PETROGENESIS OF IGNEOUS ROCKS IN THE EMINENCE KNOBS REGION: SOUTHWESTERN ST. FRANCOIS MOUNTAINS, MISSOURI

Recent USGS 7.5 minute quadrangle mapping in the Eminence Knobs region of southeastern Missouri has identified the location of approximately 13 Mesoproterozoic (1.6-1.3 bya) volcanic and plutonic units that comprise the southwest portion of the tectonically enigmatic St. Francois Mountains granite-rhyolite province.

Representative samples of 10 of the most prominent units were collected for petrographic and geochemical analysis: Ygh (informally named here as the Ruble, Mill Spring, and Big Spring granites), Ycl, Ycu, Yltm, Ymm, Yrm, Ysc, Yscr, Ysi, and Ysm. Petrography of the rhyolitic units indicates that these rocks were predominantly lava flows (Ycl, Ycu, Yltm, Ymm, Yrm, Ysc, Yscr, Ysi, and Ysm). Two interbedded air-fall and poorly-welded ash-flow units in Ycl were encountered during field work (additional air fall and ash flow deposits within some of these units are noted on USGS maps but were not located during this field study). Ysi lavas contain up to 2% secondary fluorite; other units may only have a trace of fluorite, if any at all. The Ruble and Mill Spring plutons are Alkali Feldspar Granites and the Big Spring pluton is a true Granite (IUGS classification). All granites are leucocratic and subsolvus; the Mill Spring granite contains a trace of pyrite and the Ruble granite contains a trace of primary fluorite.

Geochemically, the igneous rocks of this region have a weak anorogenic affinity based on a Ga/Al vs. Zr plot (Whalen et al., 1987). Tectonic discrimination plots using Nb, Rb, Ta, and Yb are ambiguous in delineating the specific tectonic setting. All units are ferroan and peraluminous (ASI>1) except for the Ruble Granite which is slightly metaluminous (ASI = 0.9421) (Frost et al., 2001). All units except Ycl, have La/Lu ratios of 4-5 (Ycl: 1.5–8) which in part, is used here to suggest that the units in the region are cogenetic. Except for the Mill Spring granite, all units have uniform patterns on Spider plots and REE diagrams (all are depleted in Eu, Sr, P, and Ti indicating fractionation of feldspar, apatite, and ilmenite. Depleted Ba, K, and Rb in the Mill Spring Granite indicates potential enhanced feldspar fractionation as compared to the Ruble and Big Spring granites. Based on REE and Spider plots, all granites are inferred to be either comagmatic or cogenetic with the volcanic units in the Eminence Knobs region.