A PETROGENETIC RELATIONSHIP BETWEEN ULTRAMAFIC LAMPROPHYRES IN THE NEW MADRID SEISMIC REGION AND SIMILAR INTRUSIONS ASSOCIATED WITH THE ELK CREEK CARBONATITE COMPLEX?

Ultramafic intrusions within the New Madrid seismic region have long been noted, but their origins have not been resolved. Here we compare a dike (Wildcat Hills) cutting through a fresh exposure of the Carbondale Fm. near Eldorado, IL, with similar intrusions at other locations in southern IL (Cottage Grove dike, Williams diatreme, Will Scarlet dike) and KY (Clay Lick dike), as well as from cores drilled from the Elk Creek Carbonatite Complex (ECCC) in Nebraska. Samples were analyzed by XRF, ICPMS, and EMP. The Wildcat Hills dike (~10 m wide) has a contact zone with brecciated country rock containing carbonate ocelli and calcite-filled fractures. The dike interior contains phenocrysts of olivine (5-10%), clinopyroxene (5-10%), phlogopite (10-15%), apatite (1-3%), perovskite (~3%) and Fe-Ti oxides in a calcite-rich groundmass. The mafic phenocrysts are largely serpentinized, though some retain intact cores. The Wildcat Hills dike contains 33-34 wt.% SiO₂, 7-8% Al₂O₃, 17% MgO, 12-13% FeO_t, 9-13% CaO, and 0.8-1.2% P₂O₅. High concentrations of K₂O (2-2.4%) and TiO₂ (3.8-4.0%), and very low Na₂O (0.3-0.6%) are notable. Mg# ranges between 69-72. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of Sr, Zr, and Hf. The other samples from IL and KY share these characteristics, forming a group that closely matches the mela-aillikites of Aillik Bay, Labrador, in mineralogy and composition. In comparison, the ECCC dikes contain 25-27% SiO₂, with slightly lower Al₂O₃, and higher CaO and P₂O₅. Trace element patterns are similar to those of the IL-KY mela-aillikite group, but with higher incompatible element abundances, resulting in a good match to the aillikites of Aillik Bay. The compositions of all of the intrusions in this study are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. For the rocks of Aillik Bay, workers proposed devolatilization of “proto-aillikites” to yield carbonatite and mela-aillikite. Interestingly, a linear relationship between the major-element compositions of ECCC aillikite dikes, ECCC carbonatite, and the mela-aillikites of southern IL and KY suggests the possibility that these rocks from scattered locations along the Mid-Continent Rift may share similar origins.