservation of granulite facies mineral assemblages[END_REF], which is in line with bulk-rock geochemistry (see section 4.1 and Fig. 3). Conventional thermobarometry yields equilibration conditions of 825 ± 29°C at 7.8 ± 0.5 kbar and 833 ± 11°C at 6.9 ± 0.7 kbar for HS15-BOU01 and HS15-BOU17, respectively (calculated with the two-feldspar thermometer of [START_REF] Putirka | Thermometers and Barometers for Volcanic Systems[END_REF], and the GASP barometer of Holdaway, 2001, quoted "internal" uncertainties correspond to 2 standard deviations [SD]). The pressure estimated for HS15-BOU01 must be regarded as a minimum as sillimanite is only present as inclusions in garnet, hence possibly not in equilibrium with matrix plagioclase. These conditions fall on the solidus for both samples.

Based on the calculated pseudosection, the absence of garnet and amphibole in mafic granulite HS15-BOU07 indicates temperatures exceeding 820°C for maximum pressures of 8.5-10 kbar (Fig. 1d). Quartz is predicted in low modal amounts (<4 vol.%) at such conditions but was not observed in thin section. The two-pyroxene thermobarometer of [START_REF] Putirka | Thermometers and Barometers for Volcanic Systems[END_REF] yielded temperature-pressure conditions of 904 ± 20°C and 9.4 ± 1.4 kbar which although affected by large uncertainties are consistent with pseudosection analysis.

Zircon data

In all three samples, zircon forms transparent, yellowish to colorless crystals ranging in size from 50 to 350 µm. In felsic granulites, they generally show rounded, nearly equant shapes, with the exception of a few elongated grains (Fig. 4a,b). Crystals are more irregularly shaped to anhedral in mafic granulite HS15-BOU07 (Fig. 4c). Two main zircon domains are identified based on CL images (Fig. 4): (i) entire grains or, more frequently, rounded cores showing well-developed zoning, most commonly oscillatory in felsic granulites and patchy in the mafic granulite; and (ii) entire grains or rims mantling the former, generally showing brighter, homogeneous luminescence and no to weak zoning. The cores show characteristics of resorbed zircon with internal zoning clearly truncated by irregular boundaries, which are either sharp or consist of a dark-CL seam, generally <10 µm-but up to 50 µm-thick (Fig. 4).

Wherever visible, internal structures in the CL-bright, homogeneous rims and grains consist of sector, fir-tree and/or radial zoning, imparting a multifaceted ("soccerball") texture (Fig. 4). The volume ratio between homogeneous rims/grains and zoned cores/grains varies from a grain to another but, at the scale of the sample, homogeneous rims/grains dominate.

The results of LA-(MC-)ICP-MS analyses reveal clear differences between zoned cores/grains and homogeneous rims/grains, in terms of U-Pb dates, Hf isotopic ratios and trace element composition. In all samples, zoned cores/grains show a spread of mainly (sub-) concordant U-Pb dates (Fig. 5). In mafic granulite HS15-BOU07, most analyses (7 out of 11) range from Ordovician to Carboniferous (481 ± 9 to 307 ± 7 Ma) with a few younger spots and define a trend of decreasing εHf(t) with time from +5.0 at ~480 Ma to -2.3 at ~180 Ma (Fig. 6a).

Zoned cores/grains in felsic granulites show dates mainly from Neoproterozoic to Permian (807 ± 25 to 249 ± 8 Ma) (Fig. 5) and a wide range of εHf(t) between +5 and -16 (Fig. 6a), with few outliers at very negative values (down to -58). In contrast, homogeneous rims/grains in all samples show more clustered, Carboniferous to Permian U-Pb dates between 320 and 270 Ma (Fig. 5) with an apparent mode at 280-290 Ma. The εHf(t) values are also more homogeneous in a given sample than results from zoned cores/grains (Fig. 6a While most zoned cores/grains show Ti contents between 10 and 35 ppm regardless their U-Pb date (Fig. 6b), homogeneous rims/grains have on average higher Ti (20-70 ppm).

