Protracted, coeval crust and mantle melting during Variscan late-orogenic evolution: U–Pb dating in the eastern French Massif Central

Abstract

International audienceThe late stages of the Variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt, which makes it akin to “hot” orogens. These processes resulted in the development of large granite–migmatite complexes, the largest of which being the 305–300-Ma-old Velay dome in the eastern French Massif Central (FMC). This area also hosts a wide range of late-Variscan plutonic rocks that can be subdivided into four groups: (i) cordierite-bearing peraluminous granites (CPG); (ii) muscovite-bearing peraluminous granites (MPG); (iii) K-feldspar porphyritic, calc-alkaline granitoids (KCG) and (iv) Mg–K-rich (monzo)diorites and lamprophyres (“vaugnerites”). New results of LA-SF-ICP-MS U–Pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of ~40 million years throughout the Carboniferous, as shown by intrusion ages between 337.4 ± 1.0 and 298.9 ± 1.8 Ma for the granitoids, and between 335.7 ± 2.1 and 299.1 ± 1.3 Ma for the vaugnerites. Low zircon saturation temperatures and abundant inherited zircons with predominant late Ediacaran to early Cambrian ages indicate that the CPG and MPG formed through muscovite or biotite dehydration melting of ortho- and paragneisses from the Lower Gneiss Unit. The KCG and vaugnerites contain very few inherited zircons, if any, suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites. The KCG can be explained by interactions between the CPG/MPG and the vaugnerites, or extensive differentiation of the latter. The new dataset provides clear evidence that the eastern FMC was affected by a long-lived magmatic episode characterized by coeval melting of both crustal and mantle sources. This feature is suggested here to result from a lithospheric-scale thermal anomaly, triggered by the removal of the lithospheric mantle root. The spatial distribution of the dated samples points to a progressive southward delamination of the lithospheric mantle, perhaps in response to rollback following continental subduction, or to “retro-delamination” owing to the retreat of a south-verging subduction zone

Exhuming a cold case: The early granodiorites of the northwest Iberian Variscan belt—A Visean magmatic flare-up?

“…This early Carboniferous suite is widely represented in other areas of the WEVB (e.g., Schaltegger, 1997Schaltegger, , 2000Schulmann et al, 2002;Eichhorn et al, 2000;Cartannaz et al, 2007;Li et al, 2014;Mezger and Gerdes, 2016;Tabaud et al, 2015;Galán et al, 1997;Ledru et al, 2001;Rossi et al, 2009;Kubínová et al, 2017;Laurent et al, 2017;Žák et al, 2017), including the Ossa Morena zone of southwest Iberia (e.g., Dallmeyer et al, 1995;Ordóñez Casado, 1998;Montero et al, 2000;Ordóñez Casado et al, 2008;Romeo et al, 2006;Jesus et al, 2007;Pin et al, 2008;Pereira et al, 2009Pereira et al, , 2015Braid et al, 2012;Dupuis et al, 2014;Gladney et al, 2014;Cambeses et al, 2015). In the Ossa Morena zone rocks belonging to this suite are the volumetrically dominant granitoids, whereas in northwest Iberia the volumetrically dominant granitoids are both the previously described syntectonic leucogranites and the postorogenic granites (Fig.…”

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

“…340 Ma and ceasing in the Cévennes Area (near Alès) at ca. 300 Ma (Laurent et al, 2017). In addition, from ca.…”

“…However, this magmatism was spatially and temporally associated with significant input of mantle-derived magmas (e.g., biotite-amphibole calc-alkaline granodiorites, quartz-diorites and high-Mg and high-K mafic rocks; Fig. 1; Barbey et al, 1999Barbey et al, , 2015Couzinié et al, 2014Couzinié et al, , 2016Downes et al, 1997;Laurent et al, 2017;Solgadi et al, 2007;Williamson et al, 1996;. The formation of the Velay granite-migmatite dome at 310-300 Ma marks the gravitational collapse of the orogen, which was coeval with the development of ductile extensional shear zones and opening of the late Carboniferous (Stephanian) coal-bearing basins (Burg et al, 1990;Burg and Vanderhaeghe, 1993;Malavieille, 1993;Malavieille et al, 1990).…”

“…1). Syn-to post-kinematic plutons and batholiths surround the Velay dome (the "peri-Velay" granites; Faure et al, 2009;Laurent et al, 2017;Ledru et al, 2001). Magmatism migrated southward, beginning within the Forez and Lyonnais areas (North of Saint-Etienne; Fig.…”

“…2). This heterogeneous granite contains many fragments of country rocks (including an enclave of porphyritic biotite granite dated at 321.9 ± 1.3 Ma; Laurent et al, 2017) and is characterized by the presence of cockade-type cordierite (see Barbey et al, 1999).…”