PRE-VARISCAN EVOLUTION OF THE ECLOGITISED MAFIC ROCKS FROM THE HELVETIC BASEMENT OF THE CENTRAL ALPS

This contribution examines new data on the stratigraphy, magmatism and metamorphism of polymetamorphic mafic rocks from the oldest part of the Gotthard and Tavetsch units (central Swiss Alps). In the Gotthard and Tavetsch units, a Late Proterozoic oceanic stage is inferred from characteristic chemical signatures of some of the mafic and ultramafic rocks. The closure of this oceanic domain involved both accretion and subduction. Convergent tectonics are also responsible for both supra-subduction magmatism (island are affinity) and HP metamorphism. Wedge and are sequences experienced several phases of metamorphism after the supra-subduction gabbroic intrusions. The oldest metamorphic event occurred under HP conditions, with lawsonite as key mineral phase. During the prograde path, lawsonite was replaced by Ky + Zo + Qtz, and a higher temperature eclogite assemblage formed (in the range of 650-700 degrees C). The stable assemblage at temperature peak is Grt-Omp-Ky-Qtz-Zo-Hbl-Ilm-Mag-Py and Rt. Estimated temperatures and minimum pressures at this stage are ca. 700-750 degrees C and 1.8 GPa, respectively. The eclogitic event occurred during the Ordovician. A subsequent granulite facies event is characterized by Grt-Di-Opx-Olig-Qtz-Hbl-Ilm-Mag-Py-Ttn. This assemblage yields temperatures ranging from 600 to 700 degrees C and pressure of approximately 0.8 GPa. Hydration and partial melting of the metasediments accompanied the subsequent unroofing and cooling. Late Ordovician granitoids truncate both granulite and migmatite rocks and locally caused retrogression of the eclogites. The minimum age of the HT overprinting is constrained by these post-orogenic intrusions. The resulting path reflects the style and rate of unroofing.