EARLY ARCHEAN CRUST IN THE NORTHERN WYOMING PROVINCE - EVIDENCE FROM U-PB AGES OF DETRITAL ZIRCONS

U-Pb ages of individual detrital and metamorphic zircons from 12 Arche an metasedimentary rocks, including quartzites, from the Beartooth, Ru by, and Tobacco Root uplifts of the northern Wyoming province indicate that they were deposited between 2.7 and 3.2 Ga. Younger, metamorphic zircons are found as overgrowths and new grains in some samples, and yield ages between 2.7 and 1.9 Ga. They are, however, much less abunda nt than detrital grains, which constitute >75% of the 355 grains analy zed. The majority of the detrital grains have ages between 3.2 and 3.4 Ga; none are younger than 2.9 Ga. Grains with Pb-207/Pb-206 ages betw een 3.4 and 4.0 Ga constituted approximately 15% of all grains with an alyses within 10% of concordia, but are concentrated in samples from t he eastern Beartooth Mountains. Comparison of the average of the Pb-Pb ages of individual zircons within 10% of concordia with previously pu blished Lu-Hf chondritic model ages for some individual samples sugges ts that the age distribution recorded by the U-Pb system in these zirc ons has not been significantly disturbed by pre- or post-depositional Pb-loss. Collectively, these data suggest that the individual metasedi mentary rocks did not completely share a common provenance and that a major crust-forming cycle occurred 3.2 to 3.4 Ga. In conjunction with previously published U-Th-Pb whole-rock data, these results suggest th at rocks with a relatively high proportion of > 3.4 Ga grains may have had crust of comparable age in their provenance. (C) 1998 Elsevier Sc ience B.V.