Multi-stage Garnet in the Internal Briançonnais Basement (Ambin Massif, Savoy): New Petrological Constraints on the Blueschist-facies Metamorphism in the Western Alps and Tectonic Implications

Three types of garnet have been distinguished in pelitic schists from an epidote-blueschist-facies unit of the Ambin and South Vanoise Brianconnais massifs on the basis of texture, chemical zoning and mineral inclusion characterization. Type-1 garnet cores with high Mn/Ca ratios are interpreted as pre-Alpine relicts, whereas Type-1 garnet rims, Type-2 inclusion-rich porphyroblasts and smaller Type-3 garnets are Alpine. The latter are all characterized by low Mn/Ca ratios and a coexist- ing mineral assemblage of blue amphibole, high-Si phengite, epidote and quartz. Prograde growth conditions during Alpine D1 high-pressure (HP) metamorphism are recorded by a decrease in Mn and increase in Fe (Ca) in the Type-2 garnets, culminating in peak P-T conditions of 14-16kbar and 500C in the deepest parts of the Ambin dome. The multi- stage growth history of Type-1 garnets indicates a polymeta- morphic history for the Ambin and South Vanoise massifs; unfortunately, no age constraints are available. The new meta- morphic constraints on the Alpine event in the massifs define a metamorphic T 'gap' between them and their surrounding cover (Brianconnais and upper Schistes Lustres units), which experi- enced metamorphism only in the stability field of carpholite- lawsonite (T 5 400C). These data and supporting structural studies confirm that the Ambin and South Vanoise massifs are slices of 'eclogitized' continental crust tectonically extruded within the Schistes Lustres units and Brianconnais covers. The corresponding tectonic contacts with top-to-east movement are responsible for the juxtaposition of lower-grade metamorphic units on the Ambin and South Vanoise massifs.