USING ZIRCON TRACE ELEMENT GEOCHEMISTRY TO ELUCIDATE PALEOTECTONIC SETTINGS OF THE LATE ORDOVICIAN MILLBRIG AND DEICKE K-BENTONITES, CENTRAL TENNESSEE, USA

The Late Ordovician Millbrig and Deicke K-bentonites of eastern and central North America represent two of the oldest, largest preserved ash fall deposits in the world. With an extent of >2 x 10⁶ km² and an estimated volume of >1500 km³, the Millbrig alone formed during a catastrophic eruption similar in magnitude to that of the Toba and Yellowstone supereruptions (Huff et al. 1996; Christidis & Huff 2009). Yet despite the size and implicit impacts of these volcanic events, the paleotectonic setting in which they were produced remains unresolved.

To further evaluate the tectonic origins of the Millbrig and Deicke eruptions, we relied on zircon, a common accessory mineral in K-bentonites and a resilient, robust recorder of magmatic processes and evolution. Recent comparisons of trace element signatures in zircons from various plate tectonic settings (e.g., Carley et al. 2014; Grimes et al. in press) indicate that zircon compositions vary with, and are potentially indicative of, tectonic setting.

Our goals were to evaluate the morphology and geochemistry of zircons from the Deicke and Millbrig K-bentonites and compare zircon geochemistry to that of zircons formed in other known settings. We separated zircon from four K-bentonite samples (three Deicke, one Millbrig) collected near Gladeville, TN and Carthage, TN and imaged the grains with cathodoluminescence by SEM. Using the SUMAC USGS/Stanford University SHRIMP-RG, we conducted high spatial resolution trace element analyses on ~30 of these grains and compared the results to trace element signatures in zircons from known tectonic locations worldwide.

Zircons from both K-bentonite units are euhedral and prismatic. They generally range from 100-150 um in size and display concentric, oscillatory zoning in CL. Trace element abundances in Deicke and Millbrig zircons are similar to one another (Sc = 35–148 ppm, Ti = 6–25 ppm, Nb = 2–14 ppm, Yb = 170–1200 ppm, U = 82–905 ppm) and are characterized by distinctively low Nb and high Sc/HREE – signatures consistent with those of zircons from modern arc environments. These data suggest that the Deicke and Millbrig supereruptions were probably generated in an arc environment (analogous to Toba or Taupo, not hot spot-generated Yellowstone eruptions), and they illustrate the potential for zircon chemistry as a tectonomagmatic tracer.