Abstract

Stratified sequences of Precambrian rocks from twelve localities in Arizona and Sonora yield geologic and isotopic evidence of multiple periods of rock accumulation and orogeny. Isotopic dating indicates that these rocks were formed from 1800 m.y. to 950 m.y. ago. The oldest rocks are metamorphosed sequences of volcanic and sedimentary origin. They are separated in time from a younger sequence of metamorphosed volcanic and sedimentary rocks by batholithic plutonism dated at approximately 1700 m.y. The second period of volcanism and sedimentation was terminated by batholithic plutonism about 1400 m.y. ago.Subsequent to the 1400 m.y. event, the "Younger Precambrian" rocks of Arizona, the Apache Group and probably the Grand Canyon Series, were deposited. These dominantly sedimentary rocks are characteristically only slightly and locally metamorphosed and deformed. Their age is greater than the 1150 m.y. old diabase sills and dikes that intrude them. Some areas that are not underlain by rocks of the Apache Group were metamorphosed and possibly intruded by silicic plutons about 1000 m.y. ago.From these data it appears that the stratified Precambrian rocks of Arizona can be assigned to three periods of accumulation. The first period occurred prior to about 1700 m.y., and the second occurred between 1700 and 1400 m.y. The third period, the deposition of the Apache Group, occurred between 1400 and 1150 m.y. ago.

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