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Tectonic setting of the Jurassic Smartville and Slate Creek complexes, northern Sierra Nevada, California

Howard W. Day^,1 and M.E. Bickford^,2

¹ Department of Geology, University of California, Davis, California 95616, USA
² Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA

Jurassic ophiolitic complexes in California and Oregon are widely distributed outboard of a Jurassic and older continental margin. Whether these outboard complexes were exotic and collided with the Jurassic continental margin, or whether they were simply part of a wide forearc to a Jurassic continental magmatic arc, has long been at issue. We have studied two such complexes and crosscutting intrusions in the fault-bounded Central and Western belts of the northern Sierra Nevada. We report new and revised U-Pb (zircon) dates for the Early Jurassic and younger Slate Creek complex (ca. 203–171 Ma) in the Central belt; the Late Jurassic Smartville complex (159 ± 3 Ma) in the Western belt; and crosscutting intrusions ranging from Middle Jurassic to Early Cretaceous age. The new ages imply that the Central and Western belts have been part of the same terrane since Middle Jurassic time and that the fault separating them has accommodated only minor displacements since ca. 160 Ma. Isotopic compositions of common Pb indicate derivation from mantle sources, but evidence for inheritance of Pre-cambrian zircon is common in plutons within the Early and Late Jurassic complexes and in Middle Jurassic, Late Jurassic, and Early Cretaceous crosscutting intrusions. Although there are several potential sources for such contamination, the ages of the inherited zircon are consistent with a North American origin. This contamination would have been accomplished most plausibly near the North American margin.

Key Words: California • ophiolite • tectonics • geochronology • Smartville complex • Slate Creek complex

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