ORIGIN AND EVOLUTION OF LATE MIOCENE-PLEISTOCENE BASALT: CENTRAL MOUNT BENNETT HILLS, CENTRAL SNAKE RIVER PLAIN, IDAHO

The Snake River Plain of southern Idaho represents the track of a hot-spot. The Mount Bennett Hills lie north of the Snake River-Yellowstone track, near the intersection of the eastern and western SRP. The Central Mount Bennett Hills exhibits the most abrupt uplifting of the entire range. The range gradually rises out of the SRP to the range’s crest and then drops abruptly to the Camas Prairie. Voluminous mid-Miocene (8-13 Ma) rhyolitic ash-flows and lava flows are exposed along the Mount Bennett Hills. These rhyolites unconformably rest on Cretaceous granitic rocks of Idaho batholith and are overlain by minor Tertiary basalt flows and sediments. We have sampled eleven Late Miocene to Pleistocene basaltic lava flows from six quadrangles. Based on the Idaho Geological Survey mapping the flows seem to be mono-eruptive flows from separate small vents.

Hand samples range from black to gray with the darkest samples being aphyric and the lightest samples being plagioclase phyric. The medium gray colored samples tend to be olivine plagioclase phyric. The size of olivines are approximately 0.5-1.5 mm in size, while the plagioclases ranged from approximately 0.5 to 5 mm in size. Approximately one third of the samples were slightly to moderately vesicular.

Forty-three samples were selected for our study of basaltic volcanism in the central Mount Bennett Hills. These samples were studied petrographically and by SEM. Mineral endmembers were determined by SEM-EDS. The basalts consist of plagioclase and olivine phenocrysts set in a groundmass of olivine, plagioclase, clinopyroxene, and oxides. Olivine phenocrysts endmember ranged from Fo88-60. Plagioclase phenocryst endmembers ranged from An69-56. Groundmass pyroxene endmembers ranged from Wo45-35 En59-47 Fs18-8. Cr spinel, ilmenite, and Ti magnetite were present in many of the samples. One flow unit (Square Mt.) represents a hybrid flow with much more evolved mineral data.

The samples were analyzed for major and trace elements. The majority of samples have Mg# ranging from 65-39. One flow (Square Mt) are more evolved as indicated by Mg# ranging from 35 to 25. The high Mg# samples have the following chemical ranges: TiO2 1.2 – 3.8 wt.%; FeO 7.3 – 16.1 wt.%; Nb 5-41 ppm; Zr 60-566 ppm; Ni 10-250 ppm; La 8.7-81 ppm. All magmas exhibit LREE enrichment. LaN/LuN ratios range from 3 to 7. Our hybrid flow has a LaN/LuN ratio of 4 to 10.

Initial trace element modeling suggests 30 to 76% of olivine fractionation of our selected parent sample would result in rare earth concentrations similar to our most evolved rocks in our suite.