GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 273-4
Presentation Time: 9:00 AM-6:30 PM


RIVERA, Tiffany A., Geology Program, Westminster College, 1840 South 1300 East, Salt Lake City, UT 84105, WHITE, Craig, Department of Geosciences, Boise State University, Boise, ID 83725, SCHMITZ, Mark, Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 and JICHA, Brian R., Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706

The Western Snake River Plain (WSRP), Idaho, comprises one arm of the arc-shaped lowland long recognized as the Snake River Plain physiographic province, and contains aerially extensive and compositionally diverse Pleistocene basalts. Monogenetic volcanoes in the WSRP produced a wide variety of landforms including maars, tuff cones, scoria cones and shields, and the deposits they produced include palagonite tuffs, lahars, scoria beds, pillowed lava flows, and subaerial pahoehoe. This work presents new 40Ar/39Ar dates, geochemical data, and Sr, Nd, and Pb isotopic compositions for WSRP basalts to examine compositional variations within time and space. In general, magmas experienced a shift to lower 87Sr/86Sr and greater 143Nd/144Nd, 206Pb/204Pb, and 208Pb/204Pb beginning about 1 Ma. Similarities in the isotopic signatures of mildly alkaline basalts and tholeiitic ferrobasalts, which overlap temporally and spatially, suggest the parent magmas originated in a common source but experienced different types and degrees of crystal fractionation and crustal assimilation. MELTS computer simulations of both equilibrium crystallization and fractional crystallization show that a mildly alkaline parental melt could produce residual liquids having many of the major oxide characteristics of tholeiitic ferrobasalts. However, models could not produce the observed K2O/P2O5 ratios, thus necessitating assimilation of a high-P contaminant. Further, we use the isotopic signatures of these melts to speculate on the nature of the asthenospheric mantle from which the magmas were derived and how this source region compares to that of neighboring basaltic provinces.