ABSTRACT: The Stonyford Volcanic Complex: A Jurassic Seamount in the Northern California Coast Ranges

HANAN, BARRY B., DAVID L. KIMBROUGH, San Diego State University, San Diego, CA, and PAUL R. RENNE, Institute of Human Origins, Berkeley, CA

The Stonyford volcanic complex (SFVC) is a thick sequence (1-2.5 km) of basaltic pillow lava, hyaloclastite breccia, diabase, and interlayered radiolarian chert juxtaposed between the high P/T metamorphic rocks of the Franciscan assemblage and the sediments of the Grat Valley sequence. The SFVC consists of tholeiitic Fe-Ti basalts, alkali basalts and glass, and high-Al, low-Ti tholeiites. Rare earth element (REE) and trace element data indicate that although many of the basalts have undergone extensive hydrothermal alteration, the glasses have remained pristine. {40}Ar/{39}Ar plateau ages from the glasses indicate that they were erupted over an interval <0.5 m.y. at 164 (+/-) 1 Ma. The REE abundance ranges and patterns for the glasses are similar to alkalic oceanic island basalts ( IB). The glasses are all light-REE enriched, similar to plume-enriched mid-ocean ridge basalts (MORB) and OIB. The Pb isotopic data for the glasses are similar to Pacific MORB from off-axis seamounts. The REE, trace element, and Pb isotope data indicate that the SFVC was derived from a heterogeneous mantle source mix of at least two components, a depleted MORB asthenosphere component and an enriched OIB component. In terms of a physical model, the SFVC may represent an off-axis seamount formed either on the subducting oceanic plate, with second stage melting to produce the high-Al, low-Ti basalts, or in the forearc basin above a mantle wedge containing MORB, OIB, and island-arc components.

AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)