Petrogenesis of serpentinites from the Franciscan Complex, western California, USA

Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major and trace element compositions, relict mineral (spinel and pyroxene) compositions, and stable isotope (O, H, Cl) compositions with the goal of determining protolith origin and subsequent serpentinizing fluid sources in order to decipher the tectonic setting of serpentinization. We focused on serpentinite bodies found in the Franciscan Complex (west of Cuesta Ridge; south of San Francisco; Tiburon Peninsula; Healdsburg) (n = 12). Three samples from Cuesta Ridge (part of the Coast Range ophiolite) were also analyzed for comparison. Serpentinites from Cuesta Ridge have flat to U-shaped chondrite-normalized REE patterns and spinels with Cr# values > 0.60 implying a supra-subduction zone origin. In contrast, Franciscan serpentinites west of Cuesta Ridge and Tiburon Peninsula have positive-sloped REE patterns. This depletion in LREE is typical of abyssal peridotites. Most relict spinels have low Cr# values (< 0.3) and relict clinopyroxenes from Tiburon Peninsula have high HREE concentrations, also supporting an abyssal origin. Franciscan serpentinite samples from south of San Francisco and near Healdsburg have U-shaped REE patterns and spinel compositions that lie within the forearc peridotite field with some overlap into the abyssal field and are of more ambiguous origin. All samples are high in fluid-mobile elements with remarkable positive Ce and Y anomalies. We speculate that these anomalies may be due to involvement of highly oxidizing fluids resulting in the preferential scavenging of Ce and Y by ferromanganese oxyhydroxides during serpentinization. All samples (except those south of San Francisco) have δ18O values of + 5.4 to + 7.9‰, typical values for oceanic serpentinites formed via low-T seawater hydration on the seafloor. δD values of all samples are extremely low (− 107 to − 90‰), likely the result of post-serpentinization, post-emplacement interaction with meteoric water at low temperature. Samples south of San Francisco lie on the San Andreas fault and have high δ18O values (+ 7.2 to + 9.5‰) likely due to low-T interaction with meteoric water at high fluid–rock ratios. Most of the serpentinites have δ37Cl values between + 0.2 and + 0.9‰, typical values for serpentinites formed by interaction with seawater. Exceptions are those from the San Andreas fault and one sample from Cuesta Ridge with a high δ37Cl value (+ 1.7‰) possibly from interaction with a slab-derived fluid.

► Serpentinites from Cuesta Ridge (CRO) show geochemical evidence for SSZ origin.
► Serpentinites from Franciscan Complex are both abyssal and SSZ.
► High Ce and Y concentrations may be the result of highly oxidizing fluids.