The Conejo Volcanics and other Miocene volcanic suites in southwestern California.

Abstract

Dozens of middle Tertiary volcanic areas were active in western California during and after a transition from convergent to transform tectonics. The Conejo Volcanics in the western Santa Monica Mountains represents one of the largest middle Miocene accumulations of intrusive bodies and extrusive flows and breccias found in southern California. Samples collected from the main stratigraphic units that span the eruptive history of the Conejo lavas range in composition from basalt to dacite, belong to the calc-alkaline magma series, and are characterized by low K sub 2 O contents. Nearly all the samples exhibit moderate depletion of both Nb and Ta. Whole-rock Sr sub i ranges from 0.70316 to 0.70386, plagioclase delta super 18 O ranges from 6.1 per mil to 7.9 per mil, and whole-rock epsilon sub Nd ranges from +5.6 to +8.2. Using a variety of geologic, geographic, and petrologic evidence, we propose the following tectono-magmatic history for these lavas. Capture of the partially subducted Monterey microplate by the Pacific plate initiated clockwise rotation and rifting of the western Transverse Ranges block at ca. 19 Ma. Continued rotation and rifting of the overlying continental crust in response to transtensional Pacific-plate motion of the captured microplate led to the upwelling of underlying oceanic asthenosphere. Decompression melting of this mantle source produced primitive basaltic magmas that repeatedly intruded the overlying crust, where they underwent fractional crystallization and assimilation, possibly of Catalina Schist. Starting ca. 17 Ma, these more evolved magmas erupted along the now-southern edge of the western Transverse Ranges block as it rotated off the captured Monterey microplate. The numerous other middle Miocene igneous suites in coastal and offshore southern California likely had a similar origin.

27 samples from this reference