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

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