The tectonic implications of high-pressure metamorphism in the western Alps

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Abstract
Five mineralogical stages are recognized in the metamorphism of Alpine ophiolites from one area in the Zermatt–Saas zone. Constraints on P-T values indicate an abrupt change at the metamorphic peak (550–600 °C, 17–20 kbar) from a cold positive slope of 25°C/kbar or less, to cooling initially without discernible change of pressure. Correspondence of the prograde path to the P-T trend of T (in °C) = 150 + 23 P (in kbar) shown by a variety of Alpine units suggests that they all have P-T histories for which an England & Richardson (1977) relaxation model (also England & Thompson 1984) is inappropriate. Major tectonism at the peak pressure is indicated by association between minerals (kyanite, chloritoid, lawsonite in eclogite), vein growths, fluid action and thrust surfaces. The proposed metamorphic environment involves thrust repetition, subcreting ophiolite to the over-riding slab in a thermally mature subduction zone at a depth of approximately 70 km. Preservation is favoured when ocean closure leads to subduction of sialic crust, so providing a combination of buoyant uplift with the continued refrigeration by underthrusting of cold surface materials.

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