A PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF OFF-CRATON KIMBERLITES FROM RILEY AND MARSHALL COUNTY, KANSAS (USA)

Kimberlites are silica-poor, potassium-rich ultramafic igneous rocks that are believed to form deep within the Earth (i.e. >200km). They can carry to the surface crust and mantle xenoliths that provide geologists with information on the composition, structural state, and temperature of the deep cratonic lithosphere. The Cretaceous-age kimberlites in Riley and Marshall Counties, Kansas, are unusual in that they erupted through Proterozoic rather than Archean crust. The rocks are porphyritic with phenocrysts of olivine and smaller amounts of pyroxene, magnetite, ilmenite, pyrope garnet and/or phlogopite. Most olivine and pyroxene are altered completely to serpentine or calcite and occur in a fine-grained matrix consisting almost entirely of serpentine and calcite. The minerals that typically survive this extensive alteration include phlogopite, garnet, opaque minerals (tentatively identified as ilmenite) and possibly perovskite. Because of the extensive state of alteration, study of the kimberlites using whole rock geochemistry is problematic. We are therefore undertaking detailed petrographic and mineral chemical analysis of six of the twelve kimberlites within the two counties (Antioch, Bala, Baldwin Creek, Stockdale, Tuttle, and Winkler Crater). Of these six, the Bala and Antioch kimberlites were emplaced as hypabyssal facies, whereas the remaining samples are from the brecciated diatreme or crater facies. The hypabyssal facies rocks exhibit a fine-grained kimberlitic matrix with phenocrysts and few xenoliths. They also contain little to no phlogopite, ilmenite or garnet, based on thin section observations. Brecciated diatreme facies rocks contain abundant country rock fragments, particularly shale and limestone. They also contain greater abundances of phlogopite, garnet and ilmenite, and show less extensive alteration of the kimberlitic matrix. This study will be present new major and trace element mineral chemistry data determined by electron microprobe and LA-ICP-MS on the phlogopite, garnet and ilmenite unaltered phases in order to better characterize the kimberlites located in northeastern Kansas.