ALPINE METAMORPHISM OF THE WESTERN ALPS: I. MIDDLE TO HIGH T/P METAMORPHISM.

After an overview of the structure and lithology of the Western Alps, the Alpine metamorphic imprints of middle to high T/P ratio are examined. These are found in three different rock groups. (1) In post-Variscan ophiolitic rocks, ocean-floor metamorphism predate all Alpine regional orogenic imprints. The facies of the ocean-floor metamorphism range from granulite to very low grade. (2) In the more external zones, illite crystallinity shows the distribution of diagenetic, anchizonal and epizonal domains to be broadly parallel to the basement massifs, but with highs around these massifs and duplexes produced by thrusting. There is evidence of several stages, from Late Cretaceous to Miocene. (3) In the Internal zones, ubiquitous greenschist facies can locally be subdivided into two main imprints, the first of middle pressure and characterized by phengite, the second being characterized by high T/P trend and not associated with a new foliation. The greenschist facies are replaced by amphibolite facies toward the Central Alps. Both greenschist facies are younger than the H-P/T Alpine imprints discussed in the companion paper. The age of the first greenschist imprint is Middle Eocene, the second is probably Oligocene in age. Both are interpreted within the geodynamical frame of the assumed continental magmatic arc of Eocene-early Miocene age.