OFF-TRACK YELLOWSTONE HOTSPOT BASALT VOLCANISM NORTH OF THE EASTERN SNAKE RIVER PLAIN: PHYSICAL AND CHEMICAL CONSTRAINTS ON A MONOGENETIC VOLCANO IN THE CENTENNIAL VALLEY, MONTANA

Hotspot identification has focused on the identification of clear, time-transgressive, linear chains of volcanoes, plutons, and/or eruptive products across the crust. However, these age-progressions are unclear when volcanism along a hotspot “track” does not temporally fit in the age progression and is therefore, out-of-sequence and at times off-axis of the spatial trend. The Snake River plain-Yellowstone (SRPY) volcanic province is an ideal location to examine exactly how hotspot out-of-sequence magmatism occurs relative to hotspot magmatism, along a hotspot track. Exposed adjacent to the eastern SRPY (and far off its axis) are the remnants of <10 Ma, monogenetic volcanic fields and eruptive loci. Some of these are well-studied (e.g., Blackfoot, Idaho), but most are not. As part of a larger project to understand SRPY out-of-sequence magmatism, one of the volcanic fields we are studying is centered on the Centennial Range/valley, SW Montana. The Centennial Range (and valley) lie adjacent to the major east-west trending, seismically active, Centennial normal fault. Cenozoic mafic rocks in this region include Eocene-Oligocene units, as well as discontinuously exposed packages of ~6 to 1.5 Ma lavas mapped as basalt, which are inferred to have erupted from SRPY volcanoes and flowed north down paleovalleys into Montana. However, we document local eruption of at least one of these previously mapped lavas from an inferred fissural vent at the western end of Centennial Valley. Pervasively oxidized red-pink scoria (e.g., bombs to lapilli) crop out in close proximity to a single, ~4.5 m-thick basalt lava. Both the lava and the scoria are olivine and plagioclase-rich, and also contain up to ~1 cm bright green clinopyroxene crystals. The lava is a subalkaline, tholeiitic basalt, with Mg# = 57.8 and generally resembles eastern Snake River plain olivine tholeiites. Olivine crystals show two populations, Fo_73-74 and Fo_79-82, and LA-ICP-MS and EPMA analyses of the large pyroxenes demonstrate that they are Cr-diopside. Work is ongoing to decipher the age and petrogenesis of this basalt, including geothermometry and barometry on its crystal cargo. The pyroclastic facies provide clear evidence that at least some of the basalts in SW Montana, inferred to have SRPY sources, were instead locally erupted.