In felsic granulite samples, homogeneous rims/grains show a clear correlation between Ti content and U-Pb date, with >300 Ma spots having >50 ppm Ti, decreasing to 20-30 ppm at 260-280 Ma (Fig. 6b). Data from the mafic granulite do not show such correlation, yet overlap with the trend defined by homogeneous rims/grains in felsic granulites, at 270-300 Ma and 25-40 ppm Ti (Fig. 6b). In all samples, zoned cores/grains show more scattered and generally higher concentrations of most trace elements (Nb, Y, HREE, U, Th) than homogeneous rims/grains (Fig. 7). In felsic granulites the latter are further characterized by flat HREE profiles (GdN/YbN ~1) and pronounced negative Eu anomalies (EuN/Eu* <0.1) compared with zoned cores/grains (GdN/YbN <0.2 and EuN/Eu* ~0.05-1) (Fig. 7), as well as less pronounced positive Ce anomalies (Supplementary Figure S7). In all samples, single homogeneous rims/grains analyzed multiple times generally show identical U-Pb dates and Hf isotopic ratios within uncertainties (Fig. 4), as well as only modest variations in trace element contents. Whenever analyzable, dark seams separating cores and rims show U-Pb dates and elemental/isotopic compositions that are similar to those of the rims (see examples from HS15-BOU07 in Fig. 4) and are therefore considered together with homogeneous rims/grains in the following.

Discussion

Zircon petrochronology of the Bournac granulites

Felsic granulites

The dichotomy between zoned cores/grains vs. homogeneous grains/rims is typical of zircon from (U)HT felsic granulites and reflects inheritance from the protolith vs. metamorphic growth [START_REF] Kelly | An integrated microtextural and chemical approach to zircon geochronology: refining the Archaean history of the Napier Complex, east Antarctica[END_REF][START_REF] Kunz | Zircon ages in granulite facies rocks: decoupling from geochemistry above 850° C? Contributions to[END_REF][START_REF] Taylor | Accessories after the facts: Constraining the timing, duration and conditions of high-temperature metamorphic processes[END_REF][START_REF] Vavra | Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study[END_REF]. The range of inherited U-Pb ages is consistent with a former detrital zircon population. This range is skewed towards younger (mainly Paleozoic) ages (Fig. 5) compared with the detrital zircon age distribution in exposed meta-sedimentary rocks in the E-FMC, showing a Neoproterozoic (550-800 Ma) peak [START_REF] Chelle-Michou | Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell[END_REF][START_REF] Couzinié | Detrital zircon U-Pb-Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin[END_REF], certainly reflecting Pb loss during the Variscan (U)HT event (see Supplementary Figure S8). The crystallization of homogeneous rims/grains during Variscan metamorphism is confirmed by U-Pb dates overlapping with the Carboniferous to early Permian age of this event in the E-FMC (review in [START_REF] Vanderhaeghe | Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central[END_REF] and their distinctive trace element signature, in line with that of metamorphic zircon from (U)HT granulites (Fig. 7) (e.g. [START_REF] Kelly | An integrated microtextural and chemical approach to zircon geochronology: refining the Archaean history of the Napier Complex, east Antarctica[END_REF][START_REF] Kunz | Zircon ages in granulite facies rocks: decoupling from geochemistry above 850° C? Contributions to[END_REF][START_REF] Rubatto | Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology[END_REF]Taylor et al., 2015a[START_REF] Taylor | Accessories after the facts: Constraining the timing, duration and conditions of high-temperature metamorphic processes[END_REF][START_REF] Taylor | Interpreting granulite facies events through rare earth element partitioning arrays[END_REF].

Trace element data indicate that the homogeneous zircon grains/rims grew at suprasolidus, and up to UHT conditions. The observed low EuN/Eu* ratios (Fig. 7) generally reflects zircon growth in the presence of K-feldspar produced by biotite dehydration melting reactions [START_REF] Rubatto | Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology[END_REF]. The >1 order of magnitude lower Nb values of homogeneous grains/rims relative to zoned cores/grains (Fig. 7) is explained by the presence of rutile throughout zircon crystallization, as this mineral controls most, if not all of the Nb budget of the rocks (bulk TiO2 and Nb contents of ~1 wt.% and 13-18 ppm respectively are balanced by 1 wt.% rutile at 1400-1800 ppm Nb; see Data Table 8). This, together with quartz saturation (see Supplementary Figure S3), indicates that the obtained Ti-in-zircon temperatures of 800-970 °C (Fig. 6) are robust [START_REF] Ferry | New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers[END_REF]. Lastly, the high GdN/YbN and low Y-HREE contents (Fig. 7) argue for zircon crystallization in the presence of garnet, which is confirmed by an evaluation of zircon/garnet REE partitioning. As garnet shows homogeneous M-HREE concentrations (Fig. 8a), apparent partition coefficients were calculated for each zircon analysis relative to the median garnet composition. These show a good match to the experimentally determined zircon/garnet partition coefficients at 900-1000 °C and 7 kbar (Fig. 8b), pointing to zircon growth at UHT conditions in equilibrium with garnet [START_REF] Taylor | Interpreting granulite facies events through rare earth element partitioning arrays[END_REF].

