INVESTIGATION OF PETRO-TECTONIC SETTING(S) OF FELSIC IGNEOUS ROCKS, ST. FRANCOIS MOUNTAINS, MISSOURI

The St. Francois Mountains (SFM) complex, Southeast Missouri, is comprised of 1.38 to 1.48 billion year-old granite, rhyolite, ash-flow tuff, basaltic dikes and sills. The precise petro-tectonic setting(s)in which specific SFM felsic lithologies formed is unclear. A field, petrographic, major element, and trace element study of SFM felsic rocks investigates this problem. Lithologies under study include the Butler Hill-Breadtray granite, Graniteville granite, Silvermine-Slabtown granite, and surrounding rhyolites. Samples were collected from outcrops previously identified and mapped by Kisvarsanyi, 1979. All samples were analyzed by XRF at Middle Tennessee State University. Major element oxide analyses show that most SFM felsic rocks range between 70-80 wt % SiO2, 2-7 wt % K2O, 10-15 wt % Al2O3, 2-6 wt % Na2O, and < 2 wt % CaO. Possible SFM petro-tectonic settings are identified using trace element discrimination diagrams according to Pearce et al., 1984. A plot of Nb vs.Y shows that Silvermine granite overlaps the Volcanic Arc Granite (VAG) + Syn-Tectonic Collision Granite (Syn-COLG) and the Within Plate Granite (WPG) fields, while Butler Hill and Graniteville granites and surrounding rhyolite fall entirely within the WPG field. A plot of Y + Nb vs. Rb shows that Silvermine granite overlaps the VAG and WPG fields, while Butler Hill and Graniteville granites and rhyolites fall within the WPG field. A plot of Nb vs SiO2 shows that Silvermine granite overlaps the VAG + COLG + Ocean Ridge Granite (ORG) and the WPG + ORG fields, while Butler Hill and Graniteville granites and surrounding rhyolites fall within the WPG + ORG field. A plot of Rb vs SiO2 shows that all SFM samples fall within the WPG field. Accordingly, the Butler Hill granite, Graniteville granite, and surrounding rhyolites exhibit trace element characteristics most similar to a within-plate petro-tectonic setting. However, the petro-tectonic setting of the Silvermine granite is not as well constrained.