DECIPHERING PALEO-DEPOSITIONAL ENVIRONMENTS OF THE SIMANO NAPPE IN THE CENTRAL ALPS

The Simano Nappe, a thrust sheet located in the central Alps, today consists of metamorphosed rocks representing the transitional margin between the European plate and the Tethyan Ocean. The rock sequence is characterized by thick quartzites, metadolomites and gneisses interbedded with thin metapelites enriched in tourmaline, a boron silicate. These tourmaline-rich mica schist layers could be a record of shallow restrictive marine basins, which existed in the late Permian/ early Triassic at the Tethyan margin. These basins collected clays and muds with anomalously high boron contents, which would indicate evaporative events. This study tries to understand the paleo-depositional environments of these high boron metapelites. In addition, their repeated occurrence in carbonates and sandstones could be the record of changes in sea levels. Geochemical analysis of bulk rock has revealed boron contents in the tourmaline-rich mica layers of 600 – 6500 ppm, as compared to average published values for comparable mud rock protoliths of 50 – 130 ppm. Average boron contents in the studied quartzites and dolomites are 12 -100 ppm, which are typical for carbonate sediments and sandstones. These distinct compositional differences in finely interbedded rocks suggest that the marine margin was a dynamic system in an evaporative environment. The Simano Nappe has been extensively studied in the past focusing on the pressure and temperature conditions of metamorphism. The current study, using bulk rock compositions and field observations, tries to decipher the depositional environment of the Tethyan seashore during the late Permian.