Nevertheless, K-feldspar, rutile and garnet are stable over most of the supra-solidus P-T space (see Fig. 9 and Supplementary Figure S3), which does not elucidate along which portion of the P-T path the homogeneous zircon grains/rims were formed. Prograde zircon growth has been previously proposed to form at the onset of partial melting [START_REF] Hermann | Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust[END_REF][START_REF] Rubatto | Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology[END_REF] and/or in response to the breakdown of Zr-bearing minerals (Baldwin et al., 2007;[START_REF] Vavra | Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study[END_REF]. However, none of these processes appears likely in our case. The minimum Ti-in-zircon temperatures (ca. 800 °C) exceed the prograde solidus by >100 °C (Fig. 9) and trace element data point to zircon growth in equilibrium with, not at the expense of other Zr-bearing minerals (rutile and garnet). Most importantly, homogeneous grains/rims show a clear trend of decreasing Ti contents with U-Pb dates, entailing continuous zircon crystallization during progressive cooling of the system, from ~940-970 °C to ~800 °C (Fig. 6). This trend is best explained by zircon formation through decreasing Zr solubility of the silicate melt along the retrograde path [START_REF] Kelsey | Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites[END_REF][START_REF] Mintrone | Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes[END_REF]Taylor et al., 2015a;Yakymchuk and Brown, 2014). This interpretation is further supported by (i) a rough increase of zircon/garnet HREE partition coefficients towards younger ages (Fig. 8c), consistent with a progressive temperature decrease [START_REF] Taylor | Interpreting granulite facies events through rare earth element partitioning arrays[END_REF]; and

(ii) limited εHf(t) scatter (~5 εHf-units) compared to the cores (>20 εHf-units) and no variation of εHf(t) with age (Fig. 6), arguing for continuous crystallization from a relatively homogeneous melt phase.

The correlations between U-Pb dates and key zircon trace element indicators make the ~50 Myr spread of U-Pb dates shown by homogeneous grains/rims between 315 and 265 Ma (Figs. 5,6) indicative of a truly protracted period of zircon crystallization. Analytical artifacts, such as ablation craters mixing cores and rims, can be ruled out as multiple analyses in single domains show identical dates (Fig. 4) and mixing trends between cores and rims are absent from the data (Figs. 6,7). The low α-doses and Ti-in-zircon temperatures below their specific closure temperatures (Supplementary Figures S8 andS9) make post-crystallization disturbance of the U-Pb system equally unlikely. Therefore, we collectively interpret the zircon trace element and U-Pb data to reflect continuous, supra-solidus cooling from the thermal peak at UHT conditions (940-970 °C, highest-temperature zircons; Fig. 6) down to the solidus (~800 °C; lowest-temperature zircons).

Mafic granulite

The dominant population of dates from homogeneous rims/grains at 284.7 ± 2.7 Ma in sample HS15-BOU07 (Fig. 5 and Supplementary Figure S6) could be interpreted as reflecting either crystallization of a mafic magma or metamorphism of a pre-existing crustal rock, as is generally the case in mafic granulites (e.g. Moyen et al., 2017b). In fact, the Bournac mafic granulites were originally interpreted as metamorphosed, pre-Variscan mafic rocks [START_REF] Dostal | Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene rocks of the Massif Central, France -Implications for the lower crust[END_REF] but subsequent zircon U-Pb dating led most authors to conclude that they represent underplated, Permian mafic magmas [START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Féménias | A Permian underplating event in late-to post-orogenic tectonic setting. Evidence from the mafic-ultramafic layered xenoliths from Beaunit (French Massif Central)[END_REF][START_REF] Rossi | Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France[END_REF]. The latter interpretation would be apparently consistent with widespread Permian magmatism recorded throughout the Variscan Belt [START_REF] Boscaini | Permian post-collisional basic magmatism from Corsica to the Southeastern Alps[END_REF], notably the ~283 Ma Mafic Complex of the Ivrea-Verbano Zone in NW Italy [START_REF] Karakas | The pace of crustal-scale magma accretion and differentiation beneath silicic caldera volcanoes[END_REF]. However, the trace element composition of homogeneous rims/grains in HS15-BOU07 is distinct from that of typical igneous zircon and plots off the magma differentiation trend defined by the Ivrea-Verbano zircons (Fig. 7). In particular, Permian zircons in HS15-BOU07 are richer in Ti (25-40 ppm) than magmatic zircons from the Mafic Complex of the Ivrea-Verbano Zone (5-30 ppm) (Fig. 6), pointing to different environments and/or conditions of zircon formation.

