Petrogenesis of the Eocene Rattlesnake Hills Alkaline Magmatic Complex, Wyoming, USA: A Carbonatite-Bearing Igneous Center in the Southern Great Plains Alkalic Province

Abstract Alkaline silicate–carbonatite complexes in post-collisional settings may host a variety of ore deposits. We characterize the post-collisional, Paleogene Rattlesnake Hills alkaline magmatic complex, part of the Great Plains Alkalic Province, which was emplaced as a set of 46 small igneous centers within the central Archean Wyoming Province. It is composed of two magma series: a Central Alkalic Group of nepheline-normative alkaline rocks and sparse carbonatite dikes, and Eastern and Western Felsic Groups of quartz-normative subalkaline rocks. Nd and Sr isotopic data suggest that the parental magmas originated in the asthenospheric mantle, possibly with minor amounts of partial melt from the Archean subcratonic lithospheric mantle and crust. Only the least differentiated magmas and clinopyroxene-rich cognate inclusions contain calcite phenocrysts, suggesting that extraction of carbonatite melt from silicate magma may have occurred early. Complex zoning in clinopyroxene and amphibole suggests magma recharge early during differentiation. Sodalite group phenocrysts are present only in phonolite, relatively late in magmatic evolution. Petrographic, textural, geochemical, and isotopic evidence suggests that the Eastern and Western Felsic Group rocks formed by assimilation of Archean granitic gneiss into nepheline-normative alkaline magmas, followed by fractional crystallization. The Rattlesnake Hills alkaline complex shares many petrographic, geochemical, and isotopic characteristics with the Black Hills alkaline complex on the northeastern edge of the Archean Wyoming craton, which hosts economic rare earth element (REE) deposits. However, the Rattlesnake Hills alkaline complex silicate rocks and carbonatites are less enriched in light rare earth elements (LREE) than those in the Black Hills alkaline complex. Fractionation of clinopyroxene, titanite, and apatite phenocrysts may have more effectively sequestered REE in the Rattlesnake Hills alkaline complex compared to the Black Hills alkaline complex.