BASALTIC VOLCANISM OF THE CENTRAL SNAKE RIVER PLAIN, IDAHO
Most of the chemical variation within flows from single vents can be explained by low-pressure fractionation of the observed phenocrysts (olivine + plagioclase). Line scans of olivine phenocrysts show no reversals in composition or other evidence of magma mixing. Line scans of some plagioclase phenocrysts show minor reversals that indicate fluctuations in magma chamber fluid pressures. The occurrence of cumulate gabbro xenoliths (ol+cpx+plg+oxide) in one flow is consistent with high-pressure fractionation at mid-crustal levels, within the basaltic sill imaged seismically beneath the eastern plain. There is no correlation of K with mg#, ruling out extensive assimilation of older, felsic crust; but some of the variation in trace elements may be due to assimilation of previously injected gabbroic dikes at mid-crustal depths. We infer that these basalts represent a mixed asthenospheric-lithospheric source that formed in response to Yellowstone melting anomaly; these melts evolved by a combination of high-pressure and low-pressure crystal fractionation, with possible assimilation of previously intruded mid-crustal gabbros.