PGE ABUNDANCE AND Re-Os ISOTOPIC SYSTEMATICS OF COAST RANGE OPHIOLITE PERIDOTITES AND CHROMITITE, AND INSIGHTS INTO FORE-ARC MAGMATIC PROCESSES

We report platinum-group element (PGE: Os, Ir, Ru, Pt, and Pd) 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 forearc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have IPGE (Os, Ir, Ru) concentrations similar to estimates for the primitive mantle, and nearly chondritic relative abundances [(Os/Ir)_N ≈1.1]. Abyssal-type peridotites have slightly sub-chondritic PPGE (Pt and Pd)-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 ¹⁸⁷Os/¹⁸⁸Os values of 0.1188 to 0.1315 (γ_Os = -8.3 to +1.4), and ¹⁸⁷Re/¹⁸⁸Os ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the FMQ 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 being re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of ~1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed that may correspond to rifting of the Columbia supercontinent. The SSZ-type peridotites are refractory residues after ~5 to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites studied. The chromitite analyzed has ¹⁸⁷Os/¹⁸⁸Os value of 0.1250 (γ_Os = -3.5), and PGE-Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.