PETROGRAPHY AND GEOCHEMISTRY OF AN ULTRAMAFIC DIKE IN SOUTHERN ILLINOIS

Since at least the early 1950s workers have noted ultramafic igneous dikes and breccias within the New Madrid Rift Complex, scattered through parts of Illinois, Missouri, Kentucky and Tennessee. Nevertheless, the nature and origin of these rocks has not been fully resolved, in large part due to their susceptibility to weathering. We recently sampled a dike freshly exposed in the Cottage Grove Coal Mine (Peabody Coal) near Eldorado, Illinois, approximately 27 km northwest of Hicks Dome. This near vertical dike, approximately 10 m wide and trending N20W, is clearly silicate at its interior, but transitions to a carbonate-rich composition towards the dike margins, which are mildly brecciated. The goal of this study is to characterize the mineralogy and geochemistry of the intrusion, and to investigate the nature of the melting event that produced it, and the relationship between the silicate magma and the associated carbonate material. We present modal data for samples, which are dominated by phlogopite and serpentinized phenocrysts that appear to have been olivine. In addition, the samples contain large partially altered clinopyroxene phenocrysts, abundant apatite, occasional small garnets, Fe-Ti oxides, carbonate, and a phase that is probably perovskite. We await microprobe data that will allow us to definitively identify some of the phases. In addition, the probe data (compositions of the phlogopite and clinopyroxenes) will be helpful for naming the rock, which seems to fit the description of an aillikite. Major element composition, with approximately 33-34 wt. % SiO₂, 7-8% Al₂O₃, 11-13% FeO_t, 14-17% MgO, 9-14% CaO, 0.5-1.5% Na₂O, 2% K₂O, and 1% P₂O₅ is consistent with an ultramafic lamprophyre. We await trace element data that will further help characterize the magma, the melting event, and the source rocks. In addition, as samples were collected at 1-foot increments across the width of the dike, we plan to use trends in incompatible trace element concentrations to evaluate a possible origin for the carbonate by devolatilization from the silicate magma. Future work will involve possible geothermometry/geobarometry from mineral compositions, and comparison of this intrusion with similar rocks from within the area for potential insights into igneous activity associated with rifting in the New Madrid region.