Re-Os Isotope and PGE Abundance Systematics of Coast Range Ophiolite Peridotites and Chromitite, California: Insights into Fore-Arc Magmatic Processes

We report platinum-group element (PGE) and Re concentrations, and Re-Os isotopic data for peridotites and podiform chromitite from the mid-Jurassic Coast Range ophiolite (CRO), California. Our aim is to provide insights into the formation and evolution of the CRO in a fore-arc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have Ir-group PGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances [(Os/Ir)(N) approximate to 1.1]. Abyssal-type peridotites have slightly subchondritic Pd-group PGE (PPGE)-Re abundances and flat chondrite-normalized patterns, whereas the SSZ-type ones are depleted overall with highly fractionated PPGE-Re patterns. The CRO peridotites have Os-187/Os-188 values of 0.1188 to 0.1315 (gamma(Os) = -8.3 to 1.4) and Re-187/Os-188 ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the fayalite-magnetite-quartz buffer. The abyssal-type peridotites are residues after <= 5% melting of the primitive upper mantle and represent a remnant of oceanic lithosphere trapped in an SSZ setting but before it was re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of similar to 1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed. The SSZ-type peridotites are refractory residues after similar to 5% to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites. The chromitite has Os-187/Os-188 value of 0.1250 (gamma(Os) = -3.5) and PGE-Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.