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 Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

“…The zircon U-Pb age of 473.4 ± 2 Ma obtained in this study, similar to the age of the emplacement of the adjacent granite dated at 468.2 ± 2.7 , provides new evidence of the bimodal character of this massive magmatic event recorded during the Ordovician (e.g., Iberian Massif -e.g., Sánchez-García et al, 2003, 2008Villaseca et al, 2015;Del Grecco et al, 2016;Gutiérrez-Alonso et al, 2016;García-Arias et al, 2018; Armorican Massif -e.g.,, Ballèvre et al, 2012French Massif Centrale.g.,, Pitra et al, 2012;Chelle-Michou et al, 2017;Lotout et al, 2017;Sardinia -e.g.,, Helbing & Tieppolo, 2005;Oggiano et al, 2010;Caggero et al, 2012;Bohemian Massife.g., Košler et al, 2004;Linnemann et al, 2008), related to the extreme stretching of the lithosphere (e.g., Bard et al, 1980;Matte & Burg, 1981;Matte, 1986;Pin, 1990;Ribeiro et al, 2007;Martínez Catalán et al, 2009;Lardeaux, 2014;Schulmann et al, 2014).…”

The impact of the end-Ordovician glaciation on sediment routing systems: A case study from the Meseta (northern Morocco)

“…In particular, the assessment of Mesoproterozoic zircon grains in NW Africa is of interest (Avigad et al, 2012;Pratt et al, 2015). As such ages are generally regarded in the north African context as marking a source originating in the Sahara metacraton (NE Africa; Henderson et al, 2015;Chelle-Michou et al, 2017), multi-stage sediment recycling events (see discussion in Andersen et al, 2016) and potential mixing linked to the end-Ordovician glaciation may have severely altered the true significance of zircon provenance. Alternatively, glacially fed routing systems can introduce, or enhance, the contribution of far-travelled sediment sources that were absent or subordinate in preglacial watersheds (Doornbos et al, 2009;Hofmann et al 2015;Gürsu et al, 2018).…”

Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin

“…The pre-Variscan sedimentary materials that build up the low-grade nappes (UAU, PAU and Foldand-Thrust Belt) are mostly late Ediacaran to early Ordovician in age as shown by paleontological data (Fournier-Vinas and Debat, 1970;Guérangé-Lozes and Burg, 1990;Reitz and Wickert, 1988) and dating of interlayered volcanics (Faure et al, 2009a;Lescuyer and Cocherie, 1992). In highergrade, gneissic units (UGU and LGU), detrital zircon studies (Chelle-Michou et al, 2017;Melleton et al, 2010) together with field relationships between meta-sediments and adjacent dated orthogneisses suggest maximum depositional ages ranging from the early Ediacaran to the Cambrian (Ledru et al, 1994;Melleton et al, 2010). The conspicuous presence of volcanic rocks (meta-tuffs, meta-lavas) throughout the stratigraphic pile exposed in low-grade units (Alvaro et al, 2014;Marini, 1987;Pouclet et al, 2017) and the existence of several orthogneiss massifs of similar ages within the high-grade domains (Alexandre, 2007;Duthou et al, 1984;Melleton et al, 2010;Roger et al, 2015) testify for a protracted magmatic activity from the late Ediacaran to the late Ordovician.…”

“…(i) the Cadomian Western Pacific-type marginal orogenic system, of Cryogenian-Ediacaran age (Chelle-Michou et al, 2017;Garfunkel, 2015;Linnemann et al, 2014;Nance et al, 1991); followed by, (ii) the Variscan orogeny, a major late Paleozoic continental collision episode resulting from the convergence between Laurussia and Gondwana (Kroner and Romer, 2013;Matte, 1986) and culminating with the assembly of Pangea, the latest supercontinent of Earth's history (Rogers and Santosh, 2003).…”

“…In the eastern part of the FMC, the Variscan nappes feature high-grade gneisses and, in particular, large amounts of meta-igneous rocks, which have likely witnessed one or several pre-Variscan magmatic episodes. Somewhat outdated radiometric dating from these orthogneisses (Caen-Vachette, 1979;R'Kha-Chaham et al, 1990) as well as detrital and inherited zircon data from metasedimentary rocks (Chelle-Michou et al, 2017) and Variscan granitoids (Laurent et al, 2017) respectively, suggest that they emplaced close to the Proterozoic-Paleozoic boundary at ca. 545…”