GEOCHEMICAL AND SR-ND-PB ISOTOPE CONSTRAINTS ON CENOZOIC BASALTIC VOLCANISM IN THE UPPER WIND RIVER BASIN, WYOMING, USA

The Upper Wind River Basin (UWRB) is located in northwest WY southeast of the Yellowstone Plateau volcanic field and east of Jackson Hole, WY. While the UWRB is a manifestation of primarily Laramide tectonics, younger events have played a role in its formation, including Eocene Absaroka volcanism, Cenozoic lithospheric extension, and the migration of the North American plate over the Yellowstone hotspot. Small volumes of young (e.g., <4 Ma) volcanic rocks are observed to unconformably overlie Absaroka Volcanics and Paleozoic and Eocene sedimentary lithologies along the northern margin of the Wind River Basin. Spring Mountain is located ~14 km north of Dubois, WY and ~38 km east of Lava Mountain, a basaltic andesite-dacite shield volcano. At Spring Mountain, basalt erupted through fissures along normal faults that cut Paleozoic and Eocene strata. The relative lack of weathering and stratigraphic position, suggest that these basalt eruptions are considerably younger than the Absaroka Volcanics. The basalts are olivine, clinopyroxene, and plagioclase-phyric, with anhedral to euhedral olivine. Electron microprobe results indicate that olivine phenocrysts range from Fo70 to Fo89; ~Fo89 olivines are anhedral to subhedral and interpreted as xenocrysts. The basalts are subalkaline and tholeiitic, and normative mineralogy indicates they are olivine tholeiites. Mg# ranges from 60.8-70.0, and they are characterized by high Cr (>900 ppm) that is reflective of the modal olivine. They have LILE enrichments and HFSE depletions indicative of a source that experienced prior subduction-enrichment. For one basalt, 87Sr/86Sr=0.70608 and εNd=-18.3. Pb isotope ratios from this sample, as well as other <4 Ma UWRB volcanic rocks, fall along a 2.8 Ga isochron that represents Wyoming craton stabilization. We interpret the overall geochemical and isotope characteristics of the basalts to reflect their derivation from melting of ancient lithospheric mantle. The Nd and Pb isotope data also indicate that UWRB basalts are likely not related to Snake River Plain and Yellowstone basalts (e.g., much more negative εNd and higher 207Pb/204Pb). However, the thermal/geodynamic effect of the Yellowstone hotspot on northwest WY may have played a role in UWRB lithospheric mantle melting and associated extension.