SPATIAL VARIABILITY IN ZIRCON TRACE ELEMENT GEOCHEMISTRY OF THE CRETACEOUS CALIFORNIA ARC

Magmatism in the southwestern Cordilleran arc reached its most active period in the mid to Late Cretaceous. Between ca. 120 - 75 Ma, voluminous intrusions were emplaced in the Sierra Nevada, the Mojave, the Transverse Ranges, the Peninsular Ranges, and the presently-displaced Salinian block. Cretaceous zircon trace element geochemistry varies between these arc segments, indicating spatial differences in magmatic processes during that time. To better understand this variation, we have compiled a preliminary, regional database of zircon geochemistry from previously-dated Cretaceous plutons within different geologic and tectonic regions of the central and southern California arc. We divide our results into three regions: the Sierra Nevada, Salinia, and the Mojave, all of which would have formed a contiguous arc in Cretaceous time. Our results support previous observations that the Mojave has distinctively high Th/U and Ce/Yb, signatures interpreted to derive from partial melting of high Th/U and Ce/Yb basement. Additionally, we found Hf concentrations in Mojave zircons to be higher and to have a wider range of values, indicating greater fractionation, perhaps at slower rates and shallower crustal levels. Salinia is also distinct in having the lowest Ce/Yb and Gd/Yb ratios, which denotes heavy rare earth element enrichment. The Sierra Nevada batholith is internally variable but overlaps zircon fields from both Salinia and the Mojave. More data are needed, but our initial results, combined with isotopic data, suggest that the age and lithology of the basement into which plutons are emplaced exert a major control on zircon geochemistry. Because basement domains – and the resulting zircon geochemical fingerprints – are spatially variable, they can potentially be used as diagnostic signatures in detrital zircon provenance studies. This is especially important in arc settings where large regions experience coeval magmatism.