Abstract

U-Pb isotopic systematics of detrital zircons incorporated in a middle Archean quartzite from the Beartooth Mountains, Montana, were investigated with the SHRIMP ion microprobe. These new data reveal an extended and previously unrecognized record of crustal evolution for the northern Wyoming province. Seventy-eight analyses of 67 grains yielded a range of 207Pb/206Pb ages from 2.69 to 3.96 Ga. Concordant analyses from 43 separate grains define a maximum age for the deposition of the quartzite of 3.30 Ga; other provenance ages extend to 3.96 Ga. Ages of <3.30 Ga are generally discordant, and appear to reflect late Archean disturbance of the U-Pb system, including metamorphism at ∼2.8 Ga. The predominance off ages at ∼3.3 Ga is interpreted to represent the last major episode of crust formation prior to deposition of the quartzite. The concordant analyses of >3.30 Ga indicate that older crustal components with ages up to 3.96 Ga, or detritus from them, were also in the provenance of this quartzite. This older age is equivalent to that of the oldest known rock from the Acasta gneisses of the Slave province and is exceeded only by the >4.0 Ga age of detrital zircons off the Yilgarn block of Western Australia. These data support an increased probability for the survival of sialic crust created before the cessation of the late bombardment at 3.8 to 3.9 Ga.

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