PETROLOGY OF DEEP CRUSTAL XENOLITHS FROM THE EASTERN SNAKE RIVER PLAIN, IDAHO (GEOTHERMOMETRY, PRECAMBRIAN)
MATTY, DAVID JOSEPH
Doctor of Philosophy
Xenoliths, collected from certain hybrid lava flows and vents at three localities in the eastern Snake River Plain region of southern Idaho, were derived from underlying crustal terrains which experienced granulite-facies metamorphism at approximately 3.0 AE. Lithologically, charnockite, opdalite, enderbite, and norite meta-igneous xenoliths are predominant, but biotite-garnet gneiss and cognate noritic xenoliths derived from fractional crystallization of the host lavas also occur. The metamorphic charnockite-norite xenoliths are characterized by mineral assemblages containing compositionally homogeneous plagioclase + hypersthene +/- quartz +/- alkali feldspar +/- iron-titanium oxides +/- clinopyroxene +/- biotite. Iron-titanium oxide, two-feldspar, and two-pyroxene geothermometry calculations indicate that metamorphism of the xenoliths occurred at about 700 to 800(DEGREES)C. Corresponding pressures, estimated from garnet-plagioclase-orthopyroxene-quartz and plagioclase-clinopyroxene-quartz relationships, range from approximately 4 to 8 kbar. Entrainment of the xenoliths by their host hybrid lavas has resulted in reequilibration of mineral assemblages in certain xenoliths to the higher temperature conditions of the lavas. The high temperatures experienced by the xenoliths in their host lavas also resulted in partial melting of the xenoliths, producing interstitial glasses of varying composition. Silicic interstitial glasses are most common and have been derived primarily from melting of quartz-feldspar assemblages. They are similar in major element composition to surficial rhyolites of the Snake River Plain region. More mafic interstitial glasses formed from melting of plagioclase and ferromagnesian minerals and appear to be similar in composition to the hybrid host lavas. Trace element constraints preclude a direct genetic relationship between partial melts produced from the xenoliths and surficial volcanics. However, it is possible that rhyolites could be derived from partial melting of the lower crust, providing secondary modifying processes occur.