The Acceglio zone (Cottian Alps): a new example of eclogitized continentalcrust in the western Alps.

Petrological study on andesitic schists sampled in the Acceglio zone, allow s, for the first time, the Alpine P-T path of this metamorphosed continenta l unit to be quantified. Three successive metamorphic stages are distinguis hed, the first one under eclogite facies conditions, With a pressure of 13. 5 +/- 1.5 kbar and 3 temperature of 450 +/- 25 degrees C, A second stage, c haracterized by a moderate temperature increase at the beginning of the dec ompression, corresponds to a reequilibration near the boundary between blue schist and eclogite facies conditions. The third stage, characterized by th e costability of lawsonite and albite, documents a reequilibration at the b oundary boundary blueschist and greenschist facies conditions. These new da ta confirm the heterogeneity of the metamorphic conditions observed in tilt Piemont zone of the Western Alps. They are in agreement with structural da ta and allow; us to interpret thr Acceglio domain as a tectonic extrusion o f eclogitized continental crust within the less metamorphic calcschists of the Piemont zone. (C) 2000 Academic des sciences / Editions scientifiques e t medicales Elsevier SAS.