Forearc origin for Coast Range Ophiolites inferred from osmium isotopes and highly siderophile elements

Abstract

The Jurassic Coast Range Ophiolites, California, provide evidence for tectonic processes that formed the western margin of North America. Whether the ophiolites were formed in a mid-ocean ridge, back arc or forearc setting, however, remains unconstrained. To address this issue, a systematic grid sampling approach was taken to examine the mantle section of an archetypal ~161 Ma Coast Range Ophiolite at Point Sal, California. The Point Sal peridotites are serpentinized, with 187Re187Os evidence for a localized Late Miocene alteration event in grids closest to the lowermost bounding fault of the ophiolite. This event is interpreted to be related to tectonic changes and hydration-dehydration reactions acting on the serpentinites in response to changes in the azimuth and rate of Pacific-North American plate motion. Alteration and serpentinization have not systematically affected highly siderophile element (HSE) abundances, especially Os, Ir or Ru which, together with rare earth element abundances, record significant melt depletion (>20%) of a fertile mantle protolith (187Os/188Os = ~0.129) during ophiolite formation. Peridotites subsequently experienced localized and heterogenous addition of Re, Pt, Pd and radiogenic 187Os/188Os from Cl-rich, oxidized (QFM) fluids. Geochemical features of both Point Sal peridotites, as well as associated basaltic lavas, are consistent with high degrees of partial melting and variable addition of fluids as also observed in global forearc peridotites. A forearc origin for the Point Sal Ophiolite Complex supports models for the dominant formation process of 153–166 Ma Coast Range Ophiolite infant Arc crust terranes within a single westward facing arc along the Sierra continental-margin.