Two independent U–Pb zircon ages were measured using norite samples from the North and South Ranges of the Sudbury Nickel Irruptive. Ages of graphic and graphic Ma were obtained with a probability of fit within experimental error of 51 and 57%, respectively.Previous determinations by the Rb–Sr whole-rock method gave ages between about 1700 and 2000 Ma, with errors of the order of ± 100 Ma. The difference in precision is inherent in the two methods: zircons from the norite are highly radiogenic and almost concordant whereas Rb–Sr whole-rock ages are determined by very small increments in radiogenic 87Sr.The two zircon ages have completely different discordia so that all simple models for lead loss by continuous diffusion can be excluded.

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