SM-ND ISOTOPIC EVIDENCE ON THE AGE OF ECLOGITIZATION IN THE ZERMATT-SAAS OPHIOLITE

Rocks within the Zermatt-Saas ophiolite of the western Alps have under gone eclogite facies metamorphism during subduction prior to the Alpin e collision. The metamorphic history of these rocks is well defined, w ith eclogitic assemblages being followed by the limited growth of blue schist assemblages of glaucophane and paragonite. Subsequent greenschi st alteration occurs adjacent to faults, veins and metasediments. Away from such sources of water, retrogression is very limited. Sm-Nd isot opic analyses of an essentially unretrogressed eclogitic metabasalt su ggest that eclogite facies metamorphism occurred at 52 +/- 18 Ma. The large uncertainty is due to the presence of very small amounts of Nd-r ich epidote present as inclusions within garnet. As the closure temper ature of garnet to Sm & Nd is thought to be >600-degrees-C, resetting due to post-high-pressure diffusion is thought to be insignificant. Gi ven the fine-grained protolith to the sample analysed, and its extensi ve deformation under eclogite facies conditions, incomplete homogeniza tion of pre-metamorphic isotopic variations is also considered unlikel y to be responsible for the young age. A Tertiary age of eclogitizatio n means that models of early Alpine evolution based on the cessation o f high-pressure metamorphism in the Cretaceous need to be revised.