INHERITED PROTEROZOIC ZIRCON IMPLIES A PROXIMAL NORTH AMERICAN (?) PALEO- GEOGRAPHIC SETTING FOR JURASSIC OPHIOLITES, SIERRA NEVADA, CALIFORNIA

Jurassic ophiolitic complexes in California occur outboard of the inferred Jurassic continental margin, and include the Late Jurassic Smartville (159 Ma) and Early to Middle Jurassic Slate Creek (203 - 171 Ma) complexes. A continuing debate has been whether such complexes were exotic and collided with the margin, or whether they were simply part of a wide forearc to a Jurassic, continental magmatic arc. Evidence for inherited Proterozoic zircon is widespread in plutons associated with the Sierran complexes, and the ages of the inherited zircon components are compatible with a North American origin. We suggest that ophiolitic complexes as old as Early Jurassic evolved near North America.

We inferred the presence of inherited zircon components in Early Jurassic through Early Cretaceous intrusions on the basis of: (1) zircon xenocrysts in three samples; (2) six samples in which multiple zircon fractions define inverse discordia arrays with Proterozoic upper intercepts; (3) three samples in which air-abraded zircon fractions are older than the unabraded fractions; (4) TIMS data that commonly show ²⁰⁷Pb/²⁰⁶Pb ages that are elevated compared to similar ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U ages. Rounded zircon xenocrysts in an Early Cretaceous dike (c. 106 Ma) have a TIMS ²⁰⁷Pb/²⁰⁶Pb age of 2153 Ma. A rounded, zircon xenocryst from a Middle Jurassic dike (c. 179 Ma) yielded SHRIMP ²⁰⁷Pb/²⁰⁶Pb ages of 2243 and 2453 Ma. Multiple zircon fractions from three Middle and Late Jurassic intrusions (179 -157 Ma) form arrays with negative slopes on inverse discordia diagrams, yielding upper intercept ages of 931 ± 130 Ma, 2201 ± 360 Ma, and 1107 ± 220 Ma. Sedimentary rocks in the Shoo Fly Complex of the Sierra Nevada are a permissible source for such inherited zircon. The Shoo Fly is widespread, was part of the continental margin by Late Triassic time, and contains concentrations of detrital zircon as old as 2450-2200 Ma and as young as Lower Paleozoic .

There are several direct or indirect mechanisms by which outboard Mesozoic intrusions might have been contaminated by zircon from the Shoo Fly, so it is not possible to rule out a collisional origin for these the outboard ophiolitic complexes. Nevertheless, the most plausible interpretation is that the contamination occurred near North America.