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

摘要

The Columbia River Flood Basalts (CRB) of the northwestern USA are coeval with eruptions of several thousand km(3) 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 O-18(melt) values, ranging from +1.9 to +10.5 parts per thousand (SMOW), and in zircon Hf isotope compositions, which range from epsilon(Hf)=-39 to +9. This newly identified isotopic diversity shows that the syn-CRB rhyolites were derived from high-percentage melting of the crust. Low-O-18 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.