Radiometric ages of volcanic rocks on the fort rock dome and in the aquarius mountains, Yavapai and Mohave Counties, Arizona

The Fort Rock dome, in Yavapai County, Ariz., is a roughly circular geologic structure in plan view, 2.5 km in diameter, that is similar in many ways to an impact crater; however, it is a structural dome caused by a potassic mafic intrusion at depth, and the crater-like depression in its center is erosional in origin. The Aquarius Mountains, west of the Fort Rock dome, in Mohave County, contain a felsic volcanic center that erupted tuffs, non-welded ash-flow tuffs, and lahars following dome emplacement.

This report discusses the radiometric ages of samples of rock units from both the Fort Rock dome and the Aquarius Mountains eruptive center. The ages for all samples span a short interval of time from 22.3 to 21.7 m.y. (earliest Miocene). The individual sample ages are consistent with the stratigraphic order of the rock units in the area, and the short age span is consistent with the absence of any significant unconformities in the section.