COMPARISON OF IGNEOUS ROCKS OF THE ABSAROKA PROVINCE AND THE YELLOWSTONE VOLCANIC TERRANE

The purpose of this project is to evaluate the texture, mineral content, and chemical characteristics of the igneous rock units within the Absaroka Province in comparison to the igneous units of the Yellowstone Volcanic Terrane. These two episodes of igneous activity are different in their timing and origin even though they are close to one another. Absaroka volcanism took place 52 to 38 million years ago, while major Yellowstone eruptions occurred in close proximity from 2 million to 600,000 years ago. The Absaroka volcanism is interpreted as being subduction-related, while the Yellowstone volcanism is attributed to a continental hotspot. Seven samples were collected within the Absaroka Province including three volcanic breccias, three hypabyssal dikes (1-3 m in width), and one porphyritic andesite. A total of six different samples were collected from the Yellowstone Volcanic Terrane including 5 different volcanic tuffs and one porphyritic rhyolite. These samples were evaluated using thin section, x-ray diffraction, and geochemical analyses. Mineralogy of the Absaroka samples is intermediate in composition including andesine plagioclase, nepheline, and pyroxene as primary minerals. In comparison, the Yellowstone samples are more felsic in composition with quartz and K-feldspar as the primary minerals. Sanidine and orthoclase were each detected in different samples from the Yellowstone volcanic units according to interpretation of the XRD patterns. The textures in these two areas also vary significantly. Textures in the volcanic breccias of the Absaroka units include large angular clasts of andesite and more mafic rocks such as scoria, contained within a finer matrix. Zoning of plagioclase phenocrysts are also evident in the volcanic breccias. In comparison, samples from Yellowstone units were primarily pyroclastic in origin as tuffs with some samples containing abundant phenocrysts. Based on the data collected in this study, volcanic units from the Absaroka province are distinctly different in both mineral composition and texture from those of the Yellowstone terrane. Therefore, it is evident that the tectonic framework for the production of igneous units in the Absaroka Province, over 38 million years ago, was distinctly separate from recent activity associated with the Yellowstone hot spot.