BASALTIC VOLCANISM OF THE CENTRAL SNAKE RIVER PLAIN, IDAHO

Late Neogene basalts of the central Snake River Plain NE of Twin Falls form large shield volcanoes clustered along the axis of the plain that are distinct from younger rift-related volcanism. These basalts have MgO similar to MORB (5-10%) but with higher FeO* (12-15%), TiO2 (1.6-4.3%), P2O5 (0.4-1.2%), and Al2O3 (14-17%). They are also higher in FeO* than similar basalts of the ESRP (e.g., INEEL site) but less Fe-rich than typical ferrobasalts of the WSRP near Mountain Home. The wide range in K, P, and K/P ratios at constant MgO implies a range of parent magmas derived from a similar source by variable degrees of partial melting. Fe8 values (~13) imply deep melting or a source higher in FeO than MORB asthenosphere, while Na8 values (2.4-3.2) imply moderate but variable degrees of partial melting. Partial melting models based on 18 incompatible trace elements indicate 5-10% melting of a spinel lherzolite source similar in composition to the E-MORB source. Garnet-bearing sources are ruled out by the slope of the REE patterns, implying pressures less than 20-25 Kb (i.e., within the sublithospheric channel that has been imaged seismically).

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.