TOURMALINE-PHENGITE PARAGENESES IN METASEDIMENTS FROM THE SIMANO NAPPE, SWISS CENTRAL ALPS

The Simano nappe is one of several large folded thrust sheets in the Penninic area of the Central Alps. It consists of a polymetamorphic basement core and a late Permian to Mesozoic sedimentary cover. Paleogeographically, the Simano nappe represents a part of the southernmost European plate at the transition from the Helvetic domain (European margin) to the Tethyan ocean. After nappe stacking during the Alpine orogeny, the entire Simano nappe was metamorphosed in amphibolite facies. The sedimentary cover consists, from bottom to top, of metapelites, metaconglomerates, quartzites, and dolomite marbles. Large euhedral tourmaline is unusually abundant in all these rocks, which occur in a stratigraphic position that is thought to represent the Permian/Triassic transition. Tourmaline generally grew post-kinematically with respect to the main foliation, which represents the nappe-stacking event. Moreover, the occurrence of tourmaline in these metasedimentary rocks is restricted to layers rich in white mica. At one locality, these layers occur in both dolomite and quartzite, and contain the assemblages Dravite+Phen+Phlog+Kspar+Qtz+Dol and Schorl+Phen+Mt+Qtz, respectively. This locality is well suited for phase petrologic investigations, which are currently carried out in an effort to derive field-based thermodynamic data for the tourmaline group. Preliminary estimates of metamorphic conditions for these rocks indicate a pressure between 10 and 13 kbars, considerably higher than the 5-6 kbars reported for the main Alpine event. This indicates that the tourmaline-phengite schists record a different, probably earlier stage of the orogeny, in agreement with a recent interpretation of data for a nearby garnet amphibolite in the Simano basement (Grandjean 2001). The estimated pressure is considerably lower than the peak pressure recorded in the overlying Adula nappe, which had a different tectonic history during the earliest stages of the Alpine evolution.