Isotopically diverse rhyolites coeval with the Columbia River Flood Basalts: evidence for mantle plume interaction with the continental crust

Abstract

AbstractThe Columbia River Flood Basalts (CRB) of the northwestern USA are coeval with eruptions of several thousand km3 of rhyolite. A broad survey of major phenocryst oxygen isotopes and of O and Hf isotopes in zircons from these rhyolites reveals significant diversity in inferred δ18Omelt values, ranging from +1.9 to +10.5‰ (SMOW), and in zircon Hf isotope compositions, which range from εHf = −39 to +9. This newly identified isotopic diversity shows that the syn‐CRB rhyolites were derived from high‐percentage melting of the crust. Low‐δ18O rhyolites, which fingerprint the melting of hydrothermally altered crust, are concentrated at the edge of the North American craton. This suggests that the conditions of crustal heating, faulting, and hydrothermal alteration required for the production of these rhyolites were concentrated there by the contrasts in crustal thickness and rheology associated with the boundary between the North American craton and younger accreted terranes.