Carboniferous high‐P metamorphism and deformation in the Belledonne Massif (Western Alps)

Abstract

AbstractThe age and P–T conditions of Variscan high‐P(HP) metamorphism in the Palaeozoic basement of the western Alps remain poorly constrained, but is nevertheless crucial to build a consistent tectonic scenario for the southeastern domain of the Variscan Belt. We report here the results of a structural, petrological, thermobarometric, and geochronological investigation of an eclogite‐bearing unit exposed in the northeastern part of the Belledonne Massif (France). This unit is mostly composed of metasediments that are locally migmatized and contain decametre‐ to hectometre‐scale lenses of orthogneiss and amphibolites. SIMS U–Pb dating of magmatic zircon cores in two retrogressed ecogites yields ages at 456 ± 4 Ma and 448 ± 6 Ma, which are interpreted to date the emplacement of the magmatic protoliths. The peak pressure stage in the retrogressed eclogites is estimated to be >1.4 GPa at 690–740°C, and was followed by decompression from 1.4 to ca. 1.0 GPa at 700–800°C. By contrast, the investigated migmatitic metasediment does not present any trace of HP metamorphism, but instead preserves prograde evolution from sub‐solidus conditions (ca. 0.8–1.1 GPa and 600–700°C) to supra‐solidus conditions (1.1–1.4 GPa and 700–780°C). A later stage of retrogression below ca. 0.5–0.8 GPa and 570–610°C is recorded in both lithologies, and is taken to indicate cooling and exhumation to upper crustal levels. Metamorphism was roughly coeval in the retrogressed eclogites and in the migmatitic metasediment. Metamorphic zircon rims yield U–Pb dates scattering between 340 and 310 Ma in both lithologies. In the migmatitic metasediment, a distinct younger age at 306 ± 3 Ma is interpreted to represent late stages of melt crystallization. In the retrogressed eclogites, zircon zoning and chemical composition (Th/U and REE) indicate initial crystallization of the rims during the HP stage followed by protracted growth during decompression to granulitic/amphibolitic conditions. Rutile U–Pb dating in one eclogite sample yields an age of 340 ± 11 Ma similar to the oldest zircon ages and is interpreted to approximate the age of the peak pressure metamorphism. Retrogression in the amphibolite facies is correlated with the development of a penetrative, N30° subvertical mylonitic S2 foliation. Regionally, this deformation occurs in a dextral transpressive corridor interpreted to represent a crustal‐scale shear zone active during the mid‐late Carboniferous. We therefore suggest that this structure has driven the exhumation of eclogites from the lower crust and their mixing with mid‐crustal felsic lithologies devoid of HP assemblages.