Aaron’s Bachelors Thesis

Aaron's Bachelors Thesis

All of the rocks examined are vesicular andesitic porphyrites. The phenocrysts are predominately mid-range plagioclase and pyroxene, with 1-2% iron-titanium oxides, and small amounts of apatite. The matrix is partially glassy but does contain microlites of oxides, tridymite, and pyroxenes. The plagioclase ranges from An65 to An80. Some of the crystals are zoned with a core of An75 and a rim of An90. The pyroxenes are calcium-iron-magnesium-silicates which mainly vary in the calcium content (hi-cal/lo-cal). Many of the pyroxene crystals contain oxide inclusions. All of the opaques are iron-titanium oxides, and they occur as pure grains or as complex intermixtures of two compounds, usually titaniferous magnetite and illmenite. The glassy matrix appears to be very uniform and similar in composition to the plagioclase but has less silicon and contains potassium, an element not detected in any of the phenocrysts. Small variations in the chemical composition of the phenocrysts, especially the pyroxenes and plagioclase, and the variation in the microlite/glass ratio provide a way to characterize the individual rock flows in the stratigraphic sequence.