Alpine metamorphism of the Western Alps: II. High-P/T and related pre-greenschist metamorphism.

The Internal zones of the Western Alps contain assemblages indicative of one or more H-PIT imprints, which have been more or less overprinted in Eocene and Oligocene by the greenschist facies events described in the companion paper. In most eclogite-free zones there is a grade increase from S to N and from W to E (from higher to deeper units in the nappe pile). A lawsonite-albite-chlorite +/- carpholite facies is present in the Houillere zone S of the Aosta valley, and in the Briancon cover where Al-rich rocks contain minerals such as Mg-Fe-carpholite, Fe-chloritoid, cookeite and aragonite. In the basal thrust and shear zones of the Dent Blanche nappe and Second Diorito-Kinzigitic zone there are some indications of a H-PIT trend (phengite, Fe-Na- amphibole and greenish pyroxene). In the Briancon basement rocks and in the Combin zone a glaucophane-schist +/- jadeite facies is found from approximately the Isere-Arc valleys southwards, and epidote-blueschist associations northwards. These imprints may not have been coeval in all zones, but most if not all of them may be related to the Eocene thrusting phases.