TRACER ISOTOPE GEOCHEMISTRY AND U-PB ZIRCON GEOCHRONOLOGY OF INTRUSIVE ROCKS FROM THE WALLOWA AND OLDS FERRY ARC TERRANES, BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON AND WEST-CENTRAL IDAHO

Paleozoic through Mesozoic terrane accretion along the cordillera of western North America represents an important phase of crustal growth with a complex and regionally extensive tectonic history. The Blue Mountains Province (BMP) of Oregon, Idaho, and Washington is an important piece of this discontinuous puzzle of amalgamated litho-tectonic units stretching from northern Canada and Alaska to northwestern Nevada and northern California. Existing accretionary models for the BMP greatly depend on the inferred tectonic settings of its constituent arc terranes. Preliminary Nd and Sr isotopic data and single-grain U-Pb zircon geochronologic results for the Wallowa (WA) and Olds Ferry (OF) arc basement rocks imply: 1) the Wallowa arc developed within an intra-oceanic environment from a depleted mantle source with little or no interaction with evolved continental crust during the Late Permian and Middle Triassic, and 2) the OF volcano-plutonic assemblage represents a juvenile continental fringing arc system with some interaction with evolved continental lithosphere during the Middle and Late Triassic and Early Jurassic. Continued development of geochronologic and isotopic datasets for intrusive rocks, as well as for extensive supracrustal packages of the BMP will help to: 1) establish clearer and more accurate terrane characterization and discrimination; 2) develop comprehensive models for the tectonic evolution of individual terranes as well as for the BMP as a regional geologic province; and 3) provide a concrete basis for terrane correlation with similar assemblages along the Cordilleran margin.