A SIMPLE GEOCHEMISTRY-BASED MODEL FOR CRB VOLCANISM AND ITS RELATION TO SNAKE RIVER PLAIN MAGMATISM

The Columbia River and associated basalts constitute the first major manifestation of Yellowstone hotspot magmatism. Both the relationship of the flood basalts to the Snake River Plain-Yellowstone magmatic trend, and the petrogenesis of the basalts, have been obscured by complexities of spatial-temporal patterns of volcanism, tectonics, and geochemistry. We show that the temporal change in isotope and trace element geochemistry of CRB lavas erupted during the main phase of activity (Imnaha and Grande Ronde basalts) is consistent with a single magmatic system, most probably a large magma chamber or related group of chambers. Early Imnaha basalts represent a mantle end-member present in nearly all subsequent CRB lavas. Their isotopic and trace element characteristics closely conform to those of ‘moderately high 3He/4He' OIB found in some Pacific Ocean island groups and are most plausibly explained as reflecting an origin in a mantle plume. Imnaha and Grande Ronde lavas form a compositional continuum with considerable overlap. In Imnaha lavas the range of whole-rock Sr isotope compositions is restricted (0.7038 - 0.7044), but plagioclase phenocrysts and antecrysts span 0.7038 - 0.7060, which encompasses the entire whole-rock range of both Imnaha and Grande Ronde lavas. We deduce that these two formations represent the products of a single magmatic system. Sr, Nd, and Pb isotope and trace element data require that this system assimilated ancient continental crust and must therefore have been located at least partially to the east of the regional western Idaho cratonic boundary. The most probable location is where regional tectonic elements - the western Snake River Plain, Oregon-Idaho graben, and Chief Joseph, Steens and Monument dike swarms converge near the craton boundary at ~44°N 116°30'W. This magmatic system acted as a focus for later basaltic volcanism in which there is geochemical evidence for an increasing contribution of ancient mantle lithosphere (Saddle Mountains and Snake River Plain basalts). The location is ~250 km north of the probable mid-Miocene plume axis, but may explain why subsequent volcanism converged on the central Snake River Plain from the SW (along the hotspot track) and NW (off the hotpsot track) over the period 15 - 11 Ma.