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ABSTRACT: The Del Puerto Canyon Remnant of the Great Valley Ophiolite: Geochemical and Age Constraints on its Formation and Evolution

EVARTS, RUSSELL C., U.S. Geological Survey, Menlo Park, CA, WARREN D. SHARP, State University of New York, Stony Brook, NY, and DAVID W. PHELPS, Exxon Production Research, Houston, TX

A remnant of the Great Valley Ophiolite exposed in Del Puerto Canyon, Diablo Range, consists of peridotite tectonites; a plutonic complex composed of ultramafic and mafic cumulates cut by numerous compositionally variable dikes; and a 2.5-km-thick volcanic section depositionally overlain by nonophiolitic, andesitic volcaniclastic sediments (Lotta Creek Formation), in turn overlain by Buchia Piochii-bearing shales of the basal Great Valley sequence.

The extrusive and fine-grained dike rocks generally constitute a low-TiO{2} calc-alkaline suite ranging from basalt to rhyolite and characterized by low Ti/V (8-14), low Nb [<3 ppm), and low, unfractionated REE (less than or equal to 7x chrondites; (La/Yb)n = 0.8-1.1]; a few are boninite-like (high SiO2, MgO, Ni, Cr). These features suggest an island arc or forearc eruptive site. Two intrusive samples possess MORB-like chemistry [Ti/V = 30 (La/Yb)n - 0.5, Nb = 8 ppm].

New age determinations reveal an extended magmatic history: tonalite related to the cumulates is 157 +/- 2 Ma ({40}Ar/{39}Ar, hornblende); mafic and felsic dikes cutting the cumulates are 149 (+/-) 5 Ma and 150 (+/-) 6 Ma (fission-track, zircon); and rhyolite near the top of the volcanic section is 150 (+/-) 2 Ma ({40}Ar/{39}Ar, hornblende). An andesite boulder in the Lotta Creek Formation is also 150 (+/-) 2 Ma ({40}Ar/{39}Ar, hornblende).

The Del Puerto remnant is the product of Middle to Late Jurassic magmatism in an island-arc or fore-arc setting. Rare late dikes with MORB-like chemistry may reflect incipient rifting of the arc and creation of a back-arc basin in the latest Jurassic.


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