GEOCHEMISTRY AND SR-ND ISOTOPIC COMPOSITIONS OF PERMIAN ULTRAMAFIC LAMPROPHYRES IN SOUTHERN ILLINOIS

Ultramafic igneous intrusions in southern Illinois, exposed in coal mines, drill cores, and some surface locations, remain poorly understood. The Omaha Dome, Wildcat Hills, Cottage Grove, Will Scarlet, Williams, and Grant intrusions share similar geochemistry and can be classified as ultramafic lamprophyres. Major element compositions are typically 30-34 wt.% SiO₂, 5-7% Al₂O₃, 12-14% FeO_t, 16-19% MgO, 4-5% TiO₂, 12-16% CaO, 0.1-0.7% Na₂O, 1.2-2.6% K₂O, and 0.4-1.3% P₂O₅. The Grant intrusion is an exception, with lower SiO₂, Al₂O₃, FeO_t, MgO, TiO₂, and higher CaO, and may represent a mixture of lamprophyre and carbonatite. Typically these rocks are fine grained, with phlogopite (Al- and Ti-rich with core to rim trends to lower Al₂O₃ and TiO₂), variably serpentinized olivine (~Fo88), diopside, perovskite, magnetite, apatite, and calcite. Small, strongly zoned garnets (andradite) seem to be associated with locations of secondary mineralization. The Omaha Dome intrusion is coarser grained with cumulate textures. The Grant intrusion differs in that it contains hornblende. These rocks have been called alnöites, but melilite has not been conclusively identified. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of K, Sr, Zr, and Hf, closely matching those of the mela-aillikites of Aillik Bay, Labrador. The Grant intrusion exhibits even greater REE enrichment and notable peaks at Nb, La, and Ce. The geochemical characteristics of the southern Illinois intrusions are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. A Rb-Sr isochron age of ~230 Ma is in reasonable agreement with published ages of ~270 Ma, and highlights their coeval nature. These rocks exhibit a narrow range of initial ⁸⁷Sr/⁸⁶Sr (0.70301-0.70359), and initial ε_Nd (3.7-5.1), suggesting a uniform mantle source close to Bulk Earth. There is no trend toward high ⁸⁷Sr/⁸⁶Sr to indicate crustal contamination. T-depleted mantle model ages range from 540-625 Ma, and might correlate with timing of enrichment of a lithospheric mantle source during the breakup of Rodinia. Future work will involve additional EMP analysis of minerals to gain insights into the origins of these intrusions and their relationship to the New Madrid Seismic Zone.