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