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We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust. Nous présentons dans ce papier un modèle géodynamique-tectonique pour la genèse et le fluage des roches partiellement fondues et le magmatisme au cours de l’évolution orogénique Varisque du Silurien au Carbonifère supérieur basé sur une synthèse des données géologiques du Massif Central Français. La subduction de la marge du Gondwana hyper-étirée est enregistré par des roches mafiques et ultramafiques affectées par un métamorphisme en faciès éclogitique. Ces éclogites forment pour certaines des boudins-enclaves dans des migmatites felsiques avec des reliques de faciès granulitique de HP retrogradées en faciès amphibolitique, ce qui atteste de la subduction de la marge continentale suivie d’une relaxation thermique et d’une décompression. Nous proposons que la fusion partielle à HP ait déclenché le découplage mécanique entre la plaque plongeante et les unités continentales partiellement fondues. Ceci a permis l’exhumation de ces roches gravitairement instables qui ont entrainé sur leur passage des blocs de lithosphère océanique et de manteau sous-continental. Les données géochronologiques disponibles sur les migmatites de HP contenant des éclogites indique une mise en place diachronique de nappes du Dévonien moyen au Dévonien Supérieur. Ces nappes ont chevauché un assemblage de métapélites et d’orthogneiss affectées par un métamorphisme de MP/MT allant du faciès schistes verts à amphibolite atteignant localement la fusion partielle et attribué à l’épaississement crustal du Dévonien supérieur au Carbonifère moyen. La mise en place de laccolithes enracinés dans des zones de cisaillement décrochantes et surmontés de détachements à faible pendage de c. 345 à c. 310 Ma est synchrone de la propagation vers le Sud du front de déformation Varisque marqué par le dépôt de sédiments détritiques dans les bassins d’avant-pays. Nous attribuons ces éléments à la croissance horizontale de la ceinture Varisque associée à la formation d’un plateau orogénique par fluage latéral de la racine orogénique partiellement fondue sous l’effet de la force gravitaire. La diversité des roches magmatiques témoigne d’une variété des sources crustales avec une contribution relativement modeste mais systématique de magmas issus du manteau. Dans la partie Est du Massif Central Français, la décroissance vers le Sud des âges de mise en place des magmas dérivés à la fois du manteau et de la croûte suggère le retrait d’un panneau plongeant vers le Nord vers la bordure libre constituée par la Paléotethys et située au Sud de la ceinture Varisque. La destruction de la chaine Varisque à la fin du Carbonifère est principalement le résultat de l’effondrement gravitaire accommodé par l’activation de détachements à faible pendage et l’exhumation-cristallisation de la racine orogénique partiellement fondue formant des dômes d’échelle crustale à cœur de migmatites de BP entre c. 305 et 295 Ma, concomitante au fluage latéral des unités de la zone externe de la chaine. Les derniers magmas extraits de la zone de fusion partielle forment des dykes et des laccolithes mis en place dans des détachements à faible pendage laissant derrière eux une croûte inférieure constituée de l’accumulation de granulites réfractaires de composition felsique à mafique. Ce modèle met en valeur le rôle primordial de la fusion partielle et du magmatisme sur l’évolution tectonique-géodynamique de la ceinture orogénique Varisque. En particulier, la fusion partielle et le transfert de magma (i) déclenchent le découplage mécanique entre le panneau plongeant et les unités subductées, permettant ainsi l’exhumation de ces dernières, (ii) favorisent le développement d’un plateau orogénique par fluage latéral de la croûte partiellement fondue de faible viscosité sous l’effet de la gravité, (iii) conduisent à la formation de metamorphic core complex et de domes qui accommodent l’exhumation post-orogénique au cours de l’effondrement gravitaire de la chaine. Tous ces processus contribuent à la différenciation et à la stabilisation de la croûte orogénique.

