VOLCANIC STRATIGRAPHY OF THE CENTRAL SNAKE RIVER PLAIN NEAR SHOSHONE, IDAHO: THE OWINZA BUTTE, STAR LAKE, AND SHOSHONE SOUTHEAST 7.5' QUADRANGLES: EDMAP

Neogene basaltic volcanism in the central SRP was studied through detailed field mapping, petrologic, and geochemical analyses of three adjacent USGS 7.5' quadrangles (Owinza Butte, Star Lake, and Shoshone Southeast) located in the NE portion of the Twin Falls 1:100,000 sheet. Nine late Neogene basaltic shield volcanoes and their associated lava flows are found in the study area (Notch Butte, Owinza Butte, Wilson Butte, Rocky Ridge, Star Lake Butte, Baby Butte, Farm Butte, Sid Butte). All of the basalts are ol+plg phyric or microphyric. Major and trace element analyses exhibits a wide degree of variation: 5-9.1% MgO, 11-16.1% FeO, 1.6-4.3% TiO₂, 8.9-10.8% CaO, and 0.3-0.95% K₂O, with Rb=20-50x and Nb=10-20x NMORB. While the MgO contents are similar to MORB, low mg#s result from the high FeO contents, suggesting deep melting relative ocean ridges. The variation in chemical composition of basalt suggests derivation from separate magma types that have undergone varying degrees of low and high-pressure fractional crystallization. The Wilson Butte and Rocky Ridge basalts are related by a common source and a common petrochemical history. Type 1 and type 2 Notch Butte basalts indicate two magma sources that changed over time. Type 1 and type 2 Owinza Butte basalts imply distinct parent magmas. Zoning profiles in olivine and plagioclase phenocrysts show that magma mixing was not an important process, but variations in vapor pressure may cause some reversals in plagioclase zoning. High pressure fractionation is indicated by the occurrence of cumulate ol-cpx-plag-oxide xenoliths in the Sid Butte basalt; assimilation of previously formed gabbroic cumulates may also be important.