Petrology and Geochemistry of the Rattlesnake Hills Alkalic Intrusive Complex

The Rattlesnake Hills intrusive complex of Natrona County, Wyoming is composed of alkalic and subalkalic igneous rocks estimated to be ~44 Ma (K/Ar radiometric ages of feldspars from a quartze latite and phonolite; Pekarek et al., 1974). Previous geochemical (major, minor, and Sr-Nd isotopic) analyses revealed two phases of magmatism: a central group of alkalic intrusions surrounded and preceded by intrusions of two silica-saturated felsic magmas (Hoch and Frost, 1993).  Latite, nepheline-rich phonolite, trachyte and alkalic mela-trachyte make up the central alkalic group, whereas quartz latite and rhyolite comprise the surrounding rock units.  Previous Sr and Nd isotopic data from quartz latite and rhyolite were interpreted as the result of crustal assimilation involving arc-type magmatism, and the central alkalic group was interpreted as a partial mantle melt derived from decompressional melting of lithospheric mantle from a subduction event (Hoch and Frost, 1993).  The lithospheric mantle here and elsewhere in western North America shows evidence of chemical modification by evolution of fluid from a subducted oceanic plate.  Our current research furthers the sampling and geochemical analyses of the variety of alkalic and subalkalic igneous rocks of the Rattlesnake Hills, Wyoming, and compares our data to other alkalic igneous centers of the region which have related carbonatite magmatism.