Geochemistry and Sr-Nd isotopic compositions of Permian ultramafic lamprophyres in the Reelfoot Rift – Rough Creek Graben, southern Illinois and northwestern Kentucky

Abstract

Permian dikes, sills, and diatremes in southern Illinois and northwestern Kentucky (the Omaha, Wildcat Hills, Cottage Grove, Will Scarlet, Williams, Grant, and Clay Lick intrusions) share similar geochemistry and are classified as ultramafic lamprophyres. Major element compositions are 30–35 wt% SiO2, 6–7% Al2O3, 12–14% FeOt, 16–19% MgO, 3–5% TiO2, 11–16% CaO, 0.1–0.7% Na2O, 1.2–2.7% K2O, and 0.4–1.3% P2O5. The Grant Intrusive Breccia is an exception, with lower SiO2, Al2O3, FeOt, MgO, TiO2, and higher CaO. Typically, these rocks are fine grained, with phlogopite, serpentinized olivine (~Fo88), diopside, perovskite, Fe-Ti-spinel, apatite, and calcite. Blocky and lath-shaped pseudomorphs in some samples probably represent melilite, which would make the rocks alnöites. The Grant and Williams diatremes contain sedimentary and igneous clasts (including amphibole megacrysts) within a carbonate-rich matrix. The Grant exhibits pelletal lapilli and is characterized as a lamprophyre‑carbonatite tuffisite. 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 Intrusive exhibits even greater REE enrichment and notable peaks at Nb, La, and Ce. Geochemical characteristics, including distributions of 143Nd/144Nd and 87Sr/86Sr, are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. Derivation of the lamprophyres from carbonate-rich parental melts similar to the Grant Intrusive could be achieved by separation of carbonatite. A narrow range of initial 87Sr/86Sr (0.70301–0.70449), and initial εNd (3.7–5.1), suggests a uniform mantle source close to Bulk Earth. T-depleted mantle model ages range from 540 to 625 Ma, and might correlate with timing of enrichment of a lithospheric mantle source during the breakup of Rodinia.