PETROGRAPHY AND GEOCHEMISTRY OF IGNEOUS AND METAMORPHIC CORE SAMPLES FROM THE OMAHA DOME IN SOUTHERN ILLINOIS

Ultramafic igneous intrusions have been reported in parts of Illinois, Missouri, Kentucky, and Tennessee. These rocks and their relation to the New Madrid Seismic Zone remain poorly understood due to their exotic nature and susceptibility to weathering. Here we focus on a portion of drill core from the Omaha Dome in southern Illinois (37° 51’ 37.08” N, 88° 20’ 9.42” W), extending from 2596 to 2615 feet below the surface, and exposing 15 feet of igneous rock with 4 feet of metamorphosed calc-silicate rock below. The contact zone extends from 2608 to 2611 feet and is distinguished from the dark gray igneous rock above by a distinct mottling with dark ultramafic clots mixed into a pale carbonate-rich matrix. The igneous rock contains partially serpentinized olivine phenocrysts, magnetite, apatite, and large (> 0.4 mm) crystals of perovskite and apparent melilite pseudomorphs, within oikocrysts of phlogopite and interstitial clinopyroxene. Groundmass carbonate increases in abundance toward the basal contact. Below the contact, the calc-silicate rock is marked by horizontal bands of beige and green, as well as abundant clots of granular garnet surrounded by clinopyroxene. The heterogeneous groundmass is dominated by prismatic wollastonite. Whole-rock analysis by XRF and ICPMS indicates that the igneous rock contains 33-38 wt.% SiO₂, 6-8% Al₂O₃, 16-19% MgO, 12-14% FeO_t, 15% CaO, and 0.5% P₂O₅. High concentrations of K₂O (2.6%) and TiO₂ (4.9%), and very low Na₂O (0.4%) are notable. Mg# ranges between 70-72. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of K, Sr, Zr, and Hf. These geochemical characteristics are consistent with a near-primary melt from a metasomatized peridotite source containing phlogopite-rich veins, and are very similar to those of ultramafic dikes sampled farther to the south in Illinois (the rock in this study is slightly more primitive, less altered, and set apart by its well-developed cumulate texture). Future work will involve EMP analysis of minerals to facilitate classification of the rock, analysis of drill cuttings to help constrain the thickness of the intrusion, and further comparison with similar rocks from within the area for potential insights into igneous activity associated with rifting in the New Madrid region.