Forearc origin for Coast Range Ophiolites inferred from osmium isotopes and highly siderophile elements
Forearc origin for Coast Range Ophiolites inferred from osmium isotopes and highly siderophile elements
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DOI：
10.1016/j.chemgeo.2020.119723
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发表时间：
2020-09
期刊：
影响因子：
3.9
通讯作者：
Eric Snortum;J. Day
中科院分区：
文献类型：
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作者：
Eric Snortum;J. Day
加利福尼亚州侏罗纪海岸蛇绿岩为形成北美西部边缘的构造作用提供了证据。然而，蛇绿岩是在大洋中脊、弧后或弧前背景下形成的，仍然没有受到限制。为了解决这一问题，采用系统的网格采样方法对加利福尼亚州萨尔点约161 Ma海岸山脉蛇绿岩的原型进行了研究。Sal点橄榄岩呈蛇纹状，187ResSingle Bond 187Os证据表明，在最接近蛇绿岩最低边界断裂的格点上发生了一次局部的晚中新世蚀变事件。这一事件被解释为与太平洋-北美板块运动的方位和速率变化对蛇纹岩的构造变化和水化-脱水反应有关。蚀变和蛇纹岩作用没有系统地影响高亲铁质元素的丰度，特别是Os、Ir或Ru，它们与稀土元素丰度一起，记录了蛇绿岩形成过程中可育地幔原岩(187Os/188Os=1~0.129)的显著熔体亏损(>；20%)。随后，橄榄岩经历了Re、Pt、Pd的局部和非均相添加，以及来自富氯氧化(QFM)流体的放射性187Os/188Os。Sal点橄榄岩和与之伴生的玄武岩的地球化学特征与全球弧前橄榄岩中观察到的高度部分熔融和可变的流体添加相一致。Sal点蛇绿岩杂岩的弧前起源支持沿塞拉利昂大陆边缘向西的单一弧形内153-166 Ma海岸山脉蛇绿岩婴儿弧壳地体的主导形成过程的模式。
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, with187Resingle bond187Os 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 radiogenic187Os/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.