ON THE HYDROUS ORIGINS OF THE COAST RANGE OPHIOLITE: REVISITED

The lithological and geochemical characteristics of the mantle section from ophiolites yield important observations regarding tectonic setting (e.g., abyssal vs. suprasubduction zone), the melt extraction process (e.g., fractional vs. batch), and conditions (e.g., anhydrous vs. hydrous). The pioneering work started in the 60’s initiated decades of debate, in regards to the tectonic setting of the Coast Range ophiolite (CRO). But for the past decade our works have provided compelling evidence for hydrous-fractional partial melting, based on enriched fluid-mobile element (FME: B, Li, Be) concentrations, with origins in the garnet stability field. This establishes that the CRO represents, in large part, formation by fore-arc extension above a proto-Franciscan subduction zone, modified in part by subsequent ridge-trench interactions. The Shervais and Jean (2012; GCA 95, 270-285) algorithm calculates the addition of incompatible and fluid mobile trace-elements to the mantle wedge during melting. Our results show that high-FME concentrations result from a continuous flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab, and allow us to quantify the composition of the fluid added.

A new array of elements and isotopes further advances the forearc origins of the CRO. Lithium is sensitive to sources and fluid flow mechanisms. Bulk-rock Li abundances of CRO peridotites (δ⁷Li = –14.3 to 5.5‰; 1.9–7.5 μm/g) are indicative of Li addition via fluids from a dehydrating slab, as δ⁷Li–values are lighter than normal upper mantle values. To obtain a more complete picture of the slab to arc transfer processes, we also analyzed eclogites (δ⁷Li = –18 to 3.5‰; 11.5–32.5 μm/g) and basalts (δ⁷Li = –9.6 to 7.5‰; 0.2–32.1 μm/g). Snortum and Day (2020; Chem. Geol. 550, 119723) and Choi et al. (2024; Lithosphere 154) investigated highly siderophile elements (Ru, Pd, Pt, Re, Os) and Re−Os isotopic data recorded on CRO peridotites. Both studies reached the conclusion that the CRO formed in an SSZ environment with significant fluid-assisted melting and complementary to the formation of boninites. In connection with previous studies focused on high–grade metamorphic assemblages within the Franciscan complex, an overall framework exists to reconstruct the architecture of the Middle-Jurassic California arc.