A geochemical study of the Miocene age Conejo volcanics, Santa Monica mountains : Los Angeles county, California

A geochemical study was made of twenty-eight samples of volcanic rocks collected from the Malibu Lake area located in the central Santa Monica Mountains, western Los Angeles County, California. In this area, which is situated in the Transverse Ranges province, there is an apparent thickness of 3500-4200 meters of volcanic rock. The lower portion contains pillow lavas and interbedded marine sediments and the upper portion is composed of a massive pile of flows and pyroclastic rocks. Basalt and andesite are the predominate rock type with lesser amounts of dacite. Associated with the volcanic rocks in the Santa Monica Mountains are a succession of marine and non-marine sedimentary rocks ranging in age from Jurassic to recent, and a granitic intrusion with a 110 m. y. radiometric age. The chemical data show that the volcanic suite is unusually low in potash and has Sr87/Sr86 ratio ranging from .70317 to .70420 and averaging .7035. at 55% and 60% SiO2, the suite shows a K2O content of 0.49% and 0.61% respectively. AFM diagrams and Al2O3 –normative plagioclase plots show the volcanics are calc-alkaline. Alkali-silica (Kuno) plots show they belong to the high alumina basalt series of Kuno (1968). The suite has an alkali lime index of 60.5 and plots as calc-alkalic on peacock diagrams. Within the Circum-Pacific margin only three areas: the Kirile Islands, Izu-Mariana Islands and Kermedec show a low K2O content similar to the Conejo volcanics. All of these areas have high subduction rates and are characterized by the presence of tholeiitic volcanic suites. Plots on AFM diagrams show the Conejo volcanics appear within the calc-alkaline field in contrast to the “island arc tholeiites” from the Tonga-Kermedec area which appear within the tholeiitic field. A model is postulated in which a high subduction rate had resulted in a high degree of partial melting of oceanic lithosphere within a former trench system that had previously existed along the continental margin leading to the formation of the low potassium Conejo volcanics. Volcanism ceased with the passing of the Mendocino Transform as the Pacific plate moved northwestward during Miocene time. This model is in accord with the rotation of the Transverse Ranges hypothesized by Kammerling and Luyendyk, the volcanogenic hypothesis of Benioff and the plate tectonic model of Atwater.