NEW BULK ROCK GEOCHEMICAL DATA FOR <10 MA VOLCANISM IN JACKSON HOLE AND THE UPPER WIND RIVER BASIN, WY: COMPARATIVE GEOCHEMISTRY TO SNAKE RIVER PLAIN –YELLOWSTONE BASALTS AND PETROGENETIC IMPLICATIONS

The Upper Wind River Basin (UWRB) and Jackson Hole (JH) volcanic fields, WY host <10 Ma monogenetic volcanoes and intrusions. The timing and distribution of this magmatism corresponds to uplift and thermal input associated with the migration of North America over the Snake River Plain-Yellowstone (SRPY) hotspot. Consequently, magmatism in the UWRB and JH could represent off-axis, out-of-sequence hotspot magmatism; magmatism that does not follow the linear, time-transgressive pattern associated with hotspots, but are still associated with a hotspot structurally, thermally, or petrogenetically. We use “hotspot” here to reflect a mantle upwelling and melt anomaly, regardless of depth of origin. We are investigating the relationships between this small volume magmatism, the Wyoming craton, and the SRPY. New bulk rock geochemical data from Shadow Mountain and Flat Creek (JH) and Pinnacles Butte, Norton Point, and Havely Rock Garden (UWRB) show compositions that include basalts, basaltic andesites, trachyandesites, andesites, and a dacite. All samples are calc-alkaline. Ni concentrations in these rocks range from 12-271 ppm and Cr ranges from 13-537 ppm and are similar to other UWRB volcanoes. Compared to SRPY basalts, these rocks also show a similar range of Ni and Cr values, though the UWRB rocks generally have higher SiO₂ contents (49-66 wt%). Additionally, samples from Norton and Havely have high Mg#’s (64-71), and elevated K₂O (2.94-4.23 wt%), relative to SRPY basalts, despite their more evolved compositions. These Mg#’s are also significantly higher than other UWRB volcanoes, with the exception of Spring Mountain, which has Mg# ~70. The high Ni, Cr, and Mg#’s suggest that these magmas are relatively primary. All of these new UWRB samples also fit into two distinct Ba enrichment arrays. Ba/Nd vs. Nd reveals an array of samples with Ba enrichment with little change in Nd and an array with little change in Ba but increasing Nd. The former could represent decreasing degrees of lithospheric partial melting while the latter could be enriched in Ba through asthenosphere melts assimilating metasomatized material. This data suggests that volcanism in the UWRB and JH could reflect partial melting of subcontinental lithospheric mantle or asthenosphere melts assimilating phlogopite-rich metasomatized lithosphere.