AGE, COMPOSITION AND EMPLACEMENT OF THE SANTIAGO PEAK VOLCANICS, SOUTHERN CALIFORNIA

The Santiago Peak Volcanics of Southern California are 150 m. y., indicating active volcanism during the late Jurassic (Bushee 1963; 1974). This formation overlies an angular unconformity and Triassic sediments and is intruded by the Southern California Batholith granites, consistent with their late Jurassic age. In and around San Diego County, the Santiago Peak Volcanics crop out in a strip roughly 80 miles long and no more than ten miles wide. The Santiago Peak Volcanics are dominated by andesite lava and pyroclastic deposits, but are also accompanied by quartz latites, rhyolites, and some basalt, many of which have an aphanitic texture and phenocrysts of plagioclase. Various locations throughout the formation show that the Santiago Peak Volcanics differ in rock type and degree of metamorphism, as evidenced by the type and amount of metamorphic minerals present. This formation was emplaced by long since eroded volcanoes originally erupted in a submarine environment; these rocks now make up much of the topographic highs of San Diego. Exposed outcrops are alternating flows, tuffs and volcanic breccias from a probable early double arc system, consistent with its composition. Due to the intrusion of the Southern California Batholith, the rocks are all slightly metamorphosed causing the vesicles to be obscured or completely destroyed. Tracing the extent of the Santiago Peak Volcanics reveals varying rocks types and differing degrees of metamorphism as seen in mineral contents, textures and observation of field specimens in thin section.