MANTLE-DERIVED MASS TRANSFER TO CONTINENTAL CRUST ALONG THE YELLOWSTONE HOTSPOT TRACK

Previous studies of the Eastern Snake River Plain (ESRP) indicate large, but poorly constrained transfers of mantle-derived mafic magma into continental crust occurred during passage of the Yellowstone hotspot. To estimate the mass transferred we calculate the masses of ESRP volcanic and inferred plutonic rocks and then budget the crustal- and mantle-derived components of those rocks based upon petrologic and Nd- and Sr-isotopic criteria. We divide inputs of new mass to the crust into two groups based upon their chronology and petrological characteristics. Group I: Mantle component in silicic igneous rocks. Emplaced in a northeast trending time transgressive sequence between 10-4 Ma, the rhyolitic extrusive rocks accumulated to an average depth of ~2.5 km. e_Nd and ⁸⁷Sr/⁸⁶Sr_(i) range from -7 to -8 and 0.710 to 0.715, respectively. Mixing calculations, constrained by plausible mantle-derived and Archean crustal rock end-members, yield a minimum of ~70% mantle component in the rhyolites. Assuming a ratio of intrusive to extrusive rocks of 5:1, the total transfer to the crust was equivalent to the mass of a layer of gabbro ~10 km thick. Group II: 750 meters of ~4-0 Ma olivine tholeiite basalt flows. We model these as polybarically differentiated from a picrite parent, producing cumulates with a layer mass equal to ~300 meters of gabbro. Comagmatic mafic intrusive rocks (e.g., dike swarms) are assumed to occur at a 1:1 ratio with basaltic volcanic rocks. Isotopic characteristics of mafic volcanic rocks (e_Nd ~ -3 to -5, ⁸⁷Sr/⁸⁶Sr_(i) ~0.708) indicate that they are entirely mantle-derived. Total contributions from mantle to crust are estimated to be equivalent to a 12 km-thick layer of gabbro, contributed mostly at the onset of hot spot track volcanism.