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

Granulite facies gneisses along the southwestern margin of the San Gab riel anorthosite complex include compositionally banded, locally migma titic felsic gneisses and mafic granulites, with rare interlayered aug en gneiss and garnet+hypersthene+spinel aluminous gneiss. Previous wor kers accepted a 1.4 Ga age for these granulites and a 1.2 Ga age for t he adjacent anorthosite complex and hence that this Proterozoic baseme nt is allochthonous with respect to the southwestern United States par t of the North American craton. Solvus geothermometry in felsic and ma fic granulites yields minimum metamorphic temperatures of 900-950 degr ees C, and new U-Pb zircon data yield a minimum metamorphic age of 118 6 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 northe ast. We conclude that these Proterozoic basement rocks represent the s outhwestern margin of the North American craton and not an exotic terr ane accreted in Mesozoic or Tertiary time.