Radiometric dating of intrusive rocks in the Cottonwood area, Utah

Recently completed fission-track and K-Ar dating of zircon, sphene, apatite, muscovite, biotite, and hornblende indicate that the Clayton Peak stock was intruded 37-41 m.y. ago, the Alta stock about 32-33 m.y. ago, and the Little Cottonwood stock between 24 and 31 m.y. ago. Pb-a ages on zircon, though showing the same sequence, are about twice as great and are inferred to represent a partial
mixing of inherited Precambrian zircons. Acceptance of a middle Tertiary age for the Little Cottonwood stock requires revision of earlier
interpretations that it could be as old as Late Cretaceous. K-Ar dates also indicate that the Keetley Volcanics are 32 to 35 m.y. old, and a
single date of 37.3 m.y. was obtained on the Traverse Volcanics, suggesting that they are more closely related to volcanic activity in the
adjoining Oquirrh Mountains.