ORIGIN OF GNEISSES IN THE AUREOLE OF THE SAN-GABRIEL ANORTHOSITE COMPLEX AND IMPLICATIONS FOR THE PROTEROZOIC CRUSTAL EVOLUTION OF SOUTHERN CALIFORNIA

Granulite facies gneisses along the southwestern margin of the San Gabriel anorthosite complex include compositionally banded, locally migmatitic felsic gneisses and mafic granulites, with rare interlayered augen gneiss and garnet+hypersthene+spinel aluminous gneiss. Previous workers accepted a 1.4 Ga age for these granulites and a 1.2 Ga age for the adjacent anorthosite complex and hence that this Proterozoic basement is allochthonous with respect to the southwestern United States part of the North American craton. Solvus geothermometry in felsic and mafic granulites yields minimum metamorphic temperatures of 900-950 degrees C, and new U-Pb zircon data yield a minimum metamorphic age of 1186 Ma, coincident with that of the adjacent anorthosite. We reinterpret these rocks as a 1190 Ma anorthosite complex and its granulite-grade thermal aureole. Geochemical data suggest that the anorthosite complex intruded Early Proterozoic rocks correlative with the Mojave crustal province, representing the North American craton exposed to the northeast. We conclude that these Proterozoic basement rocks represent the southwestern margin of the North American craton and not an exotic terrane accreted in Mesozoic or Tertiary time.