Furthermore, if the rock derives from a Permian mafic magma, the zoned zircon cores with older U-Pb dates and distinct trace element compositions must be interpreted as incompletely dissolved xenocrysts. Such xenocrysts would show a random sampling of compositionally diverse zircons from the country rocks, comparable to zoned cores/grains from the felsic granulites. In contrast, the zircon cores from HS15-BOU07 define a single trend of closed-system 176 Hf/ 177 Hf radiogenic ingrowth (Fig. 6a) and a tight compositional cluster in terms of trace element concentrations (Fig. 7). Hence, these cores likely represent a coherent, single zircon population crystallized at the same time, whose present-day range in U-Pb dates only results from variable extent of Pb loss [START_REF] Laurent | A linear Hf isotope-age array despite different granitoid sources and complex Archean geodynamics: Example from the Pietersburg block (South Africa)[END_REF]. Therefore, we interpret zoned cores from HS15-BOU07 as reflecting former igneous zircon crystallized from the protolith, whereas the homogeneous rims/grains correspond to Variscan metamorphic zircon. The Ordovician age of the oldest zircon core (481 ± 8 Ma) thereby places a minimum age for the mafic protolith. Following [START_REF] Dostal | Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene rocks of the Massif Central, France -Implications for the lower crust[END_REF], the mafic granulite would thus correspond to a pre-Variscan, meta-igneous rock that underwent the same UHT metamorphic and partial melting history as the meta-sedimentary granulites. The homogeneous grains/rims in HS15-BOU07 likely formed through the same process as in the felsic granulites, i.e. crystallization from a cooling melt. This is supported by their homogeneous trace element compositions and εHf (Figs. 6a,7) and overlap with homogeneous zircon rims/grains from felsic granulites in the Ti vs. U-Pb date plot (Fig. 6b).

Although the mafic granulite contains neither quartz, nor rutile, the Ti-in-zircon temperatures are probably not significantly different from those obtained from the felsic granulites at given Ti content (using aSiO2 = 0.9 and aTiO2 = 0.7 would shift temperature up by 25 °C, which roughly corresponds to the uncertainty). In this case, the shorter range of U-Pb dates in the mafic granulite may reflect the lower Zr content of the sample (124 ppm) compared to felsic granulites (222-234 ppm), which would have the effect of delaying zircon saturation along the cooling path [START_REF] Kelsey | Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites[END_REF][START_REF] Mintrone | Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes[END_REF].

Melt production and loss during Variscan UHT metamorphism

Our new data demonstrate that the investigated granulites represent pre-Variscan metasedimentary and meta-igneous mafic rocks metamorphosed at UHT conditions during the late Variscan orogeny. Mineralogy (Fig. 2) and whole-rock compositions (Fig. 3) indicate that the rocks must have lost melt to preserve anhydrous mineral assemblages, which is generally considered to happen along the prograde branch of the P-T path [START_REF] Brown | Granite: From genesis to emplacement[END_REF][START_REF] Kelsey | On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings[END_REF][START_REF] White | Melt loss and the preservation of granulite facies mineral assemblages[END_REF]Yakymchuk and Brown, 2014a). However, the homogeneous zircon rims/grains in all samples must have formed through melt saturation upon cooling from the thermal peak to the solidus (see section 5.1), meaning that at least some melt was present along the retrograde path.

To estimate the amount of melt required to explain the crystallization of homogeneous zircon rims/grains, we have applied the melt-o-meter method of [START_REF] Mintrone | Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes[END_REF]. This method uses the Zr mass balance between the bulk sample composition, the relative proportion of zircon newly crystallized from melt and the theoretical Zr solubility of such melt at inferred peak temperatures (from the model of [START_REF] Boehnke | Zircon saturation rerevisited[END_REF], i.e. 940-970 °C here (see Supplementary Material for details). The method does not intend to precisely quantify the melt fraction but only give a first-order estimate to be compared to theoretical melt productivities at peak conditions. The calculated fractions of newly precipitated zircon (FPZ) are 67 and 83 wt.% for HS15-BOU01 and HS15-BOU17 respectively, which corresponds to 11-13 wt.% (ca. 15 vol.%) melt. For the mafic granulite, the calculated FPZ is 97 wt.%, which yields ca. 7 wt.% (ca. 10 vol.%) melt at 940-970 °C. For the two felsic granulite samples, pseudosections calculated with bulk compositions adjusted to fit the predicted peak melt volume fractions (details on the melt reintegration procedure are in the Supplementary Material) predict a near-solidus (810-840 °C) field characterized by decreasing garnet mode, breaking down to biotite (Fig. 9). This reaction is not observed in the samples and would contradict zircon/garnet REE partitioning (Fig. 8), which indicates garnet stability throughout the range of zircon crystallization temperatures. Therefore, retrograde melt loss must have taken place at temperatures only slightly above 840 °C, preventing hydration of the peak mineral assemblage yet allowing for zircon crystallization from the melt phase upon cooling.

