IMPLICATIONS OF THE GEOCHEMICAL VARIATIONS IN THE WANAPUM BASALT FORMATION (COLUMBIA RIVER BASALT GROUP) FOR A PLUME SOURCE

The Wanapum Basalt consists of at least 36 flows subdivided into 5 members and constitute ~5% of the total Columbia River basalt (CRB) volume. Emplaced ~15 Ma, they represent the final eruptions of the main CRB phase. Flows of the Frenchman Springs, Roza, and Priest Rapids Members (F-R-P) comprise >98% of the total volume of the Wanapum Basalt. The vent systems for the Wanapum flows are at the northern end of the Chief Joseph dike swarm; F-R-P vent systems straddle the inferred boundary between the accreted terranes and the craton that underlie the Blue Mountains and Palouse physiographic provinces, respectively. While the voluminous F-R-P flows are quartz-normative tholeiitic basalts showing limited compositional variability, small volume Wanapum flows range from olivine-normative tholeiitic basalt to quartz-normative basaltic andesite. Primitive mantle and ocean island basalt (OIB) normalized geochemical patterns suggest that the F-R-P flows were derived from a plume source similar to OIBs. However, the F-R-P flows are enriched in Ba, La, and Pb and depleted in Nb and Sr relative to an OIB source suggesting that the plume source was modified by material derived from subducted oceanic crust. Given the proximity of the Cascadia subduction zone to the Yellowstone plume and the long history of terrane accretion in the region, magma generated within the Yellowstone plume probably incorporated chemical components derived from sudbucted oceanic crust. Elemental variations in early small volume flows of the Eckler Mountain Member differ significantly from OIBs suggesting derivation from a different source. Overall geochemical trends within the Wanapum basalts indicate that open system processes (fractionation-assimilation-recharge) subsequently modified the parental magma. Recent geochemical work by Ramos et al (2005) and Caprarelli and Reidel (2005) suggest that Wanapum magmas were derived by partial melting of the asthenosphere with minor assimilation that probably occurred in the lower crust as is consistent with the Re-Os isotopic data of Chesley and Ruiz (1998).