MINERALOGY AND PETROLOGY OF CRETACEOUS SUBSURFACE LAMPROITE SILLS, SOUTHEASTERN KANSAS, USA

Cores and cuttings of lamproite sills and host sedimentary country roc ks in southeastern Kansas from up to 312 m depth were analyzed for maj or elements in whole rocks and minerals, certain trace elements in who le 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-incompatib le elements (light REE, Ba, Rb, Sr, Th, Hf, Ta) and have nearly homoge neous initial Sr isotopic compositions (0.707764-0.708114). These lamp roites could have formed by variable degrees of partial melting of har zburgite 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 mel ting of veins and wall rock and variable composition of the metasomati zed veins could explain the significantly different composition of the Kansas lamproites. Least squares fractionation models preclude the de rivation of the Kansas lamproites by fractional crystallization from m agmas similar in composition to higher silica phlogopite-sanidine lamp roites 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 lamp roites from one another. A magma similar in composition to the average composition of the higher SiO2 Ecco Ranch lamproite (237.5-247.5 m de pth) could, however, have marginally crystallized about 12% richterite , 12% sanidine, 7% diopside and 6% phlogopite to produce the average c omposition of the Guess lamproite (305-312 m depth). Lamproite from th e Ecco Ranch core is internally fractionated in K2O, Al2O3, Ba, MgO, F e2O3, Co and Cr most likely by crystal accumulation-removal of ferroma gnesian 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 enrichment s 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 ev idence of loss of volatiles or soluble elements into the country rock. This contrasts to the previously studied, shallow Silver City lamproi te which did apparently lose H2O-rich fluid to the country rock. Perha ps a greater confining pressure and lesser amount of H2O-rich fluid pr evented it from escaping.