This work presents an integrated structural, kinematic, and petrochronological study of the Monte Grighini dome within the Variscan hinterland–foreland transition zone of Sardinia (Italy). The area is characterised by dextral transpressive deformation partitioned into low- and high-strain zones (Monte Grighini shear zone, MGSZ). Geothermobarometry of one sample of sillimanite-bearing mylonitic metapelite indicates that the Monte Grighini shear zone developed under high-temperature (~ 625 °C) and low-pressure (~ 0.4–0.6 GPa) conditions. In situ U–(Th)–Pb monazite geochronology reveals that the deformation in the shear zone initiated at ca. 315 Ma. Although previous studies have interpreted the Monte Grighini shear zone to have formed in a transtensional regime, our structural and kinematic results integrated with constraints on the relative timing of deformation indicate that it shows similarities with other dextral ductile transpressive shear zones in the Southern European Variscan belt (i.e., the East Variscan Shear Zone, EVSZ). However, dextral transpression in the Monte Grighini shear zone started later than in other portions of the EVSZ within the framework of the Southern European Variscan Belt due to the progressive migration and rejuvenation of deformation from the core to the external sectors of the belt. Graphical abstract

We present U-Pb thermal ionization mass spectrometer (TIMS) ages of wolframite from several granite-related hydrothermal W±Sn deposits in the French Massif Central (FMC) located in the internal zone of the Variscan belt. The studied wolframite samples are characterized by variable U and Pb contents (typically <10 ppm) and show significant variations in their radiogenic Pb isotopic compositions. The obtained U-Pb ages define three distinct geochronological groups related to three contrasting geodynamic settings: (i) Visean to Namurian mineralization (333–327 Ma) coeval with syn-orogenic compression and emplacement of large peraluminous leucogranites (ca. 335–325 Ma), (ii) Namurian to Westphalian mineralization (317–315 Ma) synchronous with the onset of late-orogenic extension and emplacement of syn-tectonic granites (ca. 315–310 Ma) and (iii) Stephanian to Permian mineralization (298–274 Ma) formed during post-orogenic extension contemporaneous with the Permian volcanism in the entire Variscan belt. The youngest ages (276–274 Ma) likely reflect the reopening of the U-Pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration (e.g. ferberitization). Our results demonstrate that W(±Sn) mineralization in the FMC formed during at least three distinct hydrothermal events in different tectono-metamorphic settings over a time range of 40 Ma.

Based on new structural, petrological and U-Th-Pb geochronological data, a reappraisal of the Variscan tectono-metamorphic history of the Pelvoux Massif (External Crystalline Massif, French Alps) is proposed with the aim to understand the flow pattern and kinematics of the Variscan partially molten crust and the Eastern Variscan Shear Zone. The Pelvoux Massif consists of high-grade metamorphic rocks of middle to lower crust, mostly migmatites, that record a prominent syn-metamorphic deformation event (D 2 ) characterized by a pervasive NE-SW striking, steeply dipping, S 2 foliation, and a network of anastomosed NS and NW-SE trending shear zones, the kinematics of which indicates a sinistral transpression. Relics of an early syn-metamorphic event (D 1 /M 1 ) related to crustal thickening and top-to-the-east nappe stacking are also reported. Both the D 1 and D 2 features are interpreted as reflecting a NW-SE shortening event, firstly marked by dominant nappe stacking, and secondly overprinted by a sinistral transpression that started at peak metamorphism with the onset of crustal partial melting at ca. 650 °C during the late Visean (ca. 335–330 Ma). Ongoing sinistral D 2 transpression in the partially molten middle-lower crust of the Pelvoux involved strain partitioning between C and C’ shear zones and horizontal longitudinal flow in the range 330–300 Ma. Along the anatectic front, vertical shortening and top-to-the-NW shearing (D 3 ) is coeval with D 2 and argue for southeastward motion of the partially molten crust. The contemporaneity between NW-SE directed transpressional flow and vertical shortening is supported by our radiometric data of D 2 and D 3 and attests for strain partitioning between the suprastructure and infrastructure during horizontal crustal flow under transpressive regime. The exhumation of deep-seated rocks during sinistral transpression followed a near isothermal (ca. 700 °C) evolution down to pressure of ca. 0.5 GPa in the period 325–306 Ma. The sinistral transpression recorded in the Pelvoux Massif might corresponds to an antithetic shear zone coeval with the dextral East-Variscan Shear Zone, proposed for this part of the Variscan orogen. Sur la base de nouvelles données structurales, pétrologiques et géochronologiques U-Th-Pb, une réévaluation de l'histoire tectono-métamorphique varisque du massif du Pelvoux (Massif cristallin externe, Alpes françaises) est proposée dans le but de comprendre le style et la cinématique du fluage de la croûte varisque partiellement fondue et de la Zone de Cisaillement Est Varisque. Le massif du Pelvoux est constitué de roches métamorphiques de haut grade, de la croûte moyenne à inférieure, principalement des migmatites, qui enregistrent un événement de déformation syn-métamorphique prédominant (D 2 ) caractérisé par une orientation NE-SW, une foliation S 2 à fort pendage et un réseau anastomosé de zones de cisaillement d'orientation NS et NW-SE dont la cinématique indique une transpression senestre. Des reliques d'un événement syn-métamorphique précoce (D 1 /M 1 ) lié à l'épaississement de la croûte et à l'empilement des nappes à vergence Est sont retrouvées. Les caractéristiques de D 1 et D 2 sont interprétées comme reflétant un événement de raccourcissement NW-SE, d'abord marqué par l'empilement dominant des nappes, qui évolue ensuite en une transpression senestre, initiée au pic du métamorphisme avec le début de l’anatexie crustale dès 650 °C, à la fin du Viséen (335–330 Ma). Dans la croûte partiellement fondue, la déformation transpressive senestre D 2 se partitionne avec la formation de zones de cisaillement C et C’ qui accommodent le fluage longitudinal entre 330 et 300 Ma. Le long du front anatectique, un raccourcissement vertical accompagné d’un cisaillement vers le NW (déformation D 3 ) sont contemporains de D 2 et participent aussi à l’échappement vers le sud-est de la croûte partiellement fondue. La contemporanéité entre le fluage transpressif dirigé NW-SE (D 2 ) et le raccourcissement vertical à cinématique NW (D 3 ) est étayée par nos données radiométriques. L'exhumation des roches profondes lors de la transpression senestre a suivi une évolution quasi isotherme (~ 700 °C) jusqu'à une pression d'environ 0,5 GPa dans la période 325–306 Ma. La transpression senestre enregistrée dans le massif du Pelvoux pourrait correspondre à une zone de cisaillement antithétique contemporaine de la zone de cisaillement dextre Est-Varisque.

