COUNTERCLOCKWISE P-T-T PATHS FROM AMPHIBOLITES, FRANCISCAN COMPLEX, CALIFORNIA - RELICS FROM THE EARLY STAGES OF SUBDUCTION ZONE METAMORPHISM

Overprinting mineral relationships in amphibolites from the Franciscan Complex, California suggest metamorphic evolution from high-temperature amphibolite facies to the blueschist facies with increasing P/T ratio, a counterclockwise P-T-t path. Early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are, in turn, cut or rimmed by sodic amphiboles. Some amphibolites are overprinted successively by eclogite and blueschist facies assemblages. Geochronologic, textural, and petrologic data indicate that the overprinted amphibolites are probably the product of a single metamorphic event rather than a consequence of separate metamorphic episodes, but additional age dating is desirable for better confirmation. Amphibolite to blueschist facies metamorphic evolution may have taken place within 5 Ma. A model is presented for the evolution of Franciscan amphibolites. -from Author