Petrology and Geochemistry of Accreted Oceanic Crust in Northern California

北加州增生洋壳的岩石学和地球化学

• 批准号: 8816398
• 负责人: John Shervais
• 金额: $ 5万
• 依托单位: University of South Carolina at Columbia
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1989
• 资助国家: 美国
• 起止时间: 1989-01-01 至 1991-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=8816398&HistoricalAwards=false
• 关键词: Petrology; Geochemistry; Accreted; Oceanic; Crust

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.

加州的海岸山脉蛇绿岩（CRO）包括一个 最广阔的大洋地壳区域 科迪勒拉山脉，并形成了位于前弧之下的基底 大峡谷序列的沉积物。 放射性年龄数据 显示CRO在相对较短的时间内形成， 1.55 - 1.65亿年前，就在内华达山脉之前。 CRO及其相关物构成了一个重要的构造单元 在西部科迪勒拉山脉，了解它的演变是 对揭示西北地区的构造演化 美国参考 较旧的蛇绿岩岩石成因模型通常基于 假设这些物体是在一个单一的， 岩浆循环，有或没有原始的周期性流入， 岩浆 然而，最近的研究表明， 蛇（Troodos，Semail）表明，至少有两个不同的 形成这些火山岩需要岩浆活动。 这些 幕是由火山单位， 地球化学上不同的，和年轻的，通常超镁铁质 侵入较老的变形辉长岩的深成岩套。 这些 研究证明了多阶段岩浆 历史上的发展，蛇，并暗示，他们的 构造演化必定发生在一个动态的、快速演化的 构造环境 长老溪残余的CRO包括唯一的 北方加州的一种完整的蛇绿岩。 初步 地球化学数据和实地勘察表明， 这条蛇绿岩有一个复杂的岩浆历史， 存在两个或可能三个不同的岩浆岩套。 证据包括辉长岩伟晶岩脉（对于深成岩） 它横切分层累积，层压（变形？） 辉长岩以碎屑形式出现在岩浆角砾岩中， 火山岩）的发生火山研究， 不同的地球化学特征 这些之间的关系 岩浆岩套尚不确定，但其中一个火山岩套可能 与斯托尼福德建筑群有关 斯托尼福德遗址 CRO位于沿着罢工以南仅35英里处，但它 在岩石学、地球化学和 构造关系，要求它形成于构造 与埃尔德溪蛇绿岩截然不同 建议使用现场关系，岩石学， 地球化学来揭示 这两个蛇绿岩碎片，他们的岩浆演化，以及如何 它们在同一构造位置上并列。 我计划 特别关注多阶段岩浆作用 蛇绿岩成岩过程 的一些变化 观察到的结果可能是由于构造并列 最初不相关的岩石，但其他人必须从变化中产生 在一个蛇绿岩中， 扩散中心