Petrology and Geochemistry of Accreted Oceanic Crust in Northern California

加利福尼亚海岸山脉蛇绿岩（CRO）是科迪勒拉西部最广阔的海洋地壳之一，形成了大峡谷序列弧前沉积物的基底。放射性年龄数据显示，CRO形成于1.55亿至1.65亿年前的相对较短的时间内，就在内华达造山运动之前。CRO及其相关构造是西科迪勒拉地区重要的构造元素，了解CRO及其演化对揭示北美西部构造演化具有重要意义。蛇绿岩岩石成因的旧模型通常基于这样的假设，即这些岩体是在一个单一的、长期的岩浆循环中形成的，有或没有原始岩浆的周期性涌入。然而，最近对暴露良好的特提斯蛇绿岩（Troodos, Semail）的研究表明，形成这些地块至少需要两个不同的岩浆期。这些时期的代表是地球化学上不同的火山单元，以及侵入较老的变形辉长岩的较年轻的、通常为超镁铁质的深部岩浆套。这些研究证明了多阶段岩浆历史在蛇绿岩发育中的重要性，并暗示蛇绿岩的构造演化必须发生在一个动态的、快速演化的构造环境中。CRO的Elder Creek遗迹包括北加州唯一完整的蛇绿岩。初步的地球化学数据和野外勘测表明，该蛇绿岩具有复杂的岩浆历史，至少存在两个甚至可能三个不同的岩浆套。证据包括（对于深岩体）辉长岩伟晶岩脉与层状堆积相交叉，层状（变形）辉长岩以碎屑形式出现在辉长岩角砾岩中，（对于火山岩）出现具有不同地球化学特征的火山研究。这些岩浆套之间的关系尚不确定，但其中一个火山套可能与斯通福德杂岩有关。CRO的Stonyford遗迹沿着走向向南仅35英里，但它在岩石学、地球化学和构造关系上表现出不同，这表明它形成于与Elder Creek蛇绿岩不同的构造环境中。我们建议利用野外关系、岩石学和地球化学来揭示这两个蛇绿岩碎片的对比地质历史、它们的岩浆演化以及它们是如何在同一构造位置并置的。我计划特别关注多阶段岩浆过程在蛇绿岩岩石成因中的作用。观察到的一些变化可能是由于最初不相关的岩石的构造并置造成的，但其他变化一定是由于在单个蛇绿岩扩张中心内岩浆成分随时间的变化造成的。

The Coast Range ophiolite (CRO) of California comprises one of the most extensive tracts of oceanic crust in the western Cordillera, and forms the basement which underlies fore-arc sediments of the Great Valley sequence. Radiometric age data show that the CRO formed in a relatively brief time span between 155-165 million years ago, just prior to the Nevadan orogeny. The CRO and its correlatives form an important tectonic element in the western Cordillera, and understanding its evolution is central to unraveling the tectonic evolution of western North America. Older models of ophiolite petrogenesis are commonly based on the assumption that these bodies form during a single, long-lived magmatic cycle, with or without the periodic influx of primitive magma. However, recent studies of well-exposed Tethyan ophiolites (Troodos, Semail) show that at least two distinct magmatic episodes are required to form these massifs. These episodes are represented by volcanic units which are geochemically distinct, and by younger, commonly ultramafic plutonic suites that intrude older deformed gabbros. These studies document the importance of multi-stage magmatic histories in the development of ophiolites, and imply that their tectonic evolution must occur in a dynamic, rapidly evolving tectonic environment. The Elder Creek remnant of the CRO comprises the only complete ophiolite in northern California. Preliminary geochemical data and reconnaissance field surveys suggest that this ophiolite had a complex magmatic history, and that at least two and possibly three distinct magmatic suites are present. Evidence includes (for the plutonic rocks) gabbro pegmatite dikes which cross-cut layered cumulates, and laminated (deformed?) gabbros occur as clasts in agmatite breccias and (for the volcanic rocks) the occurrence of volcanic studies with different geochemical signatures. Relationships between these magmatic suites are uncertain, but one of the volcanic suites may be related to the Stonyford complex. The Stonyford remnant of the CRO lies only 35 miles farther south along strike, but it displays differences in its petrology, geochemistry, and structural relationships which require that it formed in tectonic setting distinct from the Elder Creek ophiolite. It is proposed to use field relationships, petrology, and geochemistry to unravel the contrasting geologic histories of these two ophiolite fragments, their magmatic evolution, and how they came to be juxtaposed in the same tectonic position. I plan to focus in particular on the role of multi-stage magmatic processes in ophiolite petrogenesis. Some of the variations observed probably result from tectonic juxtaposition of originally unrelated rocks, but others must result from changes in magma composition through time within a single ophiolite spreading center.