Alkaline intrusions in a near-trench setting, Franciscan Complex, California: Constraints from geochemistry, petrology, and 40Ar/39Ar chronology

Abstract

Small gabbroic bodies intruding metagraywackes and cherts of the Late Mesozoic Franciscan Complex represent small-scale magmatic activity in a trench sedimentary environment or in the accretionary prism itself. At Leech Lake Mountain, located in the eastern Franciscan belt of northern California, interbedded radiolarian cherts and graywackes are locally intruded by Ti-rich alkalic diabase sills. The thicker sills are internally differentiated and display cumulate textures of olivine and augite. Although the diabases and surrounding graywackes were overprinted by blueschist metamorphism as indicated by jadeite-rich clinopyroxene and glaucophane, they show little deformation and good preservation of the mafic igneous minerals. The sills comprize alkali basaltic to hawaiitic compositions, all related by fractionation of olivine (Fo₈₄), augite, and titanomagnetite. The immobile trace elements Th, U, Nb, and the LREE (light Rare Earth Elements) are strongly enriched compared to MORB (Mid-Ocean Ridge Basalt), whereas Y and the HREE (heavy Rare Earth Elements) are depleted. Trace element modeling is consistent with either small degrees of melting of primitive mantle or with 6 to 8 percent of partial melting of depleted MORB mantle, which is enriched by up to 8 percent of a metasomatic melt. About 6 percent of garnet in the mantle source of the diabases can account for the observed HREE fractionation and suggests a depth of melting below the garnet-spinel transition zone at 60 to 80 km. This is in agreement with the reconstructed age and thickness of the oceanic plate the diabases intruded in the Early Cretaceous. Incompatible trace element ratios of the diabases are consistent with a HIMU (high μ)-type mantle source. The Early Cretaceous tectonic setting of the Leech Lake Mountain intrusive rocks along the California continental margin is dominated by rapid, highly oblique subduction. If melt is present beneath the oceanic lithosphere without a thermal anomaly, it may readily percolate upward through conduits opened by transtensional tectonics. Asthenospheric up- welling beneath a fracture zone that is migrating along the continental margin may be a viable alternative. ⁴⁰Ar/³⁹Ar dating of amphibole from a diabase olivine cumulate yields a plateau age of 119.0 ± 0.4 Ma, which is the first reported igneous age for the northern California Franciscan Complex. This age implies rapid subduction and metamorphism a few million years after intrusion of the diabases similar to the southern California Ortigalita Peak intrusion. However, the Leech Lake Mountain intrusive rocks are 25 my older and demonstrate that correlations on the basis of similar lithologies and metamorphic grade are problematic.