PETROLOGY AND GEOCHEMISTRY OF THE PRECAMBRIAN ILLINOIS BASEMENT AND RELATED ROCKS FROM ST. FRANCOIS MOUNTAINS, MISSOURI

The granite-rhyolite province (GRP), formed between 1.5-1.3 billion years ago, is the Precambrian basement that underlies the Illinois Basin. It is an extensive suite of rhyolite and granite extending from the Great Lakes region to the southwest, and represents a belt of felsic magmatism that extended through Laurentia and into the modern southwestern United States. Rocks of the GRP are only locally exposed, and so have largely been studied through drill cuttings and cores. The GRP is comprised of volcanic rocks with associated epizonal granites and mafic intrusions and encompasses five major rock units: quartz syenite, rhyolite, granite, gabbro, and quartz monzonite. Deep seismic reflection studies, as well as sparse deep-hole basement penetrations, report heterogeneity; however, petrologic and geochemical data are limited. Core and cuttings from 26 widely-spaced wells throughout Illinois were examined to better characterize the variation and architecture observed in the basement, as were rocks from the St. Francois Mountains in SE Missouri (~1.47 Ga). The principal rock types are alkali-feldspar granite, syenogranites, and rhyolites. The rhyolites are further subdivided into quartz-bearing, feldspar-bearing, and aphanitic. All granites are magnetite bearing, with ilmenite present as exsolution lamellae within magnetite crystals. Accessory minerals include apatite, sphene, and zircon. Magmatic allanite is rare. Granites range from fine- to coarse-grained, with rare instances of pegmatitic textures. Heterogeneity is demonstrated both laterally and vertically across the Illinois Basin. Quartz-feldspar intergrowths (graphic texture) characterize shallower granites and suggest hypabyssal emplacement. Major element geochemistry demonstrates K₂O wt % ranging from 8.6-3.8 wt % in granites, whereas rhyolites ranged from 9.6-5.4 wt %. When plotted in Nb-Y and Ta-Yb tectonic discrimination plots, the data scatter across collisional, ocean-ridge, and intra-plate tectonic settings, implying that these discrimination techniques are not sufficient to determine tectonic environments.