The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Abstract

Jurassic age volcanic rocks of the Stonyford volcanic complex (SFVC) comprise three distinct petrological groups based on their whole-rock geochemistry: (1) oceanic tholeiites; (2) transitional alkali basalts and glasses; (3) high-Al, low-Ti tholeiites. Major and trace element, and Sr–Nd–Pb isotopic data indicate that the oceanic tholeiites formed as low-degree partial melts of normal mid-ocean ridge basalt (N-MORB)-source asthenosphere similar in isotope composition to the East Pacific Rise today; the alkalic lavas were derived from an enriched source similar to that of E-MORB. The high-Al, low-Ti lavas resemble second-stage melts of a depleted MORB-source asthenosphere that formed by melting spinel lherzolite at low pressures. Trace element systematics of the high-Al, low-Ti basalts show the influence of an enriched component, which overprints generally depleted trace element characteristics. Tectonic discrimination diagrams show that the oceanic tholeiite and alkali suites are similar to present-day basalts generated at mid-oceanic ridges. The high-Al, low-Ti suite resembles primitive arc basalts with an enriched, alkali basalt-like overprint. Isotopic data show the influence of recycled components in all three suites. The SFVC was constructed on a substrate of normal Coast Range ophiolite in an extensional forearc setting. The close juxtaposition of the MORB-like olivine tholeiites with alkali and high-Al, low-Ti basalts suggests derivation from a hybrid mantle source region that included MORB-source asthenosphere, enriched oceanic asthenosphere, and the depleted supra-subduction zone mantle wedge. We propose that the SFVC formed in response to collision of a mid-ocean ridge spreading center with the Coast Range ophiolite subduction zone. Formation of a slab window beneath the forearc during collision allowed the influx of ridge-derived magmas or the mantle source of these magmas. Continued melting of the previously depleted mantle wedge above the now defunct subduction zone produced strongly depleted high-Al, low-Ti basalts that were partially fertilized with enriched, alkali basalt-type melts and slab-derived fluids.