NEW DATA ON KIMBERLITES AND LAMPROITES IN EASTERN KANSAS, U.S.A.

A cluster of thirteen kimberlites occurs along the trace of the 1.10 Ga NNE-trending Midcontinent Rift System in Riley and Marshall counties in northeastern Kansas (Fig. 1). Three of these were discovered and drilled within the past three years utilizing donated aeromagnetic coverage of the area followed by detailed groundmagnetic surveys. Modeling of the magnetic anomalies shows that final emplacement of the kimberlite bodies is controlled by N40 o W trending structures, rather than NNE-trending structures and that the contacts between the sides of the kimberlite bodies and the Paleozoic country rock are steeply dipping. The kimberlites are Late Cretaceous (about 90 my) and range from crater and diatreme facies in Riley County to hypabyssal facies in Marshall County. It is likely that as a result of Cretaceous tectonic activity, strike-slip movement was initiated along NW-SE-trending faults, thus opening pathways for the kimberlites to rise to the surface. Continuous core taken to a depth of 300 ft (91 m) was obtained from each of the three newly discovered kimberlites: Tuttle, Antioch, and Baldwin Creek. Thin-section examination of samples from the Tuttle and Antioch kimberlites show two textural types. The most usual type displays pseudo-detrital (pseudo-conglomeratic), inequigranular texture and contains abundant kimberlite xenoliths, various phenocrysts as well as country rock xenoliths. Kimberlite xenoliths are pelletal lapilli that represent magma droplets typically composed of a thin selvage of kimberlite material surrounding a