INVESTIGATING less than 10 MA OFF-AXIS MAGMATISM IN THE CENTENNIAL VALLEY, MONTANA AND UPPER WIND RIVER BASIN - JACKSON HOLE, WYOMING: YELLOWSTONE HOTSPOT - LITHOSPHERE INTERACTIONS AT THE LEADING EDGE OF THE SNAKE RIVER PLAIN (U.S.A.)

A migrating ~100-200-km-wide topographically high region of active continental extension (e.g., Yellowstone Crescent of High Terrain, YCHT), is coincident with the leading edge of Snake River plain-Yellowstone (SRPY) volcanic province and now centered on the Yellowstone Plateau volcanic field. Remnants of small-volume, <10 Ma magmas are exposed adjacent to the Yellowstone Plateau volcanic field, both to the south (in ID-WY) and west (MT) and are spatially associated with the YCHT. West, in the Centennial Valley region, are tholeiitic lavas with chemical and Sr-Nd-Pb isotope characteristics identical to Snake River olivine tholeiites that exemplify SRPY basaltic volcanism over the last <12 Ma. Some of these basalts were locally erupted and reflect the areal footprint of SRPY magma generation, north of the commonly accepted SRPY extent. South of Yellowstone, in Wyoming and adjacent Idaho, are a suite of <10 Ma intrusives, volcanoes, and lavas. ⁴⁰Ar/³⁹Ar ages from newly collected samples range from 7.9 to 1.2 Ma, with the youngest sample from a basalt intrusion southeast of Jackson Hole, WY. This suite, which includes the Jackson Hole (JH) volcanics and magmas of the Upper Wind River Basin volcanic field (UWRB), define two groups, a K-rich, alkaline to transitional group, and a calc-alkaline group. Trace element differences exist between the groups; however, all rocks have large ion lithophile element enrichments, high field strength element depletions, and other geochemical characteristics associated with subduction zone magmatism, even though they formed in an intraplate setting. These characteristics and new Sr-Nd-Pb isotope data, show that these rocks are distinct from SRPY basalts. Isotope data from the least evolved rocks (basalt and lamprophyre) suggest that the magmas were sourced from Archean Wyoming Craton lithospheric mantle. Similar small volume off-axis volcanism has been identified adjacent to oceanic hotspots and also continental and oceanic rifts, where voluminous mantle upwelling occurs regardless of whether the cause is a mantle plume or not. Thus, UWRB-JH magmatism south of Yellowstone, may represent an example of this phenomena, where off-axis magmatism occurs due to mantle melting and associated YCHT lithosphere extension, which facilitates small volume melt production and eruption.