Abstract: Volcanic Rocks on the Northern Channel Islands

WEIGAND, PETER W., KAREN L. SAVAGE, BARBARA D. CHINN, and TOM REID, Department of Geological Sciences, California State University, Northridge, CA 91330-8266

Volcanic rocks crop out on six of the eight Southern California Islands. We report here compositional data on volcanics from Santa Barbara, San Miguel, and Santa Rosa Islands. Al1 exposed rocks on Santa Barbara Island are volcanic. They are 14.8 to 16.8 Ma (K-Ar; Luisian-Relizian) and are bimodal in composition, consisting of basalt-basaltic andesite and andesite flows. San Miguel Volcanics on San Miguel Island are probably Saucesian in age and are also bimodal in composition, consisting of basalt and andesite flows. Santa Rosa Island Volcanics (new name) on Santa Rosa Island are probably Zemorian-Saucesian in age and consist entirely of basalt flows and shallow intrusions. Volcanic clasts in the upper San Onofre Breccia and Beecher's Bay Formation on Santa Rosa Island are 15.8 Ma (Ar-Ar; Luisian) and are polygenetic in composition, ranging from andesite to rhyolite. The mafic rocks from these three islands are geochemically indistinguishable and might be consanguinous, although more precise dating may show them to be unrelated.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California