Contribution of Petrologic Studies from the Western Alps to the Understanding of the HP / UHP Metamorphism

The Western Alps are an ideal area for the study of the subduction-related metamorphism for a number of reasons, such as the good exposures, and the widespread occurrence of the high-P mineral assemblages, which cover the entire field of the relevant P-T conditions. This explains why many significant investigations on the HP/UHPM have been done in the Western Alps.

Let's first mention the discovery in the Austroalpine Sesia-Lanzo basement of the Monte Mucrone metagranite recrystallised under eclogite-facies conditions (Compagnoni & Maffeo, 1973), and the finding of coesite in continental crust (Chopin, 1984). Both occurrences show that relatively large portions of continental crust may be subducted to mantle depths, in seemingly disagreement with Plate Tectonics, whereas the coesite finding opened the new fascinating field of the ultrahigh-pressure metamorphism, which is giving new compelling evidence that in Collisional Orogenic Belts tectonic mechanisms may exhume rock slabs from unexpectedly great depths.

A number of other important petrological results may be also mentioned, such as the discovery that Mg-carpholite (Goffé et al., 1973) and the Mg-Fe-carpholite_ss (Goffé & Saliot, 1977) are often the only way to recognize HP metamorphism in low-grade metamorphics, lacking other blueschist-facies index minerals, that both the talc + phengite assemblage (Chopin, 1981) in micaschists, and the forsterite + Ti-clinohumite + chlorite assemblage in serpentinites (Scambelluri et al., 1991) indicate eclogite-facies conditions.

Before concluding, it cannot be ignored that Most Alpine HP/UHP tectonic units preserve portions unaffected by pervasive deformation, where pseudomorphic and coronitic reactions have been studied in detail and modelled (Bruno et al., 2001). In some areas the lack of deformation is accompanied by preservation of primary minerals, seemingly unmetamorphosed. This uneven distribution of metamorphism, sometimes ascribed to localized tectonic/fluid overpressure, has been explained by Rubie (1990), who found in a relict plagioclase nanocrystals of kyanite.