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

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-cuttingVariscan 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 backarc 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 from a bimodal rift-related magmatism at ~480 Ma, 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 (~350-340 Ma).Finally, several generations of granite dikes of which inherited zircons display characteristics of both the UGU and the LGU were 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 ACCEPTED MANUSCRIPTA C C E P T E D M A N U S C R I P T various components from the Gondwana continent deposited in Ediacaran times, with no evidence for the involvement of an older autochthonous crust.

From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France)

“…In the French Massif Central (FMC), a complex stack of nappes is intruded by numerous late-Carboniferous granitoids (e.g. Faure et al, 2009;Lardeaux, 2014;Chelle-Michou et al, 2017). Eclogites are found in several localities throughout most of the FMC, but they are generally poorly outcropping.…”

Multistage development of a hydrothermal W deposit during the Variscan late-orogenic evolution: the Puy-les-Vignes breccia pipe (Massif Central, France)

“…1A), which belongs to the internal zone of the European Variscan belt formed during the continental collision between Gondwana and Laurussia through the Upper Paleozoic (Faure et al, 2009a;Lardeaux et al, 2014). The geologic structure of the FMC consists of a stack of three major metamorphic nappes that are from top to bottom (Faure et al, 2009a and references therein): (i) the Upper Gneiss Unit (UGU) composed of ortho-and paragneisses derived from Cambrian to Early Ordovician protoliths (530-470 Ma;Alexandrov et al, 2001;Berger et al, 2010;Chelle-Michou et al, 2017;Lotout et al, 2017) having experienced MP-MT amphibolite-to granulite-facies metamorphism at 360-340 Ma (Chelle-Michou et al, 2017); (ii) the Lower Gneiss Unit (LGU) consisting of ortho-and paragneisses similar to those of the UGU that were inherited from late Ediacarian protoliths (620-540 Ma; Alexandre, 2007;Melleton et al, 2010;Chelle-Michou et al, 2017;Couzinié et al, 2017Couzinié et al, , 2019; and (iii) the Para-Autochtonous Unit (PAU) that consists of lowgrade metasedimentary rocks (micaschists, metagreywackes, and quartzites) that were derived from Neoproterozoic to Early Cambrian protoliths (650-530 Ma; Melleton et al, 2010). These metamorphic units were intruded by numerous granitoids during the Carboniferous between 360 and 300 Ma (Faure et al, 2009a;Lardeaux et al, 2014;Couzinié et al, 2017;Moyen et al, 2017;Villaros et al, 2018).…”