Granulite xenoliths erupted in Neogene basalts, and a rare outcrop of Precambrian basement along the northern margin of the Snake River Plain (SRP) can be used as windows into the origin and stabilization of the lower crust of southern Idaho. Previous work to determine the nature of the lower crust beneath the Snake River Plain has been conducted on a suite of xenoliths exposed in Southern Idaho at Square Mountain (SM), Craters of the Moon National Monument and Preserve (CRMO), and the Spencer-Kilgore (SK) area (Leeman, 1979; Leeman et al., 1985; Matty, 1984; Wolf et al., 2005), as well as on a basement outcrop at House Mountain, near Mountain Home, ID (Alexander, 2006). This study uses U/Pb geochronology and Hf isotope geochemistry to determine the history of formation and stabilization of the lower crust of the SRP with the context of surrounding Precambrian North America. Results of this investigation reveal that two distinct terranes comprise the lower crust of Southern Idaho. The Kilgore-Craters terrane formed and stabilized  ca 2.8 Ga and the Square Mountain terrane at ca 2.5 Ga. Hf isotope ratios and U-Pb geochronology of inherited cores of zircons reveal that the Snake River Plain formed in part from older, reworked crust. Archean geologic events and Hf signatures of the Snake River Plain match those of the Wyoming Province, indicating the Snake River Plain is a westward extension of the Wyoming Province. The Kilgore-Craters terrane is equivalent to the northern Beartooth-Bighorn magmatic zone and Montana Metasedimentary province, and the Square-Mountain/Grouse Creek terrane is a westward extension of the Southern Accreted Terranes of the Wyoming Province.
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