The calculated peak melt proportions (10-15 vol.%) are lower than theoretical maximum melt productivities at 5-10 kbar and 940-970 °C (30-60 vol.% and 20-40 vol.% for meta-sedimentary vs. mafic rocks, respectively; [START_REF] Palin | High-grade metamorphism and partial melting of basic and intermediate rocks[END_REF]Webb et al., 2015 and references therein). This confirms that the E-FMC granulites have also lost substantial amounts of melt along the prograde path. Melt reintegration calculations indicate that up to 18 to 24 vol.% melt would have been lost along the prograde path for HS15-BOU01 and HS15-BOU17, respectively. This also explains the preservation of protolith zircon despite reaching UHT conditions, as prograde melt loss tends to shift up-temperature the dissolved zircon fraction isopleths [START_REF] Kelsey | Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites[END_REF]Yakymchuk and Brown, 2014b). Melt-reintegrated compositions also highlight the prograde growth of garnet as a peritectic phase (Fig. 9). Zircon thus begins to form at peak temperature in the presence of both garnet and residual melt, explaining the well-equilibrated zircon/garnet REE patterns (Fig. 8).

In summary, we propose that the Variscan P-T metamorphic history of the Bournac granulites involves: (i) prograde heating and partial melting from the solidus up to 940-970 °C (at 8 ± 2 kbar) with concomitant dissolution of protolith zircon, together with garnet growth and melt drainage reaching a cumulative loss of minimum 18-24 vol.% melt in the felsic granulites; followed by (ii) cooling with <15 vol.% residual melt and zircon crystallization, with garnet remaining stable owing to at least one last melt drainage event close to ca. 840 °C in the felsic granulites.

Timescales of melt production, extraction and crystallization in the crust

Data from our granulite xenoliths record prograde melt production and loss, followed by cooling associated with zircon crystallization and limited further melt drainage. Although other lower crustal regions might have recorded more complex histories of melt residence and transfer, the clear timing of zircon crystallization relative to the simple sequence recorded in our samples allows to temporally benchmark the thermal evolution of the lower crust and intracrustal differentiation (Fig. 10). Indeed, based on these interpretations, the oldest homogeneous zircon rims/grains analyses from meta-sedimentary granulites should provide a minimum age for the Variscan lower crustal thermal peak in the E-FMC. The ages obtained from analyses statistically defining single populations (and comprising the spots with highest Ti-in-zircon temperatures) are 313.5 ± 3.6 Ma (MSWD = 1.8; n = 9) for HS15-BOU01 and 312.8 ± 3.1 Ma (MSWD = 1.5; n = 27) for HS15-BOU17 (Fig. 10a).

The co-genetic nature of peraluminous granites from the EFMC with the investigated felsic granulites has been established by phase equilibria modelling [START_REF] Villaros | Plutons and domes: the consequences of anatectic magma extraction-example from the southeastern French Massif Central[END_REF], which is confirmed by the overlapping zircon εHf(t) between granites and granulites (Fig. 6a).

The latter show on average more negative εHf(t), but this is expected whenever zircon remains a residual phase [START_REF] Laurent | A linear Hf isotope-age array despite different granitoid sources and complex Archean geodynamics: Example from the Pietersburg block (South Africa)[END_REF]. Based on the theory of melt loss in migmatites, the granites should have formed prior to or synchronously with the lower crustal thermal peak [START_REF] Rosenberg | Experimental deformation of partially melted granite revisited: implications for the continental crust[END_REF][START_REF] White | Melt loss and the preservation of granulite facies mineral assemblages[END_REF]Yakymchuk and Brown, 2014a). This is confirmed by the distribution of zircon and monazite U-Pb ages from granites within an area of ~50 km radius centered on the Bournac pipe (Fig. 10b). The number of U-Pb dates indeed increases progressively from 340 to 310 Ma, reflecting increasing melt loss along the lower crustal prograde path, with a maximum at 320-310 Ma that strikingly coincides with the inferred 313 ± 3 Ma age for the thermal peak (Fig. 10b). This is clear evidence for direct genetic and temporal links between lower crustal granulites and upper crustal granites within the framework of the melt loss theory.

In contrast, the mid-crustal Velay diatexites and granites show a highest density of U-Pb ages at 310-300 Ma, interpreted as reflecting their crystallization some 5-10 Ma after the lower crustal thermal peak (Fig. 10b). Numerical models considering the fundamental role played by melt transfer on the thermal structure of the orogenic crust predict that peak temperature conditions should be attained synchronously in the lower and the middle crust [START_REF] Depine | Near-isothermal conditions in the middle and lower crust induced by melt migration[END_REF]. Following this, it can be inferred that the Velay magmas (of mixed lower and middle crustal origin) were formed/accumulated at the latest by 313 ± 3 Ma. Their delayed crystallization would thus indicate the prevalence of a partially molten mid-crustal layer for several Myr [START_REF] Vanderhaeghe | Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central[END_REF], in contrast to the faster cooling and crystallization of granitic plutons at higher structural levels. Additionally, melts extracted along the lower crust retrograde path possibly fed the mid-crustal partially molten layer. These melts could also have formed the small-volume silicic volcanic rocks/tuffs exposed throughout the EFMC in early Permian (299-283 Ma), post-orogenic sedimentary basins [START_REF] Michel | Chronostratigraphy and Paleoclimatology of the Lodève Basin, France: Evidence for a pantropical aridification event across the Carboniferous-Permian boundary[END_REF].

