Abstract

The Vernal Mesa-type quartz monzonite is an igneous body of batholithic proportions of Precambrian age in western Colorado and eastern Utah. A Rb-Sr age of 1,480 ± 40 m.y. has been previously reported; other Rb-Sr data reported suggest that the rock might be as much as 200 m.y. older than this. The U-Pb age of the suite of zircons is 1,443 ± 22 m.y., confirming that the Vernal Mesa was formed during the igneous activity which was widespread in the southwestern and central United States 1,450 to 1,500 m.y. ago.

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