Petrology and Geochemistry of Accreted Oceanic Crust in Northern California

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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.