ABSTRACT: U/Pb Ages of the Coast Range Ophiolite: a Critical Reevaluation Based on New High-Precision Pb/Pb Ages
MATTINSON, JAMES, and CLIFFORD, HOPSON, University of California at Santa Barbara, Santa Barbara, CA
U/Pb dating of zircon from plagiogranite has long been the dating method of choice for ophiolites, and most remnants of the Coast Range Ophiolite (CRO) have been so dated. The majority of reported zircon ages from plagiogranites have been reported as "concordant- based on the apparent agreement (within uncertainties) between measured U/Pb and Pb/Pb ages for individual fractions of zircon, and on reasonably good agreement in measured ages between different fractions of zircon from the same sample. New techniques that permit better precision for Pb/Pb ages, coupled with better understanding of the complex isotopic behavior of zircon systems reveal, however, that earlier age interpretations were oversimplified. At least some zircons previously reported as having "concordant- ages, in fac , have lost small amounts of Pb. Thus, the reported ages are too young. For example, in the earliest U/Pb CRO dating work, we reported an apparently concordant age of about 160-161 Ma for the Point Sal Ophiolite. New results indicate that the Point Sal remnant is most likely in the range of 165-173 Ma. The ophiolite remnant at Cuesta Ridge is also substantially older than previously thought. We are reevaluating other ophiolite remnants as well. The new ages have important implications for the tectonic history of the CRO and its associated sedimentary cover rocks.