ORIGIN AND EVOLUTION OF QUATERNARY BASALTS: CAMAS PRAIRIE RIFT BASIN, CENTRAL SNAKE RIVER PLAIN, IDAHO

The Camas Prairie, an elongate EW-trending valley that formed as a rift basin during passage of the Yellowstone hotspot, is situated between the late Cretaceous to early Tertiary granitoids of the Idaho Batholith to the north and the Mount Bennett Hills basalt overlying rhyolite basement to the south. The rift basin consists of Pliocene-Holocene sediments derived from the Idaho batholith, interbedded with Quaternary volcanic units that flowed from eruptive centers located in the southern and northern parts of the basin. Basin formation occurred between 5 and 1.8 Ma, based on limited age control on rifted silicic and basin-filling basalts. The Camas Prairie rift basin resembles other extensional basin-and-range systems.

Hand samples range from dark black to light 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 typical size of olivines in the olivine phyric samples were approximately 1mm in size, while the plagioclases ranged from a few millimeters to approximately 5mm in size. Approximately one third of the samples were slightly to moderately vesicular.

The samples were analyzed for whole rock major and trace elements. The major elements were determined by EMPA analysis of glass beads. The trace elements were determined by ICP-MS techniques. Results of major element analyses show ranges of: wt.% SiO2 = 44.8-54.5; wt.% TiO2 = 1.68 - 3.19; Mg# = 45.46 - 60.19; wt.% K2O = 0.36 - 2.21. Norm calculations resulted in all but one sample being olivine normative, the one sample being quartz normative. Trace element concentrations indicate that all samples are light rare earth enriched with no Eu negative anomaly. LaN ranged from 48 to 103 and LuN ranged from 9 to 15. This leads to LaN/LuN of 5.2 to 6.9.

Initial trace element modeling suggests that approximately 55% of olivine plus plagioclase fractionation of our selected parent sample (lowest rare earth concentration of our samples) would result in rare earth concentrations similar to our most evolved rocks in our suite.