PETROLOGICAL AND MINERALOGICAL VARIATIONS IN 2500-2000 YR BP LAVA FLOWS, CRATERS OF THE MOON LAVA-FIELD, IDAHO

Basaltic lava flows were erupted 2500-2000 yr B.P. along the Great Rif t volcanic rift zone at the Craters of the Moon lava field in the Snak e River Plain volcanic province, Idaho. These flows form a diverse sui te of rock types near the northern edge of the Snake River Plain but t ypical Snake River Plain olivine basalts were erupted nearly contempor aneously at the center of the plain. The Craters of the Moon lava flow s range from basalt to trachyte, whereas rocks typical of the Snake Ri ver Plain are olivine basalt and rhyolite with no intermediate types. The Craters of the Moon lava flows have chemical and mineralogical fea tures that do not fit conveniently into any of several currently recom mended rock classifications. Suggested names for the Craters of the Mo on rocks, based on color index, SiO2, P2O5 and TiO2 contents, modal fe ldspar plus glass contents, Fa contents of olivine, and feldspar compo sitions, are basalt, trachybasalt, tristanite, and trachyte. Both the Craters of the Moon and Snake River Plain rocks are dominated by plagi oclase and olivine and contain minor or lesser amounts of pyroxene. Ho wever, the Craters of the Moon rocks lack the Mg-Al and Cr-rich spinel s and the Mg-rich olivines found in Snake River Plain basalts. Instead , the Craters of the Moon lava flows contain Fe-rich olivine, rare Fe- rich pyroxene, and minor alkali feldspar. Accessory minerals in the Cr aters of the Moon rocks are zircon and apatite. Chemical and mineralog ical data, including Thompson space representations and mass balance c alculations, suggest a genetic link between the Snake River Plain basa lts and the Craters of the Moon lava flows. Thermodynamic modeling sug gests that augite could appear on the liquidus of the Craters of the M oon lava flows at high pressures and including a more magnesian (earli er?) pyroxene in the fractionating assemblage improves the least-squar es mass balance result. Processes of crystal fractionation and accumul ation, overprinted by contamination by pumiceous rhyolite, can account for much of the diversity within the rock types from the Craters of t he Moon lava field.