Lithologic Variation of Proterozoic Rocks in the San Gabriel Mountains, Wrightwood, California

The Transcontinental Proterozoic province (1.8-1.6 Ga) extends into the Mojave desert region of eastern California, but Mesozoic and Cenozoic tectonism has made the interpretation of Mojave and Yavapai province boundaries difficult. Rocks of presumed Proterozoic age are exposed in the San Gabriel Mountains to the east of the San Andreas Fault near Wrightwood, California, but their province affiliation remains unknown. Based on reconnaissance field mapping of rocks exposed at Table Mountain, we document a sequence of mafic gneisses, intrusive rocks and calc-silicate rocks. We observe biotite-rich mafic gneiss with foliation that dips moderately to the northeast, and alternating sequences of quartzites and marbles with relict bedding planes that dip moderately to the northeast. Garnet-bearing granites are exposed in sills parallel to relict bedding planes and in cross-cutting dikes. We observe diopside- and actinolite–bearing steeply-dipping dikes that cut across the granites and calc-silicate metasediments. A similar sequence of rocks is associated with the layered gneisses of the Mojave province, suggesting that this region of the San Gabriel Mountains may be affiliated with the Mojave province. In the future, U-Pb dating of detrital zircons from the quartzites and of igneous zircons from the granite may permit testing of this hypothesis.