Abstract

Igneous rocks of medial Miocene age crop out on all eight of the Channel Islands of the southern California coast. We report here compositional data on volcanics from Santa Rosa, San Miguel, and Santa Barbara Islands. Santa Rosa Island Volcanics (new name) on Santa Rosa Island erupted about 19 Ma (Ar/Ar; Saucesian) and consist mostly of basaltic breccia and minor basalt flows and shallow intrusions. Volcanic clasts in the San Onofre Breccia and Beechers Bay Formation on Santa Rosa Island are 15.8 Ma (Ar-Ar; Luisian) and are polygenetic in composition, ranging from andesite to rhyolite. San Miguel Volcanics on San Miguel Island erupted about 18 to 17 Ma (Ar/Ar; Saucesian) and are also bimodal in composition, consisting of basalt and andesite flows. All exposed rocks on Santa Barbara Island are volcanic. They are 17 to 15 Ma (K-Ar and Ar/Ar; Luisian-Relizian) and are bimodal in composition, consisting of basalt-basaltic andesite and andesite flows. Each of the four igneous suites from these three islands is compositionally unique. However, the mafic rocks from these islands are geochemically indistinguishable and might be consanguineous; detailed isotopic studies may clarify their interrelationships. The middle Miocene igneous rocks of coastal and offshore southern California probably originated from decompression melting of the mantle during transtensional tectonics related to the rotation of the western Transverse Ranges block.