Tectono-metamorphic evolution of UHP Zermatt-Saas serpentinites: a tool for vertical palaeogeographic restoration

Abstract

ABSTRACT Within the Zermatt-Saas Zone (ZSZ, northwestern Alps), Ti-chondrodite- and Ti-clinohumite-bearing assemblages in serpentinites indicate UHP conditions. Multiscale structural analysis (1:20 scale mapping) and petrological investigation of serpentinites at Créton (upper Valtournanche) evidenced a polyphasic deformation and metamorphic history. In this locality and at regional scale, S2 is the dominant foliation that developed under HP-UHP conditions. Pre-D2 mineral and textural relicts are preserved despite the pervasiveness of S2. Pre-D2 olivine + Ti-chondrodite + spinel assemblage implies re-equilibration at 2.8–3.3 GPa and 600–630°C, in agreement with conditions recorded by coesite- and microdiamond-bearing rocks in the Cignana Lake Unit. The PT conditions inferred for syn-D2 assemblages at Créton are similar to those estimated for D2 in the surrounding serpentinites, which were dated at 65 ± 5.6 Ma. These results suggest that portions of ZSZ were subducted at high depth before 70 Ma and widen the time span during which ZSZ recorded PT peak conditions. The comparison of these data with results of a numerical model of an ocean-continent subduction system gives insights on coupling stages of this UHP unit with the surrounding ZSZ rocks during the Alpine convergence and vertical palaeogeography over subsequent time steps.