Geology along the Bedretto tunnel: kinematic and geochronological constraints on the evolution of the Gotthard Massif (Central Alps)

Abstract

The unlined Bedretto tunnel crosses large parts of the pre-Triassic basement of the Gotthard massif (Central Alps), giving the possibility to study late-Variscan plutonic rocks (Rotondo granite) and their Caledonian (poly-)metamorphic host rocks (Tremola and Prato series). The Rotondo granite consists mostly of an equigranular, fine-grained granite and to a lesser extent of a porphyritic granite. Commonly, the Rotondo granite is massive or only slightly foliated. Ductile deformation is localized along discrete shear zones composed of granitic or quartz-biotite-rich lithologies. This paper reviews the geology of the Bedretto tunnel with emphasis on the Rotondo granite and presents constraints based on kinematic, microstructural, and U–Pb geochronological evidence, which can be summarized as follows: (1) Both granitic and quartz-biotite-rich shear zones (QB-SZ) in the Rotondo granite generally dip moderately to steeply towards north and are related to top-to-south reverse shearing, indicating south-verging backthrusting during the exhumation of the Gotthard massif. (2) Zircons from both the equigranular and porphyritic Rotondo granite show overlapping {}^{206}hbox {Pb}/{}^{238}hbox {U}-age ranges of 285–319 Ma and 280–335 Ma, respectively, which indicate that both are part of the same late-Variscan magmatic episode. Almost no older inherited cores are reported. (3) In zircons from a QB-SZ, 30% of the concordant age spots scatter between 339 and 589 Ma. This suggests that the parent material of the QB-SZ is unrelated to the magmatic episode that formed the Rotondo granite, but rather that the QB-SZ represent sheared xenoliths within the granite.