Elk Creek, Nebraska, carbonatite: strontium geochemistry

Subsurface carbonatite at Elk Creek, Nebraska has been recognized in drill core taken from a depth interval of 630 to at least 950 ft. The core in this interval consists of carbonated breccia and phlogopite-bearing carbonate rock. Total REE, P2O5 and Nb2O5 data are consistent with 'average' values for carbonatite. 87Sr/86Sr ratios from the carbonate fraction range from 0.7030 to 0.7055 for fifteen of eighteen samples (total Sr varies from 300 to 3500 ppm; X=1800 ppm); the remaining three samples have 87Sr/86Sr and total Sr values of 0.7085: 40 ppm; 0.7064: 92 ppm; 0.7067: 252 ppm; these samples may be mixed with sedimentary carbonate and/or contaminated by other non-carbonatite material. The Elk Creek carbonatite is of special interest because of its position with respect to tectonic elements in basement rocks. It occurs in the center of gravity and magnetic anomalies over the approximate axis of the Nemaha anticline and is apparently aligned with the Riley County, Kansas, carbonatite-bearing kimberlites. It is far removed from the E-W-trending '38th parallel' lineament along which occur numerous kimberlites and carbonatites. (Auth.)