INVESTIGATING THE PALEOTECTONIC SETTING OF ORDOVICIAN K-BENTONITES IN SOUTHEASTERN US USING ZIRCON TRACE ELEMENT GEOCHEMISTRY

The Ordovician Millbrig and Deicke K-bentonites are altered volcanic ash beds deposited across eastern and central North America. They provide evidence of two massive supereruptions in Phanerozoic history (Huff, 2007). Previous work suggest that these units were generated in a continental volcanic arc likely during the closure of the Iapetus Ocean (Samson et al., 1989; Huff, 2007; McDowell et al., 2015). This study presents detrital zircon geochronology and trace element geochemistry to further investigate the paleotectonic setting of the Millbrig and Deicke eruptions. Using LA-ICP-MS, we analyzed grains from bentonite units in the southeastern US (Tennessee, Alabama, Kentucky) for U-Pb isotope and trace element compositions. To more directly understand the conditions in which the source magmas evolved, we applied new Ti-dependent partition coefficients of rare earth elements in zircon (Claiborne et al., 2018) to calculate the corresponding melt compositions. Zircon in the Deicke bentonite are 444.13±0.96 Ma, and the Millbrig unit zircons indicate an age of 437.71±1.13 Ma. The two units are similar, though not identical in trace element composition. Preliminary comparison of zircon U/Yb using the discriminant diagrams of continental and oceanic crust zircon from Grimes et al. (2007) suggests a continental source for the two ashfalls. Calculated melt rare earth element (REE) patterns are characterized by depleted MREEs, which may indicate fractionation of amphibole. In order to further evaluate the tectonic environment of the two eruptions, we explore various methods for using Ce anomaly in zircon as a proxy for source magma oxygen fugacity. Taken as a whole, the trace element data suggest a continental arc environment for the Millbrig and Deicke K-bentonites, which supports previous interpretations. This is consistent with massive arc volcanism occurring at the continental boundary during the closure of the Iapetus Ocean.