BASALTIC VOLCANISM IN THE WESTERN SNAKE RIVER PLAIN AND BOISE RIVER VALLEY: FERROBASALTS, FLOTATION CUMULATES, AND THE CHANGE TO K-RICH OCEAN ISLAND BASALTS 750,000 YEARS AGO

Basaltic volcanism in the western Snake River Plain and the Boise River drainage area presents a conundrum for plume models of SRP volcanism: the WSRP lies at a steep angle to the “hotspot track” (ESRP trend), and it erupted in two episodes separated by 5 million years. The WSRP is a graben with over 3 km of structural relief, filled with lacustrine sediments from the Plio-Pliestocene “Lake Idaho” system; fluvial-deltaic sediments, subaqueous hyaloclastites, and subaerial basalts are less voluminous.

Volcanic activity in the WSRP began with silicic volcanic rocks along the northern and southern margins of the basin between 11.8 - 9.2 Ma. Basaltic eruptions spanned two time periods: 9-7 Ma and < 2.2 Ma. The older basalts occur as basalt flows intercalated with late Miocene sediments. Pleistocene volcanic activity (< 2.2 Ma) consists of two phases: (1) tholeiitic shield volcanoes that form the plateau around Mountain Home, and canyon-filling flows along the Boise River South Fork; (2) younger shield/cinder cone vents of K-rich (>1% K2O) transitional alkaline basalt. The early Pliocene, pre-Lake Idaho basalts from the Mountain Home AFB core are higher in Fe, Ti, HFSE, REE, but lower in alkalis, silica, and phosphorus than basalts of the ESRP. This suggests a source that is depleted relative to the ESRP basalt source in LILE, but enriched in Fe and Ti, possibly due to Fe-Ti metasomatism. The younger tholeiitic basalts (2.2 to 0.75 Ma) are Fe-Ti basalts with up to 17 wt% FeO* and 4.3 wt% TiO2. Plagioclase flotation is common and may drive Fe-enrichment. Isotopic compositions of the Pleistocene tholeiites lie on mixing lines between OIB and EM2 compositions, suggesting mixing of plume-like source with metasomatised lithosphere. The younger (<0.75 Ma) alkali basalts have K2O >1, are higher in Mg#, Si, Cr, Ni, and lower in total REE than the tholeiites. Their isotopic compositions are similar to OIB, suggesting an uncontaminated plume component.