Abstract Six K-Ar ages on phlogopite from minettes in and near the Buell Park diatreme establish that minette and coeval kimberlitic volcanism occurred 25 m.y. ago. Two altered phlogopites yield younger ages, while on phlogopite sample with an age of 34 m.y. may contain excess argon. Published apatite fission-track ages for inclusions in the kimberlitic tuff at Buell Park are too old, perhaps due to partial retention of pre-eruption tracks. Published ages for other minette localities on the Colorado Plateau are insufficient to precisely define the duration of volcanism, but some eruptions may be as old as 30 m.y. Potassic volcanic rocks similar to the felsic minettes at Buell Park erupted at about the same time at Chino Valley, Arizona, in the transition zone of the Colorado Plateau. The correlation between the two localities of unusual potassic rocks is evidence that these rocks may reflect mantle conditions associated with the stability of the Plateau, and that these conditions contrasted with those in adjacent tectonic provinces at least as early as 25 m.y. ago.