SOURCES OF CORDILLERAN GRANITIC ROCKS AND SECULAR VARIATION: INSIGHTS FROM TRACE ELEMENT AND PB-U GEOCHEMISTRY OF ZIRCON

The Cordilleran magmatic arc swept inboard and outboard across the southwestern margin of the Proterozoic North American craton from Permian through Cretaceous time. It has long been recognized that individual arc segments are characterized by distinctive suites of plutonic rocks, yet it is unclear how the distinctive traits of these rocks were generated. Do they reflect secular variation in parental magma sources, or varying differentiation processes within evolving crust in a long-lived continental margin arc setting? To test whether secular variations are a result of changing parental magma sources, we are using the SHRIMP-RG to measure trace element and isotope geochemistry of magmatic and pre-magmatic zircons in a suite of plutons from southern California, with the aim of characterizing the provenance of zircon-bearing magmas across space and through time. Previous studies of zircon populations have broadly characterized source rock ages; the high resolution of the SHRIMP-RG allows us to geochemically subsample discrete geochronologic domains within individual pre-magmatic, magmatic, and polygenetic zircons from plutonic rocks. Initial measurements of magmatic grains and overgrowths yield good correlations of Hf and Ti (e.g. Lowery et al., 2006), and a factor of 2 or more variation in Hf/Zr, reflecting both regional and temporal variations in source rock composition. In contrast, pre-magmatic zircons are geochemically as well as texturally and isotopically distinct. Premagmatic zircons and cores of polygenetic grains typically yield concordant Pb-U ages, and are consistently Ti-enriched but Eu and HREE depleted in comparison to magmatic grains and overgrowths. This preliminary data set shows that this technique can identify and characterize magmatic and premagmatic compositions, and monitor regional and temporal changes in magma sources.