PETROGRAPHIC EXAMINATION OF VOLCANIC ROCKS FROM HUGHES MOUNTAIN, SE MISSOURI

Hughes Mountain Natural Area is an exposed igneous knob of Precambrian volcanic rocks located in the Saint Francois Mountains of southeastern Missouri. This area is known for its macroscale columnar jointing that formed as the volcanic rock cooled and contracted. For this study, ten samples were collected from various locations at this exposure, and were prepared for petrographic analysis using facilities and equipment at the UALR Rock Preparation Laboratory. Petrographic analysis of prepared thin sections show that the rocks contain abundant phenocrysts of feldspar, fiamme (i.e. flattened and elongate pumice clasts), and bands of quartz contained within microscale glass flow banding. Textures observed were sericitic and perthitic alterations within the feldspars, and devitrification of fiamme, often with spherulitic growth of quartz and potassium feldspar. Though previously classified as rhyolite, observations from this study indicate that these rocks from Hughes Mountain are more specifically described as a devitrified rhyolitic ash flow tuff.