Petrologic relationship between lamprophyres, carbonatites, and heavy rare-earth element enriched breccias at Hicks Dome

Abstract

New petrological, geochemical, and P–T modelling results from igneous samples clarify how carbonatite-lamprophyre magmatism, fluorite and rare earth element (REE) enrichment are petrogenetically related in southern Illinois. P–T modelling reveals that igneous rocks derive from a deep mantle carbonated source, that is consistent with trace element signatures for a fluorine-rich transition zone origin. Major element systematics suggests liquid-immiscibility with lamprophyric melts as the origin for Ca-carbonatites. Heavy REE (HREE) enrichments in Hicks Dome breccias likely formed through preferential partitioning and transport of HREE by brine-melts, exsolved from a deep carbonatite body. Brine-melts redistributed HREEs throughout the system along brecciated pathways where they reprecipitated as HREE-rich phosphate/fluorcarbonate minerals (e.g. xenotime, florencite, synchesite) in host bedrock. The diversity of igneous rocks in southern Illinois highlights the area as an excellent natural laboratory to study carbonated melt petrogenesis and evolution.