Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)

Kévin Fréville (1, 2) , Pierre Trap (3) , Michel Faure (1, 2) , Jérémie Melleton (4) , Xian-Hua Li (5) , Wei Lin (6) , Olivier Blein (4) , Olivier Bruguier (7) , Marc Poujol (8)

1 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
2 Métallogénie - UMR7327
3 LCE - Laboratoire Chrono-environnement (UMR 6249)
4 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
5 SKL - State Key Laboratory of Lithospheric Evolution
6 CAIQ - Chinese Academy of Inspection And Quarantine
7 Géosciences Montpellier
8 Terre, Temps, Traçage

A structural and petrochronological study was carried out in the southern part of the Belledonne crystalline massif. A first tectonometamorphic event, Dx, corresponds to the eastward thrusting of the Chamrousse ophiolitic complex characterized by a low-temperature–moderate-pressure metamorphism reaching 0.535 ± 0.045 GPa and 427.5 ± 17.5 °C. A subsequent D1 deformation is defined by a penetrative S1 foliation that mostly dips toward the west and displays an E–W- to NE–SW-trending mineral and stretching lineation L1. D1 is associated with a top-to-the east shearing and is responsible for the crustal thickening accommodated by the eastward nappe stacking and the emplacement of the Chamrousse ophiolitic complex upon the Rioupéroux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa ± 0.06; 608 ± 14 °C) that attains partial melting at the base of the nappe pile (0.78 ± 0.07 GPa; 680.5 ± 11.5 °C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Rioupéroux-Livet unit constrain the age of D1 to 337 ± 7 Ma. The D2 tectono-metamorphic event is characterized by NE–SW trending, upright to NE-verging synfolial folding. Folding associated with D2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S2 foliation. In particular, D2 involves the uppermost weakly metamorphosed Taillefer unit. LA-ICP-MS U-Pb dating performed on detrital zircon grains shows that the Taillefer conglomerates was deposited during the Visean. A zircon SIMS U-Pb age of 352 ± 1 Ma from a plagioglase-rich leucocratic sill of the Rioupéroux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D2 event took place between 330 Ma and 310 Ma. We propose a new interpretation of the tectonometamorphic evolution of the southern part of the Belledonne massif, focusing on the Middle Carboniferous stages of the Variscan orogeny.

Keywords: Zircon and monazite dating; Thermobarometry; Variscan Belt; Alpine External Crystalline Massifs; Belledonne Massif; SIMS and LA-ICP-MS dating

Domains: Tectonics

Kévin Fréville, Pierre Trap, Michel Faure, Jérémie Melleton, Xian-Hua Li, et al.. Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France). Tectonophysics, 2018, 726, pp.14-42. ⟨10.1016/j.tecto.2018.01.017⟩. ⟨insu-01698809⟩