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 A lpine metamorphic imprints of middle to high T/P ratio are examined. These are found in three different rock groups. (1) Zn post-Variscan ophiolitic rocks, ocean-floor metamorphism predate all Alpine regional orogenic imprints. The facies of the ocean-floor metamorph ism 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 t he basement massifs, but with highs around these massifs and duplexes produ ced by thrusting. There is evidence of several stages, from Late Cretaceous to Miocene. (3) In the Internal zones, ubiquitous greenschist facies can locally be sub divided into two main imprints, the first of middle pressure and characteri zed by phengite, the second being characterized by high TIP trend and not a ssociated with a new foliation. The greenschist facies are replaced by amph ibolite facies toward the Central Alps. Both greenschist facies are younger than the I-I-PIT 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 are of Eocene-early Miocene age.