In-situ LA-ICP-MS analysis of pyroxene in the peridotite section of the Coast Range ophiolite: Diverse trace element compositions and cryptic garnet field melting in the Cordilleran mantle wedge

Abstract

The Coast Range Ophiolite (CRO) is tectonically dismembered and widely distributed in western California. We measured trace element, rare-earth element (REE), and fluid mobile element (FME) contents of pyroxenes in peridotites from four mantle sections from the CRO. This study utilized Laser Ablation- Inductively Coupled Plasma-Mass Spectrometry. The pyroxenes record magmatic processes characteristic of both mid-ocean ridges and supra-subduction zone (SSZ) settings. On average, the abyssal group display enriched clinopyroxene REE concentrations [Gd (0.938-1.663 ppm), Dy (1.79-3.24 ppm), Yb (1.216-2.047 ppm), Lu (0.168-0.290 ppm)], compared to SSZ clinopyroxene [Gd (0.048-0.055 ppm), Dy (0.114-0.225 ppm), Yb (0.128-0.340 ppm), Lu (0.022-0.05 ppm)]. The differences between these geochemical signatures could be a result of differing upper mantle composition, or different degrees of partial melting of the upper mantle (Shojaat et al., 2003). Fractional melting was important in the generation and evolution of CRO peridotites. Models call for 2% dry partial melting of MORB-source asthenosphere in the garnet lherzolite field for abyssal peridotites. SSZ peridotites, represents further partial melting (18%-23%) in the spinel lherzolite field, possibly formed by hydrous melting

evidenced by elevated FME concentrations (Li, B, Rb, Pb). These geochemical variations and occurrence of both styles of melting regimes within close spatial and temporal association (Barth et al., 2007) suggest that certain segments of the CRO may represent oceanic lithosphere, attached to a large-offset transform fault and that east-dipping, proto-Franciscan subduction may have been initiated along this transform (Choi et al., 2008).