JURASSIC FORMATION AND EOCENE SUBDUCTION OF THE ZERMATT-SAAS-FEE OPHIOLITES - IMPLICATIONS FOR THE GEODYNAMIC EVOLUTION OF THE CENTRAL AND WESTERN ALPS

The Zermatt-Saas-Fee ophiolites (ZSFO) are one of the best preserved s lices of eclogitic oceanic crust in the Alpine chain. They formed duri ng the opening of the Mesozoic Tethys and underwent subduction to HP/U HP conditions during Alpine compression. A cathodoluminescence-based i on microprobe (SHRIMP) dating of different zircon domains from metagab bros and oceanic metasediments was carried out to constrain the timing of formation and subduction of this ophiolite, two fundamental questi ons in Alpine geodynamics, The formation of the ophiolitic sequence is constrained by the intrusion ages of the Mellichen and the Allalin me tagabbros (164.0 +/- 2.7 Ma and 163.5 +/- 1.8 Ma) obtained on magmatic zircon domains. These data are in line with the maximum deposition ag e for Mn-rich metasediments which overlie the mafic rocks at Lago di C ignana (161 +/- 11 Ma) and at Sparrenflue (ca. 153-154 Ma). An Eocene age of 44.1 +/- 0.7 Ma was obtained for whole zircons and zircon rims from an UHP eclogite and two metasediments at Lago di Cignana. One of the Eocene zircons contains a rutile inclusion indicating formation at HP conditions. As the temperature and pressure peak of these rocks ne arly coincide, the Eocene zircons probably constrain the age for the d eepest subduction of the ZSFO. This Eocene age for the UHP metamorphis m implies that the ZSFO were subducted later than the Adriatic margin (Sesia-Lanzo Zone) and before the Late Eocene subduction of the Europe an continental crust below Apulia. A scenario with three subduction ep isodes propagating in time from SE to NW is proposed for the geologica l evolution of the Central and Western Alps.