NEW INSIGHTS FROM LA-ICP-MS ANALYSES OF COAST RANGE OPHIOLITE REFRACTORY PERIDOTITE

The Coast Range ophiolite (CRO) of California is one of the most extensive middle Jurassic ophiolite terranes in North America and has long been central to our understanding of Jurassic Cordilleran tectonics. Laser ablation analyses of pyroxenes record magmatic processes characteristic of both mid-ocean ridge and supra-subduction zone settings. MOR-type clinopyroxene display enriched REE concentrations [e.g., Gd (0.938–1.663 ppm), Dy (1.79–3.24 ppm), Yb (1.216–2.047 ppm), and Lu (0.168–0.290 ppm)], with overall REE patterns that can be modeled by 3% dry partial melting of MORB-source asthenosphere in the garnet lherzolite field. This affinity was further confirmed by isotope analysis. These clinopyroxenes have ⁸⁷Sr/⁸⁶Sr = 0.70250–0.70385, εNd = +4.5 to +14.3, ²⁰⁶Pb/²⁰⁴Pb = 18.50–18.77, ²⁰⁷Pb/²⁰⁴Pb = 15.58–15.59, ²⁰⁸Pb/²⁰⁴Pb = 38.08–38.28, and εHf = +23.3 to +29.0; all similar to Pacific Jurassic MORB. SSZ-type clinopyroxene have depleted REE concentrations [e.g., Gd (0.048–0.055 ppm), Dy (0.114–0.225 ppm), Yb (0.128–0.340 ppm), and Lu (0.022–0.05 ppm)] and can be modeled by 15–20% further partial melting in the spinel lherzolite field, possibly by hydrous melting.

After subduction became the dominant tectonic environment, subduction factory processes controlled the chemical characteristics of the CRO. Again, by applying the laser ablation technique to CRO relic pyroxenes, results show that all samples display enrichments in fluid-mobile elements, i.e., B ~20 – 80x DMM and Pb ~4 – 20x DMM. In contrast, melting models based on the RE and HFS elements show that fluid-mobile elements should have extremely low concentrations. These results allow us to create a new method of calculating the amount of fluid necessary to change refractory clinopyroxene compositions to those that have been fluid-influenced. Application of this new calculation method shows that the amount of fluid added to our DMM-source was consistent; boron was added in the 100’s of ppm range and lithium, thorium, and lead was added in the 10’s of ppm range. This method represents the total addition of material to the mantle wedge source region, may be calculated for each element of interest, and is an independent determination of the subduction component that requires few assumptions.