Triassic pegmatites in the Mesozoic middle crust of the Southern Alps (Italy): Fluid inclusions, radiometric dating and tectonic implications

Abstract

The schists in the northern part of the South-Alpine crystalline basement along Lake Como record Barrovian syn-kinematic metamorphism of Variscan age. They cooled below the Rb-Sr whole rock closing temperature at ca. 300 Ma and were exhumed by ca. 6-7 km before the Late Permian. In the Middle Triassic a thermal perturbation affected the South-Alpine middle crust leading to the widespread transformation of garnets into biotite + sillimanite aggregates under static conditions. Anatectic pegmatites were emplaced roughly contemporaneous with the peak temperature conditions. Rb-Sr mineral ages on pegmatites, schists and marbles between 229 and 194 Ma show the crust was again cooling during the Late Triassic, when continental rifting started. Stretching leading eventually to the opening of the Ligurian-Piemont ocean continued until Middle Jurassic times. Fluid inclusion data from the pegmatites establish that only limited decompression took place during Late Triassic to Early Cretaceous cooling. As a result of Alpine shortening, the rocks were eventually exhumed to the surface