Within the Zermatt–Saas ophiolite zone of the western Alps a record of polystadial high-pressure metamorphism is well preserved. Early blueschist assemblages, retained as inclusions in garnets, were succeeded by eclogitic assemblages which in some rocks contained lawsonite, kyanite, talc and chloritoid. These eclogitic assemblages formed by prograde reaction from the early blueschists. Subsequently, reaction of these eclogites with a mixed H2O-CO2, vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite-garnet paragenses by assemblages containing glaucophane, paragonite and ankerite. High-pressure, eo-alpine metamorphism took place under conditions of 550–600°C and 17.5–20 kbar. Values of aH2O between 0.55 and 1 are high enough to accommodate equilibration with a water-rich vapour phase under the highest-grade conditions. The presence of such a fluid phase is locally indicated by the presence of quartz-rich veins containing omphacite and kyanite. This vapour phase was absorbed during retrogression. Later, rehydration is limited to areas close to albite veins, tectonic contacts and bodies of metasediment.

The metamorphic conditions determined for the Zermatt-Saas zone are compatible with those suggested for over- and under-lying units. These conditions, and the P–T path inferred by comparing the reaction histories with a petrogenetic grid for the system Na2O-CaO-MgO-A12O3-SiO2-H2O, suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the Alpine orogeny.

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