2 SUBDUCTION EVENTS IN A POLYCYCLIC BASEMENT - ALPINE AND PRE-ALPINE HIGH-PRESSURE METAMORPHISM IN THE SURETTA NAPPE, SWISS EASTERN ALPS

This study is essentially based on coupling macrostructures, microstru ctures and metamorphic petrology in polymetamorphic mafic rocks from t he Swiss Eastern Alps (Suretta nappe, Penninic domain). Petrographic c riteria are used in conjunction with structural analysis and microprob e work to define crystallization/deformation relationships and to esta blish a relative but precise sequence of tectonometamorphic events. A first eclogite facies overprint and related exhumation occurred before emplacement of late Palaeozoic intrusives. During the Alpine cycle, t he Suretta nappe was part of the thinned European continental margin. The Tertiary burial due to subduction and collision is responsible for D1 ductile thrusting and blueschist facies metamorphism. Late deforma tion phases, related to exhumation, are responsible for the developmen t of extensional structures under greenschist facies conditions. Quant itative metamorphic petrology based on Gibbs free energy minimization (DOMINO by de Capitani) gives a constraint on the P-T conditions durin g the polymetamorphic and polycyclic evolution. The first high-P metam orphic event related to pre-Alpine structures occurred at c. 700 degre es C and at least 2.0 GPa. These conditions are compatible with pre-Al pine high-P re-equilibration already described in several Alpine units . The Alpine high-P metamorphism occurred under blueschist facies cond itions at c. 400-450 degrees C and 1.0 GPa. Similar high-P, low-T cond itions have already been described in the Mesozoic and Permian rock ty pes. The two high-P events are clearly related to two different geothe rmal regimes and geodynamic environments.