PROGRADE HIGH-PRESSURE TO ULTRAHIGH-PRESSURE METAMORPHISM AND EXHUMATION OF OCEANIC SEDIMENTS AT LAGO-DI-CIGNANA, ZERMATT-SAAS ZONE, WESTERN ALPS

Pelagic metasediments and MORE-type metabasalts of the former Tethyan oceanic crust at Cignana, Valtournanche, Italy, experienced UHP metamo rphism and subsequent exhumation during the Early to Late Tertiary. Ma ximum PT conditions attained during UHP metamorphism were 600-630 degr ees C, 2.7-2.9 GPa, which resulted in the formation of coesite-glaucop hane-eclogites in the basaltic layer and of omite-aragonite-lawsonite- coesite-phengite-bearing calc-schists and garnet-phengite-coesite-schi sts with variable amounts of epidote, talc, dolomite, Na-pyroxene and Na-amphibole in the overlying metasediments. During subduction the roc ks followed a prograde HP/UHP path which in correspondance with the Ju rassic age of the Tethyan crust reflects the thermal influence of rela tively old and cold lithosphere and of low to moderate shear heating. Inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle-pl astic transition in the quartz-aragonite-dominated rocks, induced conv ection in the asthenospheric mantle wedge and/or heat consumption by e ndothermic reactions over a restricted PT segment during subduction. A fter detachment from the downgoing slab some 50-70 Ma before present, the Cignana crustal slice was first exhumed to ca, 60 km and concomita ntly cooled to ca. 550 degrees C, tracing back the UHP/HP prograde pat h displaced by 50-80 degrees C to higher temperatures. Exhumation at t his stage is Likely to have occurred in the Benioff zone, while the su bduction of cool lithosphere was going on. Subsequently, the rocks wer e near-isothermally exhumed to ca. 30 km, followed by concomitant deco mpression and cooling to surface conditions (at < 500 degrees C, < 1 G Pa). During this last stage the UHPM slice arrived at its present tect onic position with respect to the overlying greenschist-facies Combin zone. In contrast to the well-preserved HP/UHPM record of the coesite- glaucophane eclogites, the HP/UHP assemblages of the metasediments hav e been largely obliterated during exhumation. Relies from which the me tamorphic evolution of the rocks during prograde HP metamorphism and t he UHP stage can be retrieved are restricted to rigid low-diffusion mi nerals like garnet, dolomite, tourmaline and apatite. (C) 1998 Elsevie r Science B.V.