POST-CA. 5 MA EXTENSION-RELATED MAGMATISM IN THE UPPER WIND RIVER BASIN, WYOMING (USA), ASSOCIATED WITH THE YELLOWSTONE HOTSPOT TECTONIC PARABOLA

The upper Wind River Basin in northwest Wyoming (USA) is located ∼80–100 km southeast of the Yellowstone Plateau volcanic field. While the upper Wind River Basin is a manifestation of primarily Cretaceous to Eocene 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 tail. New ⁴⁰Ar/³⁹Ar ages coupled with existing K-Ar results from intrusives and lavas in the upper Wind River Basin show that igneous activity younger than ca. 5 Ma occurred locally. Field and geochemical data show that these <ca. 5 Ma upper Wind River Basin magmas were either erupted or emplaced along normal fault zones at different locations and range in composition from tholeiitic basalt to calc-alkaline basaltic andesite through dacite, and include a lamprophyre intrusion. Together, these igneous rocks define the Upper Wind River Basin volcanic field (UWRB). All UWRB rocks have large ion lithophile element enrichments, high field strength element depletions, and other geochemical characteristics associated with subduction even though they erupted in an intraplate setting. Our results suggest that UWRB magmatism, the Miocene Jackson Hole volcanics, and other small-volume, similarly aged intermediate to felsic magmatism in eastern Idaho, are the result of the interaction between the North American plate and the progression of the tectonic parabola associated with the Yellowstone hotspot tail. As a result, it appears that in multiple locations across most of the length of the Yellowstone hotspot track, major silicic activity was preceded, at a given longitude, by typically smaller-volume volcanism, likely associated with uplift and extension. We suggest that similar relations should characterize other examples of continental hotspots and can be used to identify them in other locations in both the modern and past geological records.