Age of emplacement of Riley County, Kansas, kimberlites and a possible minimum age for the Dakota Sandstone

Field evidence suggests that the kimberlites of Riley County, Kansas, were emplaced into Lower Permian rocks in post-Dakota Sandstone time. The Dakota Sandstone in Kansas is thought to be earliest Late Cretaceous in age; thus the maximum age of emplacement of the kimberlites is approximately 100 ± 20 m.y. K-Ar dates on chloritized biotite and phlogopites from the kimberlites range from 112 ± 6 m.y. to 380 ± 40 m.y.; the dates earlier than 100 m.y. (6 of 7 samples) are attributed to either the xenocrystalline nature of the chlorites and/or excess Ar and low temperature of intrusion. Fission track dates from two apatites from granite xenoliths are 115 ± 12 m.y. and 123 ± 12 m.y.; these dates reflect cooling at about 120 m.y. ago, which may place a maximum age of emplacement on the kimberlites.