The existence of pieces of the Variscan belt in the Alpine basement has been acknowledged for a long time but the correlation of these massifs to the litho-tectonic domains established in Western Europa outside the Alpine chain is still disputed. Due to their ubiquitous character, the abundant late Variscan migmatites and granites are useless to reconstruct the Variscan architecture in the Alpine basement. Ophiolitic sutures, high- and low-grade metamorphic units, and foreland basins provide a preliminary reconstruction of the Variscan orogen exposed in the Alpine basement. The longitudinal extension of the Armorican and Saxo-Thuringian microcontinents between Laurussia and Gondwana is proposed independently of the Intra-alpine and Galatian terranes. The litho-tectonic units of the Corsica-Sardinia segment are correlated to the Moldanubian, Armorican and Saxo-Thuringian Domains. In the Alpine Helvetic and Penninic Domains, the Chamrousse ophiolites are ascribed to the Tepla-Le Conquet suture, whereas the Lepontine, and Stubach ophiolites represent the Rheic suture. The south-directed nappe stack of the South Alpine Domain is similar to the Moldanubian French Massif Central. In the Austroalpine nappe stack, the Ritting ophiolites separate Saxo-Thuringia and Armorica continental blocks. Disentangling the Variscan belt in the Alpine basement suggests a concave-to-the-East arcuate structure called here the Variscan Alpidic orocline.

Variscan granitoids and associated mafic rocks exposed in the External Crystalline Massifs (ECM) of the Western Alps document the Variscan stages from the early Carboniferous collision to the early Permian post-collisional setting. Our study focuses on the Central part of the ECM, synthesizing newly acquired and existing geochronological, whole-rock geochemical and isotopic data. We identified two distinctive magmatic series: (i) high-K calc-alkaline granitoids, which range from magnesian (MgG) to ferro-magnesian (FeMgG) rocks; (ii) ultra-high-K metaluminous (UHKM) rocks (“durbachites”). These series were emplaced roughly simultaneously between ca. 350 and 300 Ma, with two main episodes during the Visean (ca. 348–335 Ma) and the late Carboniferous (305–299 Ma), with a more limited activity in between. A younger Permian event at ca. 280–275 Ma has also been identified in one granitoid pluton. Contemporaneous emplacement of these two series reflects concomitant crustal anatexis and melting of LILE–LREE-rich metasomatized lithospheric mantle. Trace elements and Nd–Sr isotopes reveal significant hybridization between these two magmatic end members, by magma mixing, or assimilation of crystallized mafic ultrapotassic enclaves in the high-K calc-alkaline granitoids. Granitoid composition evolves over time, especially SiO 2 , Mg#, Sr/Y, La/Yb and Nb/Ta, possibly explained by increasing differentiation of magmas over time, changes in the crust versus mantle sources mass-balance, and decrease in melting pressure due to the orogenic collapse. The εNd i values of both high-K calc-alkaline granitoids and durbachites decreases from [− 3.8; − 2.9] to [− 6.4; − 5.2] between 345 and 320 Ma, possibly indicating an increasing influence of subducted/relaminated crustal material contaminating the lithospheric mantle source. εNd i values then rise to [− 3.7; − 0.5] during the late Carboniferous, possibly due to progressive exhaustion of the enriched mantle source, or advection of the asthenosphere during the post-collisional stage. Graphic abstract Possible geodynamic scenario along the central-eastern segment of the Variscan Belt, which may account for the temporal evolution of Variscan magmatism in the External Western Alps.

