INTERMEDIATE VOLCANISM ASSOCIATED WITH THE YELLOWSTONE HOTSPOT, SOUTHEASTERN IDAHO AND WESTERN WYOMING

The Miocene to Recent Yellowstone-Snake River Plain (YSRP) volcanic system is known to be exclusively bimodal (either basalts or rhyolites) with the exception of the Craters of the Moon ferrolatites. Along the southeastern flank of the YSRP “hotspot” track, exists a coeval set of low-volume volcanics which contain flows of intermediate composition (54-70% SiO₂). These include the Carlton Creek Volcanics (Ar-Ar 6.62 Ma), the Dacite of Pony Creek (Ar-Ar 6.59 Ma), the Calamity Point Andesite (K-Ar 6.3 Ma), the Jackson Hole Volcanics (K-Ar 8.06-8.48 Ma; Ar-Ar 7.36 Ma), Lava Mountain (K-Ar .48 Ma), Pilot Knob (K-Ar 3.4 Ma), Crescent Mountain (K-Ar 3.6 Ma), and various basalt flows and dikes near Grand Targhee and Dubois, Wyoming.

Compared to the YSRP, these suites are enriched in MgO, Ni, and Cr. They are depleted in HREE with respect to LREE and in high field strength elements (Nb, Ta). The Jackson Hole Volcanics have an isotopic signature (low ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd) indicative of Archaean lithosphere. These features suggest a regionally extensive (if small volume) melting of a deep-seated Archaean mantle source. Seismic attenuation tomography suggests that limited melting occurs within the Yellowstone mantle plume at ~250 km, where lithospheric mantle, hydrated during Laramide flat slab subduction, crosses the wet peridotite solidus. The degree of melting would be insufficient for melt segregation at this depth, but melt and residue would continue to ascend with the plume, which tomography shows to be inclined to the southeast. In the core of the plume, the initial melt would be completely overprinted by voluminous melting at the dry peridotite solidus at ~110 km. On the periphery to the northwest the initial melt, cooled by contact with overlying lithosphere, would likely solidify at depth. To the southeast, however, heat from the overhanging plume might serve to maintain these partial melts, allowing the intermediate magmas to reach the surface.