MAGMATIC CONDITIONS AND PROCESSES RECORDED IN THE 1.4-1.5 GA GRANITE-RHYOLITE TERRANE, ST. FRANCIS MOUNTAINS, MO: INSIGHTS FROM ZIRCON TRACE ELEMENT GEOCHEMISTRY

The geology of a large province extending from California to Labrador is, in part, characterized by 1.4-1.5 Ga year old silicic volcanic and intrusive rocks of the Granite-Rhyolite Terrane (GRT). These rocks are buried by younger sediment throughout most of the mid-continent but are well exposed in the St. Francis Mountains of southeastern Missouri. The eastern portion of the St. Francis exposure is comprised mainly of rhyolitic ash flow tuffs from the Butler Hills caldera accompanied by a suite of ~coeval shallow granites. Prior research conducted on the granites and rhyolites in this area include whole rock elemental analyses and U-Pb zircon dating, which established general relationships among and ages of rocks of the Butler Hills caldera and related granites (Bickford et al., 1981; Van Schmus et al., 1996).

Our research aims to further constrain magmatic conditions and processes chronicled by silicic units in the St. Francis Mountains, which in turn may explicate the development of the GRT as a whole. To do so, we rely on high spatial resolution trace element analysis of zircon, a ubiquitous, resilient accessory mineral that concentrates U, Th, REEs and other important trace elements as it crystallizes. Trace element compositions of individual zircon grains reflect magmatic composition; moreover, core-to-rim zircon geochemistry can reveal how magmatic conditions changed during zircon crystallization. As demonstrated in recent studies, Ti concentrations in zircon can serve as proxies of magmatic temperature.

We separated zircon from 3 rhyolite samples (~40 zircon grains) and 5 granite samples (~84 zircon grains) collected from representative exposures in the St. Francis Mountains and imaged them with cathodoluminescence by SEM at Vanderbilt. Both granitic and rhyolitic grains generally display euhedral cores surrounded by euhedral oscillatory zoning and in some cases anhedral, possibly inherited cores. Using the SUMAC USGS/Stanford University SHRIMP-RG, we will analyze core-to-rim trace element compositions (including Ti, U, Hf, and Nb) of individual zircon grains and calculate Ti-in-zircon temperatures (Ferry & Watson, 2007). Our data will help elucidate compositions of GRT magmas, constrain GRT magmatic conditions and evolution, and allow us to explore connections between GRT granites and rhyolites.