Hot metamorphic core complex in a cold foreland

Abstract

The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Viséan/Namurian boundary (B320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D 1 ), all structural levels shared the same tectonic evolution: D 2 (extension and exhumation), D 3 (refolding) and post-D 3 dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M 2 ). M 2 occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D 2 effects defines a bulk prolate strain in a ''pinched pull-apart'' setting. Ductile D 2 deformation during M 2 dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D 1 is only represented by the inverted position of bedding (overturned limbs of recumbent D 1 folds) and by refolded D 1 folds. U-Pb monazite and zircon ages and K-Ar muscovite ages are in accord with Ar-Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at C330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shearzone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward

Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire, French Massif Central)

“…Protoliths of the gneiss have been dated by U–Pb zircon analysis at c. 520 Ma and c. 470–450 Ma (Cocherie et al, ; Ducrot, Lancelot, & Marchand, ; Pitra, Poujol, van den Driessche, Poilvet, & Paquette ; Roger, Respaut, Brunel, Matte, & Paquette, ; Roger et al, ; Trap, Roger, Cenki‐Tok, & Paquette, ). High‐ T metamorphism and deformation of gneiss/schist has been determined by U–Th–Pb monazite ages at c. 315–300 Ma (Roger et al, ; Trap et al, ) and by U–Pb zircon dating of a syntectonic felsic dike at c. 309 Ma (Franke, Doublier, Klama, Potel, & Wemmer, ).…”

“…Demange () calculated conditions of ~700°C, 0.9 ± 0.2 GPa for the dome‐margin eclogite using jadeite‐in‐Cpx barometry and Grt–Cpx Fe–Mg exchange thermometry. Another study described a phase diagram (pseudosection) constructed using the whole‐rock composition of the Cabardès (dome‐margin) eclogite from Demange () and reported P – T conditions of >650°C, >1.4 GPa but did not show the phase diagram or describe how it was calculated (Franke et al, ). To determine P – T conditions of eclogite facies metamorphism of the dome‐margin eclogite and compare results with the dome‐core eclogite using the same methods, we constructed pseudosections for the least retrogressed dome‐margin eclogite and applied Zr‐in‐rutile and Ti‐in‐zircon thermometry.…”

The Kulm Facies of the Montagne Noire (Mississippian, southern France)

“…The Montagne Noire itself is characterized by a complex polyphased Variscan deformation and syntectonic metamorphism (e.g. Franke et al, 2011;Chardon et al, 2015), and is one of the rare areas south of the Variscan suture, which has not been incorporated into Alpine orogenies (Feist & Flajs, 1987). Engel et al, 1982) Demange, 1994).…”

Environmental controls on the development of Mississippian microbial carbonate mounds and platform limestones in southern Montagne Noire (France)

“…(A) General location of the Montagne Noire in southern France. (B) Structural units and nappes in the Montagne Noire (for details of the units see Engel et al ., , and Franke et al ., ). (C) Geological map of the Mont Peyroux nappe and Cabrières slices (for details see Vachard et al ., ).…”