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

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 degrees C. A subsequent D-1 deformation is defined by a penetrative S-1 foliation that mostly dips toward the west and displays an E-W- to NE-SW-trending mineral and stretching lineation L-1. D-1 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 Riouperoux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa +/- 0.06; 608 +/- 14 degrees C) that attains partial melting at the base of the nappe pile (0.78 +/- 0.07 GPa; 680.5 +/- 11.5 degrees C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Riouperoux-Livet unit constrain the age of D-1 to 337 +/- 7 Ma. The D-2 tectono-metamorphic event is characterized by NE SW trending, upright to NE-verging synfolial folding. Folding associated with D-2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S-2 foliation. In particular, D-2 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 Riouperoux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D-2 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.