PETROLOGY OF QUATERNARY RHYOLITE DOMES OF THE BIMODAL BLACKFOOT VOLCANIC FIELD - SOUTHEAST IDAHO

The Blackfoot Volcanic Field (BVF; our informal name) is a region of Quaternary, bimodal basalt-rhyolite volcanism infilling parts of two northwest-trending Tertiary grabens bordering the southern margin of the Eastern Snake River Plain. The BVF extends from Blackfoot, ID, 90 km to the south near Soda Springs, ID, over an area of ~1500 km². Located near the center of the dominantly basaltic volcanic field are two spatially separated groups of rhyolite lava domes. The southernmost, youngest group (K/Ar dated at ~50 Ka in previous work) consists of three mineralogically and compositionally identical domes and associated tephra cones aligned N 33^o E over a distance of 5.3 km. The southernmost of these domes is largest (~0.5 km3) and has produced a small coulee. The older (~1.4 Ma) dome field is located ~25 km to the north. Three domes are aligned S 60^o E over a distance of 6.2 km. Largest of the three is estimated to have a volume of 0.38 km³. Alignment of domes in the respective dome fields diverge from coeval, spatially overlapping NW-trending fault patterns. Rhyolites within the respective dome fields are essentially identical in phenocryst mineralogy and bulk major and trace element composition. Both sets of domes are high silica (~76.5 wt.%), potassium-rich (4.7 - 5.0%) and metaluminous. Phenocryst assemblages are similar consisting of ~2 mm sized, euhedral to subhedral sanidine and embayed quartz, euhedral to subhedral 2 mm-sized biotites, 1 mm-sized, weakly normal zoned plagioclase, 0.5 mm-sized hornblende and opaques (mostly magnetitess). One sample from the southern dome field yielded bulk-rock ⁸⁷Sr/⁸⁶Sr(i)=0.71065 and a eNd*=-11.0. While the major element chemical compositions are similar for the two dome sets they are distinguishable in a variety of major and most trace elements (e.g., Rb=490 vs 330 ppm; U=31 vs 13 ppm, respectively in the central and more northerly dome fields). Rhyolites of the BVF resemble nearby coeval rhyolites on the Eastern Snake River Plain in some respects (e.g., high K2O and Rb, and low Sr) but they differ in other important respects (e.g., having lower FeO_T, Zr). In this preliminary work we speculate that differences between the two proximal rhyolite systems reflect a greater degree of interaction between parental basalt magma and Archeon crust in the BVF.