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