K/Ar chronology of the Tucson Mountains, Pima County, Arizona

Abstract

K-Ar age determinations on rocks in the Tucson Mountains indicate two main periods of volcanic activity. The Cat Mountain Rhyolite, Biotite Rhyolite, and Shorts Ranch Andesite, which comprise and older group, range in age from 57 m.y. to 70 m.y. and are Late Cretaceous to early Tertiary Laramide. K-Ar ages of 71 m.y. to 75 m.y. for the intrusive rocks near Amole Peak are close to that of the Cat Mountain Rhyolite. Rocks in the younger group range in age from 20 m.y. to 30 m.y. and are late Oligocene to Miocene. Similar isotopic ages of the mid-Tertiary rocks near Safford Peak and the Tumamoc Hill-'A' Mountain area suggest a closer relationship between these two sequences than previously thought.

15 samples from this reference