Zircon geochronology of Precambrian rocks in southeastern Wyoming and northern Colorado

Archean gneisses and Early Proterozoic metasedimentary rocks of the Wyoming Province are separated from Proterozoic eugeoclinal metamorphic rocks by a major east-west–trending shear zone called the Cheyenne belt. U-Pb zircon ages of Archean tonalites north of the Cheyenne belt denote an intrusive event at 2,700 Ma. Detrital zircons from Proterozoic metasedimentary rocks north of the Cheyenne belt define an apparent age of 2,450 Ma for the source rock, similar to an age of 2,430 Ma obtained for a local granite. A metagabbro plug, which intruded the metasedimentary rocks about 2,100 Ma, constrains their deposition within this 350 m.y. period. Ages for key units just south of the Cheyenne belt in Wyoming delineate at least three magmatic events at 1,780; 1,750; and 1,625 Ma. Ages for large plutons in the northern Colorado area define pulses of granodioritic to granitic intrusions at approximately 1,720 and 1,670 Ma.

A U-Pb zircon age of 1,792 ± 15 Ma for a Proterozoic metavolcanic rock in the Sierra Madre is greater than ages reported for other Proterozoic metavolcanic rocks in the U.S. Rockies. However, ages for Proterozoic plutons in southeastern Wyoming are similar to other ages for plutonism and volcanism for rocks exposed in the central Colorado Rockies and are coeval with suturing of Proterozoic crust with the Archean Wyoming Province along the Cheyenne belt. Although at present the accretionary history for these Early Proterozoic rocks is not well understood, it is evident that there exists a progressive decrease in age for volcanism as well as plutonism from north to south.