MAJOR AND TRACE ELEMENT VARIATIONS IN A LARGE PROTEROZOIC IGNIMBRITE DEPOSIT, ST. FRANCOIS MOUNTAINS, MISSOURI

The objective of this study is to characterize chemical variations in one of the most extensive caldera collapse-related rhyolitic ash flow tuffs in the St. Francois Mountains—the Grassy Mountain Ignimbrite. The Mesoproterozoic St. Francois Mountains igneous terrane makes up an area of 40,000 km² and is part of the larger Eastern Granite-Rhyolite Province which spans much of the Midwestern and Eastern United States (Kisvarsanyi, 1980). The Grassy Mountain Ignimbrite was later emplaced at ~1.46 Ga, associated with the collapse of the Butler Hill Caldera (Du Bray, 2021).

The St. Francois Mountains igneous terrane serves as one of the only well-exposed, easily accessible examples of a volcanic-plutonic complex in the Midwestern U.S. exhibiting tuffs, lava flows, dikes, shallow intrusives, and various igneous contacts. The central location and variety of outcrops available make the St. Francois Mountains a valuable field area for high school and university-level geology and earth science groups. However, the Grassy Mountain Ignimbrite, which is thought to be the largest rhyolitic unit associated with the Butler Hill caldera collapse, has not yet been sampled comprehensively and systematically. A better understanding of this unit could benefit from further sampling and geochemical analyses to characterize the heterogeneity, or homogeneity, of the unit. Around 15 samples of the exposed rhyolite unit will be collected laterally and possibly vertically. The samples will then be analyzed for major and trace elements. Spider diagrams, trace elemental ratio diagrams, and discrimination diagrams will be generated from the geochemical analyses in order to compare samples within the unit and with other volcanic units in the terrane. Constraining the variation in geochemistry of the Grassy Mountain Ignimbrite will help provide a better understanding of the petrogenesis of the unit, the potential zoning in the unit, and its relationship to other volcanic and plutonic units.