Petrological constraints on the eclogite‐ and blueschistfacies metamorphism and P‐T‐t paths in the Western Alps*

Abstract

Phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high‐P rocks in the nappes of the Western Alps: very high‐P eclogite‐facies (including kyanite eclogites and coesite‐pyrope assemblages), eclogite‐facies (paragonite‐zoisite eclogites), high‐T blueschist‐facies (glaucophane‐garnet ± lawsonite assemblages) and low‐T blueschist‐facies (glaucophane‐lawsonite ± pumpellyite assemblages). The blueschist‐facies‐eclogite‐facies transition is promoted chiefly by increasing T, low bulk XMg and relatively low μH2O. The variety of assemblages and the heterogeneous approach to equilibrium observed in the Alpine rocks are not only constrained by the intersection of the reaction surfaces in P‐T‐X space, but also by the effect and timing of the processes which control kinetics (i.e. pervasive deformation and fluid infiltration). The faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures (and times), all may have induced μH2O gradients and contributed to the heterogeneous distribution of deformation through a process of reaction‐enhanced ductility. Also mass‐transfer may have been an effective process in determining the type of high‐P assemblage in particular rock volumes.As regards the P‐T‐t paths, only the post‐climax histories are recorded well in the Alpine nappes. Post‐eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early‐Alpine (Cretaceous) event. In contrast, more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early‐Alpine event and the subsequent meso‐Alpine (Palaeogene) collision between the ‘European’and ‘African’plates. In the Western Alps, prevalent eclogite‐facies conditions were attained during the metamorphic climax of the early‐Alpine subduction, while blueschist‐facies recrystallization characterizes the early‐Alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso‐Alpine continental collision.