Precise U–Pb zircon dating using the chemical abrasion – isotope dilution – thermal ionization mass spectrometry (CA-ID-TIMS) method constrains the age of the Central Sudetic Ophiolite (CSO) in the Variscan Belt of Europe. A felsic gabbro from the Ślęża Massif contains zircon xenocrysts dated at 404.8 ± 0.3 Ma and younger crystals dated at 402.6 ± 0.2 Ma that determine the final crystallization age of the gabbro. An identical age of 402.7 ± 0.3 Ma was determined for plagiogranite from the Nowa Ruda–Słupiec Massif, and plagiogranite from the Braszowice–Brzeźnica Massif yields a similar, but less reliable, age of > 401.2 Ma. The different massifs in the CSO are therefore considered as tectonically dismembered fragments of a single oceanic domain formed at c. 402.6–402.7 Ma (Early Devonian – Emsian). The magmatic activity recorded in the CSO was contemporaneous with the high-temperature/high-pressure metamorphism of granulites and peridotites in the Góry Sowie Massif, separating dismembered parts of the CSO. This suggests geodynamic coupling between the continental subduction recorded in the Góry Sowie and the oceanic spreading recorded in the CSO. Regional geological data indicate that the CSO was obducted before c. 383 Ma, less than 20 Ma after its formation at an oceanic spreading centre. The CSO is shown to be one of the oldest and first obducted among the Devonian ophiolites of the Variscan Belt. The CSO probably originated in an evolved back-arc basin in which the influence of subduction-related fluids and melts increased with time, from negligible during the formation of predominant mid-ocean-ridge-type magmatic rocks to strong at later stages, when rodingites, epidosites and other minor lithologies formed.

