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 187Re 187Os 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.

中文翻译：

---

从锇同位素和高度亲铁元素推断的海岸蛇绿岩的弧前成因

摘要 加利福尼亚州侏罗纪海岸山脉蛇绿岩为形成北美西部边缘的构造过程提供了证据。然而，蛇绿岩是在大洋中脊、后弧或前弧环境中形成的，仍然不受限制。为了解决这个问题，采用了系统的网格采样方法来检查加利福尼亚州萨尔角的原型 ~161 Ma 海岸范围蛇绿岩的地幔部分。Point Sal 橄榄岩被蛇纹石化，187Re 187Os 证据表明在最靠近蛇绿岩最低边界断层的网格中发生了局部晚中新世蚀变事件。该事件被解释为与作用于蛇纹岩的构造变化和水合脱水反应有关，以响应太平洋-北美板块运动的方位角和速率变化。改变和蛇纹石化并未系统地影响高度亲铁元素 (HSE) 的丰度，尤其是 Os、Ir 或 Ru，它们与稀土元素丰度一起记录了肥沃的地幔原岩 (187Os/188Os = ~ 0.129) 在蛇绿岩形成期间。随后橄榄岩经历了来自富氯氧化 (QFM) 流体的 Re、Pt、Pd 和放射性 187Os/188Os 的局部和异质添加。Point Sal 橄榄岩以及相关玄武岩熔岩的地球化学特征与在全球弧前橄榄岩中观察到的高度部分熔融和流体的不同添加一致。