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 t he Gotthard and Tavetsch units (central Swiss Alps). In the Gotthard a nd Tavetsch units, a Late Proterozoic oceanic stage is inferred from c haracteristic 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-s ubduction magmatism (island are affinity) and HP metamorphism. Wedge a nd are sequences experienced several phases of metamorphism after the supra-subduction gabbroic intrusions. The oldest metamorphic event occ urred under HP conditions, with lawsonite as key mineral phase. During the prograde path, lawsonite was replaced by Ky + Zo + Qtz, and a hig her temperature eclogite assemblage formed (in the range of 650-700 de grees 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. Th e eclogitic event occurred during the Ordovician. A subsequent granuli te facies event is characterized by Grt-Di-Opx-Olig-Qtz-Hbl-Ilm-Mag-Py -Ttn. This assemblage yields temperatures ranging from 600 to 700 degr ees C and pressure of approximately 0.8 GPa. Hydration and partial mel ting of the metasediments accompanied the subsequent unroofing and coo ling. 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 intru sions. The resulting path reflects the style and rate of unroofing.