PETROLOGY OF BASALTIC PILLOW LAVAS IN TETON CANYON, IDAHO, USA

Three basalt lava flows are exposed near the mouth of Teton Canyon, Idaho, U.S.A., each with a base of pillow lavas. These flows erupted from a shield volcano on the Rexburg bench, south of the canyon and dammed the river at three levels. The absence of paleosols between the flows indicates they were emplaced shortly after one another–likely from the same eruptive event. The basalts contain phenocrysts of euhedral plagioclase, euhedral to subhedral olivine, & Fe-Ti oxides. The oldest & youngest flows also contain euhedral clinopyroxene. Like other normal Snake River Plain basalts, these flows are ferroan, calc-alkalic, medium-K₂O, and Nb-rich. The younger flows contain progressively smaller plagioclase; more euhedral olivine phenocrysts; lower Cr & Ni concentrations; and higher Rb, Zr, & Ba concentrations--indicating that the magma system became progressively less evolved as the eruption progressed. This study is part of a larger study to understand the nature of basalt-river interactions in southeast Idaho.