Forearc Basin Provenance Analysis by SHRIMP Detrital Zircon Geochronology

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Project Title: Forearc Basin Provenance Analysis by SHRIMP Detrital Zircon GeochronologyPI: Stephan GrahamABSTRACT The composition of grains in sandstone can be diagnostic of the original source of those grains, as in the case of volcanic grains eroded from volcanic arcs. Conventionally, such analyses are conducted with a standard optical microscope, but the broad compositional similarity of sand from all volcanic arcs often results in ambiguity as to specific sources. Greater interpretational leverage would be provided by an ability to determine the ages of individual grains, and that capability exists in the Sensitive High Resolution Ion Microprobe (SHRIMP). The application of SHRIMP technology to a geologically well-characterized arc-related sedimentary basin (i.e., one for which the sources of sediment are independently known) could serve as a standard against which studies of poorly known but analogous basins might be gauged. The Mesozoic strata of the Great Valley basin of California present such an opportunity, because the bulk-grain and trace element compositions of sediment are already determined, and the sources of sediment in the adjacent Sierran and Klamath arcs are generally known. Thus, SHRIMP study of the ages of Great Valley zircon sand grains will form the basis of a model of sediment source relations in an ancient arc-related basin, as well as provide insights into long-term rates of erosion of volcanic arcs and sedimentation of derivative sediments. The Great Valley model subsequently will be compared with results of SHRIMP analysis of zircon grains from the Mesozoic Methow basin of Washington and British Columbia. If consistent with the Great Valley study, the Methow results may demonstrate the broader utility of the analytical method, as well as contribute to solving some specific problems of the geologic evolution of that region.