Mineralogy and petrology of cretaceous subsurface lamproite sills, southeastern Kansas, USA

Cores and cuttings of lamproite sills and host sedimentary country rocks in southeastern Kansas from up to 312 m depth were analyzed for major elements in whole rocks and minerals, certain trace elements in whole rocks (including the REE) and Sr isotopic composition of the whole rocks. The lamproites are ultrapotassic (K₂O/Na₂O = 2.0–19.9), alkalic [molecular (K₂O/Na₂O)/Al₂O₃ = 1.3-2.8], enriched in mantle-incompatible elements (light REE, Ba, Rb, Sr, Th, Hf, Ta) and have nearly homogeneous initial Sr isotopic compositions (0.707764-0.708114).

These lamproites could have formed by variable degrees of partial melting of harzburgite country rock and cross-cutting veins composed of phlogopite, K-Ti richterite, titanite, diopside, K-Ti silicates, or K-Ba-phosphate under high H₂O/CO₂ ratios and reducing conditions. Variability in melting of veins and wall rock and variable composition of the metasomatized veins could explain the significantly different composition of the Kansas lamproites.

Least squares fractionation models preclude the derivation of the Kansas lamproites by fractional crystallization from magmas similar in composition to higher silica phlogopite-sanidine lamproites some believe to be primary lamproite melts found elsewhere. In all but one case, least squares fractionation models also preclude the derivation of magmas similar in composition to any of the Kansas lamproites from one another. A magma similar in composition to the average composition of the higher SiO₂ Ecco Ranch lamproite (237.5–247.5 m depth) could, however, have marginally crystallized about 12% richterite, 12% sanidine, 7% diopside and 6% phlogopite to produce the average composition of the Guess lamproite (305–312 m depth).

Lamproite from the Ecco Ranch core is internally fractionated in K₂O, Al₂O₃, Ba, MgO, Fe₂O₃, Co and Cr most likely by crystal accumulation-removal of ferromagnesian minerals and sanidine. In contrast, the Guess core (305–312 m depth) has little fractionation throughout most of the sill except in several narrow zones. Lamproite in the Guess core has large enrichments in TiO₂, Ba, REE, Th, Ta and Sc and depletions in MgO, Cr, Co and Rb possibly concentrated in these narrow zones during the last dregs of crystallization of this magma.

The Ecco Ranch sill did not show any evidence of loss of volatiles or soluble elements into the country rock. This contrasts to the previously studied, shallow Silver City lamproite which did apparently lose H₂O-rich fluid to the country rock. Perhaps a greater confining pressure and lesser amount of H₂O-rich fluid prevented it from escaping.