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Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell

Laurent, Oscar
Moyen, Jean-François
Block, Sylvain
Paquette, Jean-Louis
Couzinié, Simon
Gardien, Véronique
Vanderhaeghe, Olivier
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Published in Gondwana research. 2017, vol. 46, p. 170-190
Abstract The East Massif Central (EMC), France, is part of the internal zone of the Variscan belt where late Carboniferous crustal melting and orogenic collapse have largely obliterated the pre- to early-Variscan geological record. Nevertheless, parts of this history can be reconstructed by using in-situ U-Th-Pb-Lu-Hf isotopic data of texturally well-defined zircon grains from different lithological units. All the main rock units commonly described in the EMC are present in the area of Tournon and include meta-sedimentary and meta-igneous rocks of the Upper Gneiss Unit (UGU) and of the Lower Gneiss Unit (LGU), as well as cross-cutting Variscan granitoid dikes and a heterogeneous granite coring the major Velay dome. Herein we demonstrate that the UGU and the LGU have markedly distinct zircon records. The results of this study are consistent with deposition of the protoliths of the paragneisses within a back-arc basin that was located adjacent to the Arabian-Nubian shield and/or the Saharan Metacraton during the late Ediacaran and collected detritus from the Gondwana continent. At ~545 Ma some of these sedimentary rocks were affected by a first melting event that formed the protoliths of the LGU orthogneisses, those of which subsequently remelted at ca. 308 Ma to form the Velay granite-migmatite dome. Protoliths of the UGU result mainly froma bimodal rift-relatedmagmatismat ~480Ma, corresponding to melting of the Ediacaran sediments and depleted mantle. Zircon rims from the UGU additionally provide evidence for a metamorphic/migmatitic overprint during the Lower Carboniferous (~350–340Ma). Finally, several generations of granite dikes of which inherited zircons display characteristics of both the UGU and the LGUwere protractedly emplaced from ~322 Ma to ~308 Ma, the youngest of which being coeval with the formation of the Velay dome. Our data further show that the vast majority of crustal material ultimately involved in the Variscan orogeny, which forms the present-day basement in the EMC, was derived from a sedimentary mixture of various components from the Gondwana continent deposited in Ediacaran times, with no evidence for the involvement of an older autochthonous crust.
Keywords North Gondwana marginVariscan orogenyMassif CentralPan-African/Cadomian orogeniesZircon
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Research group Isotope Geochemistry, Geochronology and Thermochronology
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CHELLE-MICHOU, Cyril et al. Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell. In: Gondwana research, 2017, vol. 46, p. 170-190. doi: 10.1016/j.gr.2017.02.010 https://archive-ouverte.unige.ch/unige:93143

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Deposited on : 2017-04-05

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