SILICIC VOLCANISM OF THE YELLOWSTONE HOTSPOT – A LITHOSPHERIC PROBE

Silicic volcanism defines three episodes in the history of the Yellowstone hotspot. 1) Volcanism from 16.5-15.2 Ma is characterized by frequent eruptions (40/myr) of high-temperature rhyolite from multiple and dispersed volcanic centers situated in the accretionary terrain west of the Precambrian continent. These rhyolites are chemically and isotopically distinct from younger products. 2) Volcanism from 15.2-8 Ma is characterized by moderately frequent eruption (15-20/myr) of mostly high-temperature rhyolite from localized volcanic centers. High temperatures and frequent eruption imply rapid transport from source to surface with minimal opportunity for fractionation and/or assimilation. This process may be facilitated by significant crustal extension. 3) Volcanism from 8-0 Ma is characterized by markedly lower rates of eruption (~3/myr) of cooler and more chemically evolved rhyolite and an overall lower volumetric rate of eruption. The transit of the hotspot onto the Precambrian continent can be tracked precisely by a rapid shift in neodymium and hafnium isotopic ratios that center the crustal melting anomaly directly on the lithospheric boundary at 15.2 Ma. The slope of the isotopic shift constrains the lateral dimension of the crustal melt source to ~70 km. The transition at 15 Ma from widely dispersed centers of volcanism to localized centers is consistent with a model for truncation or shearing of a plume head from its conduit upon encountering a step in the topography of the continental lithosphere.

Differences in protoliths in accreted terrain and Precambrian continent result in compositional and isotopic differences in silicic magmas. Early-erupted magmas were produced either by extreme fractionation of mantle-derived basalt or by partial fusion of prior underplated basalt. Rhyolites bear chemical affinities to residual glass in Columbia River basalts. Silicic magmas younger than 15.2 Ma are hybrid melts with contributions from Precambrian crystalline basement and mantle-derived basalt. Isotopic evidence requires the mantle contribution to vary at a minimum from 20-40% along the hotspot path. Mass balance indicates substantial crystallization of basalt is necessary to yield a fractionated component that admixes with the crustal fraction to produce hybrid silicic melts.