Geochemical and isotopic evidence for the origin of continental flood basalts with particular reference to the Snake River Plain Idaho, U. S. A

Abstract
Voluminous outpourings of olivine and quartz tholeiite cover vast tracts of the western U.S.A, around the Columbia and Snake Rivers. Voluminous eruptive units within each province are petrographically and chemically homogeneous and generally lack significant lateral or temporal variation. These features suggest relatively homogeneous source regions. A possible scenario for the Snake River Plain involves extraction of tholeiitic melts from enriched spinel lherzolite mantle (87Sr/86Sr > 0.7058,143N d/144Nd < 0.51252) which contains at least a component of 2.5 Ga material. Subsequent fractionation of olivine, plagioclase, apatite and magnetite in crustal magma chambers and simultaneous assimilation of crust (ca. 20%) accounts for the isotopic variability in the more evolved ferrolatites and ferrobasalts. Unlike the olivine tholeiites these evolved volcanic rocks exhibit all the classic elemental and isotopic correlations consistent with an origin involving combined assimilation and fractional crystallization.