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

The St. Francois Mountains (SFM) complex in Southeast Missouri is comprised of 1.38 to 1.48 billion year-old granites, rhyolites, ash-flow tuffs, basaltic dikes and sills. The precise petro-tectonic setting(s) in which specific SFM lithologies formed remains unclear. An ongoing field, petrographic, major element, and trace element study of SFM felsic rocks investigates this problem. 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, and by comparison of SFM geochemical signatures to those of similar rocks from known tectonic settings. Notably, a plot of Nb versus Y shows that felsic SFM samples overlap the Within Plate Granite (WPG) and the Volcanic Arc Granite (VAG) + Syn-Tectonic Collision Granite (Syn-COLG) fields, with most samples plotting in the WPG field. A plot of Nb versus SiO2 shows that most samples plot in WPG and Ocean Ridge Granite (ORG) fields, as opposed to VAG, Collision Granite (COLG), and ORG fields. A plot of Rb versus SiO2 shows that all samples plot in the WPG field. Accordingly, trace element data suggest a within plate origin for most SFM rocks under study. Additional investigation of major and trace element data is underway to further correlate specific SFM lithologies to their petro-tectonic setting(s).