A Rb-Sr isochron age of 1635 ± 24 m.y. was obtained for whole-rock samples from the Gunflint Iron Formation and the Rove Formation in Ontario, using a value of 1.39 × 10−11 yrs−1 for the decay constant of Rb87. This date may be interpreted as being approximately the time of deposition and diagenesis of the Animikie Series in the Thunder Bay district or as the time of low-grade metamorphism. A number of arguments are presented in support of the former alternative interpretation.

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