Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central

Abstract

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and 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. These features reflect the horizontal growth of the Variscan belt and the 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 Paleothethys 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 correspond to post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.

Strain Partitioning Within Bending Orogens, New Insights From the Variscan Belt (Chiroulet‐Lesponne Domes, Pyrenees)

“…While those from the NPM are similar to the gneiss domes that were developed in the Pamir syntax (i.e., syn‐convergence), those from the eastern Massif Central look different. Indeed, this massif only recorded N‐S extension at 305–300 Ma, leading to the development of widespread sedimentary basins and locally extensional domes (Figure 13a) (Ducassou et al., 2019; Faure et al., 2009; Vanderhaeghe et al., 2020, and references therein). This suggests a regional evolution with its own dynamics in response to an asymmetric delamination process (Laurent et al., 2017).…”

“…km wide and km long continuous shortened belt. c) Conceptual model of lateral escape having taken place in the Armorican massif, after Gumiaux et al, (2004) and Gapais et al, (2015); d) Conceptual model of transverse extension in the Eastern Massif Central, after Vanderhaeghe et al, (2020). Further explanation in the text.…”

Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

“…Early to middle Carboniferous pervasive MP/MT Barovian metamorphism has been attributed to the transition from continental subduction to collision [22,23] and the late Carboniferous exhumation of large migmatite domes associated with LP/HT metamorphism is considered to represent the gravitational collapse of the Variscan belt [24][25][26][27]. The evolution of the Variscan belt is marked by the intrusion and emplacement of a variety of magmas reflecting (i) the transition from an active margin with back-arc basins from the Devonian to the early Carboniferous and (ii) protracted partial melting of orogenic root coeval with the input of mantle-derived magmas from the middle Carboniferous to the Permian [28][29][30][31][32][33][34].…”

Metabasic rocks from the Variscan Schwarzwald (SW Germany): metamorphic evolution and igneous protoliths

“…The uppermost 'Granulite (Gföhl) Unit' or 'Upper Gneiss Unit (UGU)' in the FMC consists of relic HP-UHT granulites with intercalations of UHP-UHT garnetbearing peridotites and eclogites (e.g. Fuchs 1976;Thiele 1984;Rey et al 1992;Gardien et al 1997;Berger et al 2010;Faryad et al 2013;Altherr and Soder 2018;Sorger et al 2020;Vanderhaeghe et al 2020). Not only the garnet peridotites and eclogites but also the granulites of this unit locally contain relic UHP minerals such as coesite and diamond (Lardeaux et al 2001;Naemura et al 2011;Perraki and Faryad 2014;Jedlicka et al 2015;Thiéry et al 2015).…”