The time lag between the crystallization of the oldest upper crustal granites (ca. 340 Ma) and the thermal peak (ca. 313 Ma) constrains the duration of suprasolidus heating in the lower crust (700-970 °C) to a maximum of ca. 27 Myr (Fig. 9), from which a minimum heating rate of 10 °C/Myr can be deduced. Lithospheric mantle delamination has been proposed as a heat source for Variscan (U)HT metamorphism in the eastern FMC [START_REF] Laurent | Protracted, coeval crust and mantle melting during Variscan lateorogenic evolution: U-Pb dating in the eastern French Massif Central[END_REF][START_REF] Vanderhaeghe | Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central[END_REF]. Thermal models show that at reasonable mantle heat fluxes (<50 mW m -2 ), the resulting abrupt temperature increase at the Moho would entail crustal melting and melt-assisted heat advection upwards in the crust for 25-30 Myr until a steady-state geotherm is reached [START_REF] Depine | Near-isothermal conditions in the middle and lower crust induced by melt migration[END_REF], in excellent agreement with our timing constraints.

Conversely, lower crustal cooling from peak down to ca. 800°C is reflected by protracted zircon crystallization between 315 and 265 Ma (Fig. 10b), corresponding to a slower cooling rate of 3.0 ± 1.5 °C/Myr (Fig. 6b), similar to estimates obtained from other granulite terranes (e.g. [START_REF] Guergouz | Zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the Ivrea-Verbano and Strona-Ceneri Zones[END_REF][START_REF] Harley | The Archaean geological evolution of Enderby Land, Antarctica[END_REF][START_REF] Harley | The origins of granulites: a metamorphic perspective[END_REF][START_REF] Kelsey | Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites[END_REF][START_REF] Mezger | U-Pb geochronology of the Grenville Orogen of Ontario and New York: constraints on ancient crustal tectonics[END_REF]. Such slow cooling rate is consistent with the long-lived presence of melt in the lower crust. Indeed, melt crystallization releases latent heat and promotes lateral flow of the weakened lower crust, which hinders cooling associated with tectonic exhumation [START_REF] Vanderhaeghe | Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central[END_REF].

Conclusions and implications

Granulite xenoliths sampled by the Miocene Bournac pipe in the eastern FMC represent pre-Variscan lower crustal rocks that underwent the same Variscan UHT metamorphic episode. Partial melting started at ~340 Ma in response to lithospheric mantle delamination and was followed by heating up to peak temperatures of 940-970 °C (at 6-10 kbar) reached at 313 ± 3 Ma. Significant melt loss throughout the prograde path and at peak temperature contributed to form voluminous, 340-310 Ma upper crustal peraluminous granite plutons and mid-crustal diatexites. However, the lower crust was not completely melt-depleted at peak conditions, as the cooling (hence Zr saturation) of residual melt (~15 vol.%) is required to explain zircon formation, in particular the trend of decreasing Ti content throughout the ~50 Myr period of zircon crystallization in felsic granulites. Most of the melt left in the system was eventually drained along this retrograde path before reaching temperatures of ~840 °C (at ~280 Ma), allowing the preservation of dry assemblages in the lower crust and explaining minor volumes of late granites/rhyolites in the upper crust.

These results have important implications for the interpretation of U-Pb dates in UHT granulites and the link between granulites and granites in orogenic systems:

-Although most of the melt loss from the lower crust happens along the prograde path, some UHT granulites may not be melt-free at peak conditions and non-negligible melt loss can also take place along the retrograde path, accounting for the generally slow cooling rates observed in these settings.

-In addition to the duration spent above a certain temperature [START_REF] Harley | A matter of time: The importance of the duration of UHT metamorphism[END_REF], the zircon crystallization time span in these UHT granulites also depends on the amount of melt still present at peak conditions and the Zr budget of the partly molten system, as melt cooling and saturation is the dominant zircon-forming reaction in this case.

-As a corollary, sharp peak of zircon dates in mafic granulites may me mistaken as an igneous crystallization age, whereas it rather reflects late zircon saturation closer to the solidus due to lower Zr contents than in meta-sedimentary granulites.

