Abstract

Early Precambrian rocks near Rainy Lake, on the Ontario-Minnesota border, are comprised of a thick section of metasediments and paragneisses (Coutchiching) overlain conformably by metavolcanics (Keewatin) and intruded by Laurentian Igneous rocks. Eight of ten zircon samples from these units lie within experimental error (±30 m.y.) of a 2750-m.y. Concordia chord. We conclude that the zircon ages are primary and not a result of metamorphic resetting. There is thus no evidence for an older sialic source region from which the detrital Coutchiching zircons were derived. It is probable that these detrital zircons were derived locally from contemporaneous volcanics and shallow intrusives within the geosyncline. There is little recorded time difference (<50 m.y.) between the development of this volcanic-sedimentary pile and the intrusion of the Laurentian igneous rocks.

The Rb-Sr isochron for the volcanics gives an age of 2770 m.y. (initial Sr87/Sr86 = 0.7010) which is not statistically different from the Coutchiching isochron age of 2690 m.y. (initial ratio = 0.7002). Both isochrons are in agreement with the zircon intercept age of 2750 m.y. (using λRb87 = 1.39 X 10−11 yr−1). At least some of the paragneisses are shown to be open to strontium migration during regional metamorphism. The Coutchiching isochron age of 2690 m.y. probably represents an early stage of metamorphism during the Algoman orogeny. Samples of one Laurentian intrusive give an isochron age of 2520 m.y., which is distinctly younger than the ages of the Laurentian zircons. This intrusive has been highly sheared and the rocks have probably been reset during the shearing event.

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