Boron isotopic variations in NW USA rhyolites: Yellowstone, Snake River Plain, Eastern Oregon

The geochemistry of NW USA rhyolites correlates strongly with geography and the nature of the underlying basement terranes. Rhyolites from the Snake River Plain-Yellowstone (SRPY) province have higher Sr-87/Sr-86, Pb-207/Pb-206, and lower Nd-143/Nd-144 than those from the Oregon High Lava Plains (HLP) province, reflecting a dominant influence of Precambrian cratonic crust east of the western Idaho suture zone versus accreted oceanic terranes of Phanerozoic age to the west. Rhyolites from the cratonic domain show significant enrichments of Th, U, and LREE/HREE, whereas B concentration and especially B/Nb and B/Rb are systematically higher west of the tectonic boundary. This decoupling of B from the other incompatible elements is best explained in terms of distinctive magmatic sources east and west of the suture zone.