We present new constraints on the age, nature, and tectonic setting of mafic eclogite protoliths from the Maures-Tanneron Massif, southern Variscan belt. Whole-rock major and trace element geochemistry was combined with zircon dating using 206 Pb/ 238 U by LA‒ICP‒MS to improve the understanding of this key-target of the European Southern Variscides. Geochemical data show that protoliths of the mafic eclogites are typical MORBs, while REE and HFSE patterns suggest an E-MORB affinity. However, the geochemical study shows several signs of crustal contamination that increases with the degree of retrogression. A comparison with Sardinian eclogites, which belong to the same Variscan microplate, namely, “MECS” (Maures-Estérel-Corsica-Sardinia), demonstrates that the eclogites are included in migmatites, which is the case for the studied samples, are the most contaminated. The Maures-Tanneron mafic eclogites represent the remnant of an oceanic basaltic crust. Zircon cores display homogeneous Th/U ratios (0.3–0.4), which are consistent with a magmatic origin, and define an age peak at 499.5 ± 2.9 Ma that is interpreted as the most likely emplacement age of the basaltic protolith. This age suggests that this protolith was part of an oceanic floor that was older than the Rheic Ocean and located to the north of the Gondwana active continental margin as predicted by recent unified full plate reconstruction models. Although the studied eclogites are retrogressed, the study of mineral inclusions trapped in garnets combined with thermodynamic modelling yields a P−T range of 17.2–18.5 kbar and 640–660 °C, which is consistent with the standard oceanic subduction palaeo-geotherm. These new data suggest that eclogites recognized in the “MECS” Variscan microplate represent the closure of oceanic domains of different ages (Cambrian or Ordovician). Nous présentons de nouvelles données sur les éclogites mafiques rétromorphosées du massif des Maures-Tanneron (MTM), appartenant à la chaîne varisque méridionale d’Europe. La caractérisation géochimique en éléments majeurs et traces de ces roches a été associée à la datation dans les zircons par la méthode LA-ICP-MS sur U‒Pb, afin d’améliorer la compréhension de cette zone-clé de la chaîne varisque. L’analyse géochimique indique que les protolithes correspondent sans équivoque à des MORBs, les profils en terres rares et HFSE montrant une affinité apparente proche du pôle E-MORB. Il faut cependant envisager les effets d’une contamination crustale, dont l’intensité augmente avec le degré de rétromorphose. La comparaison de nos résultats avec les données obtenues sur les éclogites de Sardaigne, appartenant à la même microplaque varisque Maures-Estérel-Corsica-Sardinia (MECS), indique que les éclogites les plus contaminées, parmi lesquelles les nôtres, se situent toujours dans un encaissant migmatitique. Néanmoins, les éclogites les mieux préservées du MTM proviennent toutes d’un ancien plancher océanique de nature basaltique. La méthode U‒Pb par LA‒ICP‒MS appliquée aux cœurs homogènes des zircons, et dont le ratio Th/U ∼0.3–0.4 est compatible avec une origine magmatique, conduit à un âge de 499.5 ± 2.9 Ma pour les protolithes des éclogites. Cet âge est donc interprété comme l’âge le plus probable du basalte originel. Il témoigne de l’existence d’un espace océanique, qui précède l’ouverture de l’océan Rhéique et se localise à proximité d’une marge active dans le Nord du supercontinent Gondwana. Ce résultat est en accord avec les derniers modèles paléomagnétiques globaux de paléo-cinématique des plaques. Bien que ces éclogites aient subi des modifications importantes au cours de leur rétromorphose, l’étude de la chimie des inclusions minéralogiques « blindées » au sein des cœurs de grenats, combinée à la modélisation thermodynamique sur roche totale, permet de proposer un domaine de conditions P−T maximales de l’ordre de 17,2–18,5 kbar pour 640–660 °C. Ces conditions sont compatibles avec les conditions standard d’un géotherme de subduction océanique. Ces nouvelles données, combinées à celles obtenues en Sardaigne et en Corse, indiquent que les éclogites recensées dans les vestiges de la microplaque MECS témoignent de la fermeture de domaines océaniques d’âges différents (Cambrien ou Ordovicien).

Apatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only U-bearing mineral available to date mafic rocks because primary zircon and/or baddeleyite are not present. In this paper, U-Pb LA-ICP-MS dating of apatite was applied to seven different dike and sill samples of dolerite from the Variscan belt of Brittany (Armorican Massif, western France). These dolerites, which are characterized by a within-plate tholeiite geochemical signature, are organized in several dense swarms across the belt. Their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic-metamorphic settings. Their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40Ar/39Ar or the U-Pb methods on classical minerals like mica, plagioclase, or zircon. Although the closure temperature of apatite is lower than the emplacement temperature of the magma, physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less. Consequently, the U-Pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions. Our results demonstrate that, in all cases, the apatite grains do carry enough radiogenic Pb to be dated by in situ U-Pb analyses and yield a 207Pb-corrected mean age of 363.4 ± 5.8 Ma. These results reveal the existence of a major and short-lived magmatic event in the Variscan belt of Brittany during the Devonian-Carboniferous transition, a feature further highlighted by field evidence. Beyond the geological implications of these results, U-Pb LA-ICP-MS dating of apatite appears to represent an ideal tool to date small size mafic intrusions.

Understanding how the lithosphere accommodates deformation along oblique convergent plate boundaries is an important issue for unveiling orogenic belt exhumation. The Variscan belt exposed in Sardinia represents an area where it is possible to investigate the inner part of this orogen and a km-scale transpressive shear zone, i.e. the Posada-Asinara shear zone, that drove the exhumation of the metamorphic core of the belt. Even though the Asinara island offers very good exposure of main tectonic units building up the Variscan belt of Sardinia, a detailed geological map of the island is still lacking, due to a former limitation of access that lasted for many years. In this contribution, we compile a new 1:25.000 geological-structural map of the entire Asinara island useful to better understand this key sector of the Variscan belt in Sardinia.