Paper No. 9-8
Presentation Time: 9:00 AM-6:00 PM
TRACE ELEMENT CHARACTERISTICS OF THE WANAPUM AND SADDLE MOUNTAINS BASALTS, COLUMBIA RIVER BASALT GROUP
Re-organization of the dominantly bimodal Yellowstone hotspot volcanic province in the inland northwestern USA between 16 and 14 Ma was accompanied by marked changes in the geochemistry of erupted basalts. This transition occurred as contemporaneous rhyolites became less Fe-rich and the locus of silicic activity moved on-craton to produce the future Snake River Plain – Yellowstone hotspot track. The pre-16 Ma ‘main phase’ Steens, Imnaha, Grande Ronde and Picture Gorge Basalts  of the Columbia River Basalt Group (CRBG) have trace element and radiogenic isotope features that indicate derivation from a mixture of enriched mantle (plume-source), depleted upper mantle, and subduction-influenced mantle, modified by crustal assimilation . The later Saddle Mountains Basalt (~15 – 6 Ma), and basalts of the Snake River Plain, have the isotopic characteristics of ancient continental lithosphere . To aid in understanding the transition, we have begun a systematic investigation of the lithophile trace element characteristics of the Saddle Mountains Basalt and underlying Wanapum Basalt, emplaced between the Grande Ronde and Saddle Mountains at ~16 Ma, and comparison with the main-phase lavas. Both the Saddle Mountains and Wanapum Basalts display some unusual rare earth and high field strength element features. Main phase primitive lavas have Nb/Ta between 14 and 16, in common with most basalts world-wide. In contrast, some Wanapum and Saddle Mountains units have Nb/Ta ~17.5, close to the bulk silicate Earth value (17.8) but rare globally. Intriguingly, the co-erupted Asotin and Wilbur Creek Members of the Saddle Mountains have Nb/Ta = 15 and 17.4 respectively. The Umatilla Member of the Saddle Mountains is a highly unusual CRBG unit, with mild alkaline character (basaltic trachyandesite to trachyandesite), extreme enrichment in Ba (≤0.5 wt% BaO), positive Eu anomalies, and high Zr/Hf (~47). These features of the Umatilla are consistent with accumulation of alkali feldspar and zircon, but neither are observed, nor are likely, phenocryst phases. Alternatively, the Umatilla magma assimilated felsic plutonic rock or crystal mush in a crustal magma system.
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