A continental forearc serpentinite diapir with deep origins: Elemental signatures of a mantle wedge protolith and slab-derived fluids at New Idria, California

Abstract

The New Idria serpentinite body in central California has been interpreted as a diapir that was hydrated at depth in the forearc mantle wedge by aqueous fluids released from the subducted slab. This interpretation is primarily based on structural relationships and petrographic analyses of high-grade metabasaltic tectonic blocks entrained within the serpentinite. The geochemistry of the New Idria serpentinites is largely unknown other than a few boron isotope analyses that are consistent with slab-derived fluid compositions, but not definitive. In this contribution, we present whole-rock elemental analyses and petrographic examinations of a suite of ten serpentinite samples to constrain the tectonic origin of the New Idria serpentinite body. Major, minor, and trace element relationships reflect the signatures of depleted peridotitic protoliths typically associated with serpentinites from mantle wedge settings. These include high MgO/SiO2 vs. low Al2O3/SiO2, high MgO vs. low TiO2, Cr-rich chromites, and enriched Zr/Nb and Hf/Nb ratios. Unlike in abyssal serpentinites, U is not fluid-mobile and is strongly coupled to Th, with a U-Th relationship that is best explained by melt–rock interaction. Rare earth element fractionation trends are also consistent with melt-rock interaction. Fluid-mobile element (FME) concentrations (Cs, Ba, and Rb) are consistent with a slab-derived signature and are notably enriched, featuring some of the highest Cs and Ba concentrations ever reported in serpentinites. No petrographic or geochemical evidence was found to suggest that these enriched FME signatures resulted from systematic post-serpentinization fluid-rock processes or pre-serpentinization melt-rock interactions. In aggregate, the geochemical signatures of the New Idria serpentinites suggest a highly depleted mantle wedge protolith overprinted by melt-rock interactions and subsequent serpentinization by a slab-derived fluid, consistent with the forearc diapir hypothesis. Although subduction at New Idria ended around 25 Ma, geophysical evidence and simple geodynamic calculations suggest that the timing of the proposed diapiric rise plausibly coincides with the final stages of subduction in central California.