-

The relative timing of zircon crystallization in co-genetic upper crustal granites and lower crustal granulites is a key information to understand the processes by which they are linked. In particular, prograde/peak melt loss from the lower crust to form upper crustal granites, followed by zircon saturation in granulites upon retrograde cooling, is expected to decouple U-Pb crystallization ages between the upper and lower crust, as observed here. Italy [START_REF] Karakas | The pace of crustal-scale magma accretion and differentiation beneath silicic caldera volcanoes[END_REF][START_REF] Storck | Mantle versus crustal contributions in crustal-scale magmatic systems (Sesia Magmatic System, northern Italy) from coupling Hf isotopes and numerical modelling[END_REF] and granites from the E-FMC (Moyen et al., 2017b, with estimated zircon Ti contents based on magma temperatures proposed by [START_REF] Laurent | Protracted, coeval crust and mantle melting during Variscan lateorogenic evolution: U-Pb dating in the eastern French Massif Central[END_REF]. In (a) the orange arrow represents the isotopic evolution of bulk crust with a 176 Lu/ 177 Hf ratio of 0.015 (yellow); and the green arrows are Pb loss trends for zircon cores and homogeneous grains/rims from HS15-BOU07 using the average 176 Lu/ 177 Hf of each group.

Crosses labelled "2 SE" correspond to the typical 2σ uncertainties. environments (as compiled in Laurent et al., 2022), (U)HT migmatites and granulites [START_REF] Kelly | An integrated microtextural and chemical approach to zircon geochronology: refining the Archaean history of the Napier Complex, east Antarctica[END_REF][START_REF] Kunz | Zircon ages in granulite facies rocks: decoupling from geochemistry above 850° C? Contributions to[END_REF][START_REF] Rubatto | Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology[END_REF]Taylor et al., 2015a) and the Ivrea Mafic Complex [START_REF] Karakas | The pace of crustal-scale magma accretion and differentiation beneath silicic caldera volcanoes[END_REF]. For the latter, the arrows depicts the igneous evolution from the most mafic to the most evolved magmas. -Michou et al., 2017;Couzinié et al., 2014;[START_REF] Didier | Disturbance versus preservation of U-Th-Pb ages in monazite during fluid-rock interaction: textural, chemical and isotopic in situ study in microgranites (Velay Dome, France)[END_REF][START_REF] Laurent | Protracted, coeval crust and mantle melting during Variscan lateorogenic evolution: U-Pb dating in the eastern French Massif Central[END_REF] and lower crustal metasedimentary granulites (this study). The maximum at 320-310 Ma in the upper crustal rocks coincides with the inferred 313 ± 3 Ma age for the lower crustal thermal peak and reflect the paroxysm of melt production and extraction in the orogenic crust. In contrast, the highest density of U-Pb ages at 310-300 Ma in the Velay diatexite/granites point to a delayed crystallization and is consistent with the prevalence of supra-solidus conditions in the mid-crust 5-10 Ma after the thermal peak. The scattered, even younger U-Pb dates in metasedimentary granulites is interpreted as reflecting protracted zircon crystallization in residual lower crustal melts upon slower cooling from peak temperature down to ~800°C.

  ), with negative values in felsic granulites from -10.0 ± 3.3 (2 S.D.; n = 55) in HS15-BOU17 to -5.7 ± 3.0 (2 S.D.; n = 43) for HS15-BOU01; and positive values of +4.1 ± 1.2 (2 S.D.; n = 54) in mafic granulite HS15-BOU07.

Figure 4 .

 4 Figure 4. Representative cathodoluminescence images of zircon grains from the granulite xenoliths investigated in this study. The white and yellow dotted circles represent the position of the laser spots for U-Pb dating/trace element (TE) and Lu-Hf isotopic analyses respectively, with corresponding spot name (S1-XX, M1-XX, M2-XX), 238 U/ 206 Pb date ±2σ uncertainty (in Ma), Ti content (in blue), and εHf(t) ± 2σ uncertainty (calculated using the 238 U/ 206 Pb date obtained on the same domain). The value with an asterisk corresponds to the 207 Pb/ 206 Pb date as the analysis was discordant.

Figure 5 .

 5 Figure 5. Results of zircon U-Pb analyses for the Bournac granulite xenoliths shown as Tera-Wasserburg diagrams ( 238 U/ 206 Pb vs. 207 Pb/ 206 Pb) with error ellipses quoted at the 95% confidence level. The insets selectively represent data from homogeneous grains and rims with ellipses color-coded based on Ti contents from the same ablated volume.

Figure 6 .

