NEW GEOCHEMICAL CONSTRAINTS ON QUATERNARY MAFIC VOLCANISM IN THE UPPER WIND RIVER BASIN, WYOMING (U.S.A.)

The Yellowstone Plateau volcanic field is one of the most hazardous volcanic systems in the world, and is characterized by three cycles of voluminous explosive rhyolite eruptions at 2.1, 1.3, and 0.64 Ma. Yellowstone Plateau volcanic field activity includes caldera-forming silicic eruptions, effusive rhyolite lavas, and basaltic volcanism that primarily occurred at plateau margins. Some of the basalts post-date the 640 ka Lava Creek tuff (e.g. 3rd-cycle caldera formation); these eruptions occurred along N-S trending fault zones near the Yellowstone Caldera margin. Basalts erupted in the Upper Wind River Basin (UWRB), Wyoming at Lava Mt., southwest of Yellowstone, have been included in Yellowstone geological maps. Lava Mt. is a partially dissected shield volcano composed of ~25 basaltic lavas capped by a cinder cone. One of the upper-most lavas yielded a K-Ar age of 0.48 ± 0.06 Ma. If valid, this places Lava Mt. volcanism among the mafic eruptions associated with the end of the third Yellowstone eruptive cycle. Approximately 38 km east at Spring Mt., just north of Dubois, WY, undated basalts crop out and appear to have been erupted along normal faults that cut Paleozoic and Eocene strata. The basalts on Spring Mt. are texturally similar to those from Lava Mt. A reconnaissance sample suite was collected in summer 2013 from both locations; one Lava Mt. unit is actually an andesite and some of the Spring Mt. basalts are fairly primitive (Mg # ~70, Cr >900 ppm). Overall, all samples are subalkaline, tholeiitic to transitional, and have enriched trace LILE concentrations relative to MORB. UWRB mafic volcanism is overlooked in studies of Yellowstone-related volcanism and regional hazard assessments because virtually no literature exists discussing these rocks. It is possible that UWRB basalts originated from Eastern Snake River Plain-Yellowstone mantle sources, thus we plan to document the petrogensis of these lavas and their relationship to Yellowstone via further detailed work including field mapping, petrography, ⁴⁰Ar/³⁹Ar geochronology, whole rock chemistry, and isotope data, with He isotope constraints from olivine.