Very-high-pressure metamorphism in the western Alps: implications for subduction of continental crust

Abstract

The widespread occurrence of coesite and pure pyrope, the presence of several new mineral assemblages and new rock-forming minerals in common lithologies, but also crystal—chemical features such as the existence of silicate—phosphate solid solutions or the repeated occurrence among the new minerals of a new dense structure with face-sharing octahedra, are evidence that unusual pressures have been attained in part of the Dora Maira massif, western Alps, during Alpine regional metamorphism. Mineral assemblages suggest minimum pressures well in excess of 25 kbar (1 bar = 10 5 Pa) which, according to preliminary experimental data, may have reached 35 kbar in this at least 5x10 km 2 continental terrain clearly of supra-crustal origin. Obviously even continental material may be buried to depths of the order 100 km and then uplifted to the surface. Whereas the burial in a low temperature regime is readily explained by subduction of the continental lithosphere (of a peninsula?) 110 Ma ago or earlier, the uplift is much more problematic because it is petrologically constrained to proceed without significant temperature increase. The progressive migration 'in-plate’ of intra-continental thrusts once subduction was blocked, accounts for the stepwise decrease in age and grade of high-pressure metamorphism toward the external part of the chain; by the repeated underthrusting of cold material it might also have prevented a temperature increase in the most internal, early subducted zones.