Polyphase HP metamorphism in the ophiolitic glaucophanites of the lower St. Marcel valley (Aosta valley)

extural and chemical evidences give some insights on the HP tectono-metamorphic events that affected the ophiolitic glaucophanites of the St. Marcel Valley (Aosta, Italy). Pre-kinematic (early) LT blueschist conditions, consistent with lawsonite and glaucophane stability, are assumed on the basis of relict minerals enclosed in garnet cores. Syn-kinematic HT blueschist to eclogite conditions are deduced from the disappearance of lawsonite. Almandine-rich garnet - Mg-Fe chloritoid - glaucophane ± zoisite ± paragonite ± chlorite are the most common phases in glaucophanites, omphacite - garnet - rutile in eclogites. Post-kinematic, late-eclogite conditions are suggested by small recrystallized rims of garnet, growth of widespread unhomogeneous crossite and development of a new generation of chloritoid.