--> --> Abstract: Petrology, Geochemistry, and Origin of the Coast Range Ophiolite, California, by J. Shervais; #91016 (1992).
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ABSTRACT: Petrology, Geochemistry, and Origin of the Coast Range Ophiolite, California

SHERVAIS, JOHN, University of South Carolina, Columbia, SC

The Coast Range Ophiolite of California is an assemblage of ultramafic, mafic, and minor felsic igneous rocks exposed in scattered localities from Pt. Sal in the south to Elder Creek in the north. These tectonically disrupted ophiolite remnants are depositionally overlain by Upper Jurassic to Cretaceous deposits of the Great Valley Sequence, and are in tectonic contact with Jurassic and younger rocks of the Franciscan assemblage.

The Coast Range Ophiolite comprises a relatively complete ophiolite sequence with cumulate ultramafic and mafic plutonic rocks, a sheeted dike and sill complex, subaqueous volcanic rocks, and volcanogenic sediments. Field relationships show that igneous rocks of the ophiolite formed in two or more magmatic episodes, and that felsic volcanic and plutonic rocks typically are younger than the main ophiolite sequence.

Volcanic rocks of the Coast Range Ophiolite are low in high-field-strength cations (Ti, Zr, Nb, Y) and show no enrichment in TiO{2} with increasing FeO{+}/MgO, similar to volcanic rocks in island-arc suites. Volcanic rocks with high silica are common, and rocks with boninitic affinities also occur. Cumulate plutonic rocks contain calcic plagioclase and display the crystallization sequence Ol->Cpx->Plg->Ppx/Hb, typical of primitive island arc plutonic suites. Cross-cutting plutonic rocks include gabbro, diorite, and quartz diorite with calc-alkaline affinities.

The Coast Range Ophiolite probably formed by fore-arc or intra-arc rifting over a proto-Franciscan subduction zone that dipped to the east. Volcanic rocks with MORB or OIB affinities are rare, and may represent older, pre-arc crust or accreted oceanic rocks related to the Franciscan assemblage.

 

AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-Previous HitEMDTop Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)