Mineralogical and Chemical Evolution of Lamproites in Woodson and Wilson Counties, Southeastern Kansas

Abstract

The major and trace element content of 123 lamproites and associated sedimentary rock samples from six cores of the Silver City Dome and two cores from the Rose Dome in southeastern Kansas were analyzed. The lamproites are ultrapotassic (weight percent K₂O/Na₂O = 2.0 to 22.1), alkalic (molecular [K₂O +Na₂O]/Al₂O₃ = 1.01 to 3.04), and are enriched in mantle-incompatible elements (e.g., light REE, Ta, Hf, Ba, Rb, Sr, Th). The low Al₂O₃ content of these samples is consistent with lamproites formed in stable continental settings such as those from West Kimberley, Australia, and Smoky Butte, Montana. Also the lack of a significant negative Ta and Nb anomaly in mantle-normalized plots precludes the source rocks having been involved in subduction.

The samples from the Silver City cores are composed of mostly serpentinized olivine and mica with lesser amounts of K-richterite, diopside, rutile, apatite, perovskite, and spinel in a serpentinized groundmass. The Rose Dome cores contain mostly serpentinized olivine and mica with minor diopside, spinel, perovskite (more abundant than at Rose Dome), xenotime, and feldspar. Most mica compositions follow the trend of decreasing FeO and Al₂O₃ similar to trends of the Guess Core and the Wolgidite trend at West Kimberly. Micas enriched in Ba and Ti (BaO = 16.6–19.3 weight percent; TiO₂ = 26.0– 27.8 weight percent) rim some micas of more normal composition. These very high Ba and Ti-rich micas have not been found in other lamproites or other types of alkalic rocks.

The average composition of the shallow cores at the Silver City Dome are similar to one another (except for Na₂O, Li, and Cs), although the average composition of the shallow cores are different than the previously studied deeper Guess and Ecco Ranch cores in this area. The average composition of the two Eagle cores at Rose Dome are similar to one another, but they are higher in Fe₂O₃ (total), MgO, CaO, P₂O₅, Th, Co, Ni, Cr, U, Y, REE, Cu, and Cs concentrations than the shallow cores from the Silver City Dome.

Metasomatized mantle (lherzolite-harzburgite) containing varied amounts of veins containing clinopyroxene, K-Ti amphibole, phlogopite, apatite, and K-Ba-Zr-Nb titanates could have melted in varied amounts to form the lamproites. The lower SiO₂ and Al₂O₃ and higher MgO lamproites at Rose Dome likely result from melting at a higher temperature of more lherzolite-harzburgite and less vein material than the more SiO₂-rich and MgO-poor lamproites at Silver City.

Also the concentration of most elements varies vertically in the cores, and the variation in the elemental concentration can be related to mineral gravity settling, flow differentiation, or volatile transport processes within the sills. For example, at the Silver City Dome, concentrations of olivine and phlogopite can be related to the samples with the lowest SiO₂ and highest MgO, Mg#, Ni, and Co content. The portions of the cores with the highest concentrations of K₂O, Al₂O₃, and Sc may be correlated to samples with the highest phlogopite. Also Ba often decreases in amount upward in many cores at the Silver City Dome and is enriched in overlying hornfels compared to the unaltered overlying shale. Ba movement in H₂O-rich fluids into the overlying hornfels can explain this observation. Curiously, overlying limestones are not enriched in Ba, so at least locally the limestones may be impervious to the fluids.