METAMORPHIC CONVERGENCE OF THE UPPER AND LOWER PLATES OF THE VINCENT THRUST, SAN-GABRIEL MOUNTAINS, SOUTHERN CALIFORNIA, USA

The Vincent thrust of the San Gabriel Mountains, southern California, separates eugeoclinal Pelona Schist from overlying Precambrian to Meso zoic igneous and metamorphic rocks of North American continental affin ity. The thrust is generally considered to be synmetamorphic because o f similarity in structural orientations and mineral assemblages betwee n the Pelona Schist and mylonites at the base of the upper plate. In t his study, compositions of calcic amphibole and plagioclase in the upp er plate and structurally high Pelona Schist were compared to further test this interpretation. Amphibole in the schist is mostly actinolite to actinolitic hornblende with high Na/Al ratio, indicating relativel y high-P/low-T metamorphism. Individual grains are zoned, with concent rations of both Na and Al decreasing from cores to rims. Premylonitic amphibole in the upper plate is hornblende, tschermakite and pargasite with compositions indicative of low- or medium-P metamorphism. During mylonitization, this amphibole was replaced by actinolite to actinoli tic hornblende with a similar range of Na and Al as amphibole rims in the Pelona Schist, but with slightly lower Na/Al ratio. This is consis tent with the decrease of Na/AI up-section previously noted within the Pelona Schist of this area, and is considered to be the result of an inverted thermal gradient during thrusting. Convergence of composition between schist and upper plate also occurs for K and Ti contents of a mphibole and An content of plagioclase. These features provide strong evidence that mylonitization of the upper plate is closely related in space and time to metamorphism of the Pelona Schist and therefore that the Vincent thrust is a remnant of the primary fault along which the Pelona Schist and correlative units were subducted beneath North Ameri ca. Nonetheless, very fine-scale differences in amphibole composition between the schist and upper plate may indicate that metamorphic re-eq uilibration could not quite keep pace with movement on the fault.