PETROLOGY AND GEOCHEMISTRY OF THE ABSAROKA VOLCANIC PROVINCE

This study compared samples from three stratigraphic units of the Absaroka Volcanic Province in northwestern Wyoming. Distinctions were made through mineral identification, petrology, and geochemistry. Three stratigraphic groups have been identified within the Absaroka Volcanic Supergroup; the Washburn, Sunlight, and Thorofare Creek groups, from oldest to youngest respectively. Results of this study and previous research indicate the Washburn Group consists of andesite and dacite lavas as well as volcaniclastic rocks. Pyroxene-rich andesite lavas, volcaniclastic rocks, potassium-rich mafic dikes and related mafic flows compose the Sunlight Group directly east of Yellowstone National Park. Further south, the Thorofare Creek Group includes pyroxene- and biotite-rich andesitic conglomerates and breccias. Eight samples were collected within the Absaroka Volcanic Province including three rounded cobble-sized clasts, representative of earlier flows, two volcanic breccias with smaller clasts, and three hypabyssal dikes (1-3 m in width). Samples were analyzed using thin section microscopy and x-ray diffraction (XRD). In addition, a few geochemical analyses were completed for comparison with existing data. Clasts collected west of Meeteetsee, Wyoming were dominated by plagioclase which exhibited distinct oscillatory zoning, and pyroxene phenocrysts. Olivine, biotite, and chlorite were also present in lesser amounts. The two volcanic breccias, sampled from the Washburn and Sunlight groups, consisted of smaller, more angular clasts of andesite, basalt, and scoria, within a fine-grained matrix. Geochemical analysis showed the three dikes cutting volcaniclastics of the Wapiti Formation (Sunlight Group) were potassium-rich. The dikes, classified as absarokite and shoshonite, were much lower in silica but higher in potassium and sodium as indicated by the presence of nepheline in the XRD results. Changes in the mineralogy, chemistry, and texture between these three different stratigraphic groups, suggest a change in the composition of the magma source through time.