Ap. Barth et al., ORIGIN OF GNEISSES IN THE AUREOLE OF THE SAN-GABRIEL ANORTHOSITE COMPLEX AND IMPLICATIONS FOR THE PROTEROZOIC CRUSTAL EVOLUTION OF SOUTHERNCALIFORNIA, Tectonics, 14(3), 1995, pp. 736-752
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.