 6 Figure 6. Plots of zircon εHf(t) (a) and Ti contents (b) vs. 238 U/ 206 Pb date (considering only concordant data) for the Bournac granulite xenoliths, with a close-up focus on the Variscan period (240-340 Ma) on the left-hand side diagrams. Note the scale change at 500 Ma in the right-hand side diagrams. The Ti-in-zircon temperatures were calculated using the calibration of Ferry and Watson (2007) with a(TiO2) = a(SiO2) = 1. The grey and orange fields represent zircon data respectively from the lower crustal Mafic Complex of the Ivrea-Verbano Zone, NW

Figure 7 .

 7 Figure 7. Selected trace element plots comparing the compositions of zircons from the Bournac granulite xenoliths to those of zircon from various parental magmas and geodynamic

Figure 8 .

 8 Figure 8. Evaluation of zircon/garnet REE partitioning for the felsic granulites. (a) REE concentrations of zircon (Zrn) and garnet (Grt) normalized to C1 chondrite composition of McDonough and Sun (1995). (b) 'Array plot' of Taylor et al. (2017) where D i zrn/grt represents the zircon/garnet partition coefficient of element or ratio i for each zircon analysis, calculated using the median garnet composition of the sample. Stars represent experimental data from Taylor et al. (2015b) at 7 kbar and Rubatto and Hermann (2007) at 20 kbar. (c) Plot of log D Yb zrn/grt vs. 206 Pb/ 238 U date for each zircon analysis.

Figure 9 .

 9 Figure9. Simplified P-T pseudosections calculated for the estimated subsolidus (left panels) and peak (right panels) compositions of felsic granulites HS15-BOU01 and HS15-BOU17, contoured for garnet mode and where major mineral reactions are indicated. The colored frames depict the P-T field where the mineral assemblages of the samples are stable. The inferred P-T path involves (1.) prograde heating and partial melting with concomitant melt drainage (white dots), up to (2.) peak conditions of 940-970 °C (at 8 ± 2 kbar), followed by (3.) cooling with ≤15 vol.% residual melt with at least one last melt drainage event (black dot) at ≥840 °C. Further details on the melt reintegration procedure are provided in the Supplementary Material.

Figure 10 .

 10 Figure 10. Temporal relationships between granulite-facies metamorphism and granite magmatism in the eastern FMC. (a) Tera-Wasserburg Concordia diagrams for the homogeneous zircon rims/grains in the Bournac felsic granulites, with calculated Concordia ages from the oldest population as estimates of the age of the thermal peak. (b) Schematic crustal column of the eastern FMC along with the histograms and density distributions of zircon and monazite U-Pb dates obtained from upper crustal granite plutons/dykes, the Velay diatexite/granites (data from Chelle-Michou et al., 2017; Couzinié et al., 2014;[START_REF] Didier | Disturbance versus preservation of U-Th-Pb ages in monazite during fluid-rock interaction: textural, chemical and isotopic in situ study in microgranites (Velay Dome, France)[END_REF]

Figure 1 Figure

 1 Figure 1

Figure 8 Figure 9 Figure 10

 8910 Figure 8

Acknowledgements

We thank M. Charlet, C. Doucet and A. Didier for help during fieldwork and sampling; as well as B. Devouard, G. Estrade, J.-M. Hénot, C. Guilbaud, M. Guillong, M. Leisen, C. Merlet, and B. Suchéras-Marx for assistance with sample preparation and analyses. P. Bouilhol, F. Faure, V. Gardien, J.-F. Moyen and O. Vanderhaeghe are thanked for enlightening discussions. OL and LSD thank the F.R.S.-FNRS for financial support during field campaign.

SC acknowledges financial support from the "Conseil Général de l'Ardèche" while working at

ENSL. Two anonymous reviewers and the editor R. Hickey-Vargas are thanked for their very constructive and helpful comments. 2020). The crust and mantle structure along with approximate seismic velocities for the different layers are from [START_REF] Perrier | Structure profonde du Massif Central français[END_REF]. (d) Summary of P-T estimates obtained for the three investigated crustal xenoliths; shaded areas and dotted fields represent the stability fields of mineral assemblages for our samples (derived from pseudosection analysessee Supplementary Material) and for similar literature samples (compiled in [START_REF] Leyreloup | La croûte métamorphique du Sud de la France (Massif Central, Languedoc). Géologie de surface et des enclaves remontées par les volcans cénozoïques : le rôle des intrusions mafiques basicrustales dans la croûte inférieure[END_REF], while the symbols represent average conditions of equilibration for our samples based on conventional thermobarometry. [START_REF] Rudnick | 4.1 -Composition of the Continental Crust[END_REF]. The pink field shows the variability of the Ivrea stronalites [START_REF] Schnetger | Partial melting during the evolution of the amphibolite-to granulite-facies gneisses of the Ivrea Zone, northern Italy[END_REF].