GEOCHEMISTRY ACROSS THE IMNAHA-GRANDE RONDE CONTACT WITHIN THE COLUMBIA RIVER FLOOD BASALT PROVINCE, SE WASHINGTON

The Steens Basalt, Imnaha Basalt and Grande Ronde Basalt make up most of the erupted volume of the 210,000 km³ Columbia River Flood Basalt Province in the inland Pacific Northwest. A notable feature within the province is the physical and chemical transition from the Imnaha Basalt to the voluminous Grande Ronde Basalt. The Imnaha lavas are mostly more primitive, coarse, diktytaxitic and frequently plagioclase-phyric basalt flows while the Grande Ronde lavas are more evolved and phenocryst-poor, with aphanitic to glassy groundmass. The transition is well exposed in the eastern Columbia Plateau of southeast Washington. Field studies were carried out near Clarkston and Asotin, WA as well as adjacent parts of Idaho and Oregon. The lavas exhibit a jump in silica content across the transition, however other geochemical properties vary more continuously, including lithophile trace elements and radiogenic isotope ratios. Two compositional types, the American Bar and Rock Creek [1] within the Imnaha appear to be derived from two distinct mantle sources; the Grande Ronde lavas may be mostly derived from American Bar-type via assimilation and fractional crystallization. Plagioclase phenocrysts range from An₃₉ – An₇₅ in the Imnaha and An₄₀ – An₆₉ in the Grande Ronde. Minor element (Mg, Fe, Sr) variations in compositional profiles through plagioclase compositional profiles show spikes recording periodic recharge, seen more frequently in the Imnaha.

Sampling strategy included collecting vertical profiles within individual flows to search for bulk-rock geochemical variability. Inflation models for flood basalt emplacement predict vertical chemical profiles through individual lava flows if the chemistry of erupting lavas is changing. Such profiles are found elsewhere in the Grande Ronde stratigraphy [2]; however, Imnaha and Grande Ronde flows on either side of the transition appear to be mostly internally homogeneous. Incompatible trace elements determined by high-precision ICPMS analysis offer additional possibilities for flow identification and correlation, in supplement to existing approaches using XRF data [3].

[1] Hooper et al (1984) J. Pet. 25, 473–500; [2] Reidel (2005) J. Geol. 113, 1–21; [3] Hooper (2000) G^3 1, 1024