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 H,O-CO, 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 aH,O between 0.55 and 1 are high enough to accommodate equilibration with a water-rich vapour phase under the highestgrade 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 Na,0-Ca0-Mg0-A1,0,-SiO~-H20, 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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Metre to tens-of-metre wide, steeply dipping, greenschist facies shear zones that cut blueschists and eclogites of the Combin and Zermatt±Saas Zones at Ta È schalp and in adjacent areas of the western Alps were sites of extensive recrystallization driven by¯uid¯ow and deformation. Rb±Sr data imply that these shear zones formed at 42±37 Ma with a systematic younging of structures northward toward, and into, the hangingwall of the Mischabel Structure. Shearing commenced at 400± 475 uC and 400±500 MPa and continued as pressures and temperatures fell to 300±350 uC and 300±350 MPa. Individual shear zones were active for 2±3 Myr with later lower grade stages of shearing concentrated into narrow zones. Fluids that in®ltrated the shear zones were water rich (X H 2 O >0.9). Alteration zones around albite veins and at the margins of serpentinite bodies are penecontemporaneous with these shear zones and formed at approximately the same conditions. The eclogites were exhumed from c. 64 km at 44 Ma to 14 ±16 km at 42± 41 Ma implying exhumation rates of 2±5 cm yr x1 . Rapid exhumation was probably achieved by extension aided by buoyancy, following subduction of continental crust, and rapid erosion. The shear zones form part of a regional-scale extensional system responsible for a signi®cant portion of the exhumation of the subducted oceanic crust.

The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however, despite its potential importance, fluid-rock interaction during high-pressure metamorphism is relatively poorly understood. The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites, suggesting that they interacted with little pervasive fluid during highpressure metamorphism. Cover sediments also have oxygen isotope ratios within the expected range of their protoliths. In the rocks that escaped late greenschistfacies retrogression, different styles of sub-ocean-floor alteration may be identified using oxygen isotopes, petrology, and major or trace element geochemistry. Within the basalts, zones that have undergone highand low-temperature sub-ocean-floor alteration as well as relatively unaltered rocks can be distinguished. Serpentinites have d 18 O and d 2 H values that suggest that they were formed by hydration on or below the ocean floor. The development of high-pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub-ocean-floor alteration and which contained hydrated, fine-grained, reactive assemblages. Given that the transformation of blueschist-facies metabasic rocks to eclogite-facies assemblages involves the breakdown of hydrous minerals (e.g. lawsonite, zoisite, and glaucophane), and will thus liberate considerable volumes of fluids, metamorphic fluid flow must have been strongly channelled.High-pressure (quartzccalciteBomphaciteBglaucophaneBtitanoclinohumite) veins that cut the ophiolite rocks represent one possible channel; however, stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids. Fluids were more likely channelled along faults and shear zones that were active during high-pressure metamorphism. Such strong fluid channelling may cause fluids to migrate toward the accretionary wedge, especially along the slab-mantle interface, which is probably a major shear zone. This may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting. Additionally, because fluids probably interact with relatively small volumes of rock in the channels, they cannot "scavenge" elements from the subducting slab efficiently.

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The timing of high-pressure (HP) metamorphism in the internal basement massifs of the Western Alps has been contentious. In the Gran Paradiso massif silvery micaschists, thought to have developed from granitic precursors, contain assemblages indicative of pressures in excess of 18 kbar at 500-550°C. This paper presents unique geochronological data for the paragenesis of the silvery micaschist HP assemblage. Rb-Sr microsampling of an apatite-phengite pair thought to have remained closed to Rb-Sr exchange since the HP paragenesis formed has yielded an age of 43.0 ± 0.5 Ma. Greenschist retrogression occurred after 36.3 ± 0.4 Ma, probably in the interval 36-34 Ma. The localised disturbance of the Rb-Sr system in phengite, apatite and allanite during retrogression means that only in situ microsampling could obtain meaningful ages from these rocks. The new data indicating a Tertiary age for HP metamorphism in the Gran Paradiso massif agree with recent data for other internal basement massifs in the Western Alps. A model fitting the Gran Paradiso massif into the Western Alpine framework is presented.

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