1 Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: 2 the geologic record of the French Massif Central 3 4 Vanderhaeghe Olivier1, Laurent Oscar1,2, Gardien Véronique3, Moyen Jean-François4, Gébelin Aude5, Chelle- 5 Michou Cyril2, Couzinié Simon4,6, Villaros Arnaud7,8, Bellanger Mathieu9. 6 1. GET, UPS, CNRS, IRD, 14 avenue E. Belin, F-31400 Toulouse, France 7 2. ETH Zürich, Institute for Geochemistry and Petrology, Clausiusstrasse 25, CH-8038 Zürich, Switzerland 8 3. Université Lyon 1, ENS de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622, Villeurbanne, France 9 4. Université de Lyon, Laboratoire Magmas et Volcans, UJM-UCA-CNRS-IRD, 23 rue Dr. Paul Michelon, 10 42023 Saint Etienne 11 5. School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth,UK 12 6. CRPG, Université de Lorraine, CNRS, UMR7358, 15 rue Notre Dame des Pauvres, F-54501 13 Vandoeuvre-lès-Nancy, France 14 7. Univ d’Orléans, ISTO, UMR 7327, 45071, Orléans, France ; CNRS, ISTO, UMR 7327, 45071 Orléans, 15 France ; BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France 16 8. University of Stellenbosch, Department of Earth Sciences, 7602 Matieland, South Africa 17 9. TLS Geothermics, 91 chemin de Gabardie 31200 Toulouse. 18 19 [email protected] 20 +33(0)5 61 33 47 34 21 22 Key words: 23 Variscan belt; French Massif Central; Flow of partially molten crust; Orogenic magmatism; Orogenic plateau; 24 Gravitational collapse. 25 26 Abstract 27 We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for 28 magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis 29 of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks 30 records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins- 31 enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for 1 32 continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting 33 has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would 34 have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental 35 mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of 36 distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses 37 affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late 38 Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip 39 transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the 40 southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland 41 basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau 42 by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to 43 various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, 44 the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma 45 suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late 46 Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by 47 the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root 48 forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the 49 external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts 50 correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic 51 and mafic granulites in the lower crust. 52 This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution 53 of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling 54 of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an 55 orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic 56 core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these 57 processes contribute to differentiation and stabilisation of the orogenic crust. 58 Index 59 1. Introduction ....................................................................................................................................................................... 4 60 2. Geology of the French Massif Central: a window through the Variscan belt .................................................................... 6 61 2.1. The Variscan belt: continental blocs, oceanic sutures, allochthonous terranes and paleogeographic reconstructions ... 6 62 2.2. The main lithologic-tectonic units of the French Massif Central ....................................................................................... 8 63 Metamorphic nappes ....................................................................................................................................................... 9 64 Protoliths age for the high-grade rocks, basement of the Devonian to Carboniferous volcanic and sedimentary units 11 2 65 Migmatites and granulites .............................................................................................................................................. 12 66 Late Devonian to Carboniferous plutonic rocks ............................................................................................................. 13 67 Carboniferous volcanic and sedimentary deposits ........................................................................................................ 15 68 2.3. Architecture and P-T-t record of nappes of the western and eastern French Massif Central ......................................... 16 69 Western French Massif Central (W-FMC)...................................................................................................................... 17 70 Eastern French Massif Central (E-FMC) ........................................................................................................................ 21 71 3. Previous tectonic-geodynamic reconstructions and debated issues .............................................................................. 27 72 Monocyclic doubly-vergent orogen model ..................................................................................................................... 27 73 Polycyclic orogenic model.............................................................................................................................................. 27 74 Collision versus syn-orogenic extension during the Carboniferous ............................................................................... 28 75 Impact of partial melting and magmatism on Variscan tectonics? ................................................................................. 29 76 4. A new model for the geodynamic-tectonic evolution of the Variscan belt of Western Europe ........................................ 31 77 4.1. Pre-Variscan configuration: the North Gondwana hyper-extended margin .................................................................... 31 78 4.2. Late Silurian to Devonian subduction and HP partial melting of terranes issued from the North Gondwana margin ..... 32 79 4.3. Middle Devonian to early Carboniferous syn-orogenic exhumation of the partially molten subducted crustal units with 80 mantle enclaves ....................................................................................................................................................................... 34 81 4.4. Late Devonian exhumation of nappes in between retreating slabs ................................................................................ 37 82 4.5. Carboniferous (c. 345-310 Ma) building of an orogenic plateau by lateral flow of the partially molten orogenic root ..... 39 83 4.6. Late Carboniferous to Permian (c. 305-295 Ma) gravitational collapse and exhumation of the partially molten root of the 84 Variscan orogenic belt .............................................................................................................................................................. 41 85 4.7. Pertinence of the proposed geodynamic-tectonic model compared to physical modeling of the dynamics of orogenic 86 belts 46 87 5. Conclusion ...................................................................................................................................................................... 47 88 6. References: .................................................................................................................................................................... 56 89 90 91 92 3 93 1. Introduction 94 95 Migmatites and granites are the main constituents of the continental crust and their petrogenesis and emplacement 96