MAGMATIC CONDITIONS AND PROCESSES RECORDED IN 1.4-1.5 G A GRANITE RHYOLITE TERRANE, ST. FRANCIS MOUNTAINS, MO: USING TI-IN-ZIRCON THERMOMETRY TO EVALUATE RELATIVE CRYSTALLIZATION TEMPERATURES

The geology of a large province extending from California to Labrador is, in part, characterized by 1.4-1.5 Ga 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. The GRT serves as an example of classic A-type granites and rhyolites which have been considered to be generated at higher temperatures and display distinct geochemical patterns (Whalen et al., 1987).

Our research aims to further evaluate zircon geochemistry in those units, to compare recent zircon investigations in other magmatic environments, and test the hypothesis that the zircons might reveal unusually high crystallization temperatures (via Ti concentrations) and distinctive elemental signatures. To evaluate this, 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 analyzed core-to-rim trace element compositions (including Ti, U, Hf, and Nb) of individual zircon grains and calculated Ti-in-zircon temperatures (Ferry & Watson, 2007). The average Ti concentration that we measured in our zircons was 11.3 ppm, which correspond with an estimated crystallization temperature of ~809°C.