ON THE ORIGINS OF CHEMICAL AND ISOTOPIC DIVERSITY AMONG THE COLUMBIA RIVER FLOOD BASALTS

Examination of new, precise whole-rock trace element data and new and existing radiogenic isotope data from the Columbia River basalts indicates a relatively straightforward pattern of magma generation and modification during flood basalt production. Peak CRB activity at ca. 17 - 15 Ma is represented by the Steens, Imnaha, Grande Ronde, and Picture Gorge basalts. Later waning activity from ca. 15 to 6 Ma resulted in the Wanapum and Saddle Mountains flows. In Sr-Nd-Pb isotopic space, Imnaha basalts lie at the apex of radiating trends and represent a component that is present in all CRB lavas, with the possible exception of some Saddle Mountains flows. In common with previous workers, we interpret this component to represent the mantle plume. Picture Gorge basalts lie between Imnaha and Pacific MORB isotopic compositions and represent mixtures of plume and depleted mantle components. Grande Ronde lavas form a geochemical continuum with the Imnaha but are more evolved with higher Sr isotope ratios and LILE/HFSE ratios. The latter observation is inconsistent with an origin for the Grande Ronde lavas by melting of recycled oceanic crust within a mantle plume, while Pb isotope data exclude the possibility that this additional component is derived from young accreted crust; it is instead identified as ancient continental crust, now closely represented by Miocene rhyolite lavas of the western Snake River Plain that are products of later crustal melting in this region. We suggest that the Imnaha and Grande Ronde magmas were processed through crustal magma chambers located in the area where the Chief Joseph dike swarm, Oregon-Idaho graben and western Snake River Plain graben converge and that the mafic magmas had access to ancient crust; heating of the crust caused greater degrees of assimilation with time. Grande Ronde magmas moved >200 km northwards through Chief Joseph dikes to erupt from vents in the vicinity of SE Washington and hence flood the Columbia Plateau. Later Wanapum and Saddle Mountains magmas, which additionally contain a lithospheric mantle component, were also processed through this system.