Book Description

A detailed geochemical and geochronological study on rocks of the Monte Rosa Nappe (MR; Western Alps) was carried out. The MR was subject to Alpine high-pressure and greenschist-facies metamorphism. An U-Pb geochronological study of zircons and monazites of the MR granite revealed a Permian intrusion age (270 ± 4 Ma). The MR granite belongs to the post-Variscan magmatic assemblages that demonstrate the instability of the Variscan continental crust. A paleogeographic position of the MR as part of the 'Briançonnais domain' is assumed. Talc-kyanite-chloritoid-rocks ('whiteschists') are found within the MR granite. They are indicators for a high-pressure metamorphism in the MR. Mass balance calculations assess the mass transfer necessary to produce a whiteschist from a granitic protolith. A model for the evolution of whiteschists is presented here. An in-situ 40Ar/39Ar UV-laser ablation study on white micas of the Alpine mineral assemblage revealed a heterogenous age distribution. The ages can be explained by recrystallization of micas under relatively 'dry' Alpine high-pressure metamorphic conditions with accompanied loss of radiogenic argon. A similar complex evolution with partial homogenization of the isotopic system can be observed for the Sr-system. These studies yielded insight into the closure behavior of isotopic systems under high-pressure metamorphic conditions.