The Significance of K-Ar Dates on Altered Kimberlitic Phlogopite from Riley County, Kansas

Five known kimberlites intrude Lower Permian sedimentary rocks in Riley County, Kansas. Structural studies show that these kimberlites were emplaced in Late Cretaceous (post-Dakota sandstone) time. Four K-Ar age determinations on altered phlogopite from two of the kimberlites vary between $$204 \pm 20$$ m.y. and $$380 \pm 40$$ m.y. The phlogopite, therefore, crystallized much earlier than the time of emplacement of the kimberlite pipes and, in a sense, may be regarded as xenocrystic. Both the $$*Ar^{40}$$ and total K content are low due to the altered nature of the phlogopite. The pre-emplacement ages are consistent with a low temperature of injection, possibly as low as 100°-200° C, which explains the lack of pyrometamorphic contact effects in the Permian country rocks.