Geochemical constraints on Cenozoic intraplate magmatism in the Upper Wind River Basin, Wyoming (USA)

Abstract:

The Upper Wind River Basin is located in north-central Wyoming (USA). At the northwestern edge of the basin, preliminary work by others has identified <4 Ma igneous rocks (lavas and shallow intrusives in low volumes) that are exposed southeast of the Yellowstone Plateau volcanic field. Virtually no literature exists on these rocks aside from a few K-Ar ages. Pilot Knob is an augite-rich intrusive body that yields a 3.4 ± 0.06 Ma K-Ar age. Lava Mountain, which lies ~ 4 km south of Pilot Knob, is a shield volcano where ~25 lavas are exposed in what appear to be glacially truncated cliffs. At the summit, a small capping cinder cone overlies lavas; one of the youngest lavas yields a K-Ar age of 0.48 ± 0.06 Ma. Crescent Mountain lies ~6 miles northeast of Lava Mountain and one Crescent Mountain lava yielded an ~3.6 Ma K-Ar age. At Spring Mountain, ~14 km north of Dubois, WY, local eruptions of at least one thin basaltic lava occurred from fissures that cut Paleozoic and Eocene sedimentary strata. Materials sampled from all locations range from basalt to dacite and define a primarily calc-alkaline differentiation array. Pilot Knob and one Crescent Mountain sample have wt. % K₂O values between 2.7 to 3.8 at ~53 to 56 wt. % SiO₂, which are much more K-rich than any other sample. These samples are also characterized by enrichments in LILE (e.g., >2000 ppm Ba, >1500 ppm Sr), LREE (>100 ppm La, >250 ppm Ce), Zr, Pb, and HREE depletions, relative to the other samples. The least evolved basalts from Spring Mountain are primitive with Mg # ~70 and Cr >900 ppm. Preliminary field constraints and satellite imagery indicates that regional fault zones control the location of individual eruptive loci/intrusives. For example, Pilot Knob and Lava Mountain lie along the projection of a normal fault zone that extends southeast from the Yellowstone Plateau volcanic field. Work is ongoing to further physically, geochemically, and isotopically characterize these igneous rocks with the goal of determining whether they are related to mantle melting associated with the Yellowstone Plateau volcanic field and the eastern Snake River Plain (e.g., Yellowstone hotspot), or other regional lithospheric-derived melting associated with continental extension (e.g., Basin and Range faulting or other causes).