Abstract

Suites of samples from the Dore Lake Complex, the Chibougamau Pluton, and the volcanic Roy Group were analyzed for an age determination by the Rb–Sr whole-rock isochron method. The rocks of the Dore Lake Complex became a closed system with respect to Rb and Sr 2573 ± 234 m.y. ago (λ87Rb = 1.47 × 10−11 yr−1) with an initial 87Sr/86Sr ratio of 0.7009 ± 0.0005. The Rb–Sr age of the Chibougamau Pluton is 2424 ± 79 m.y. and (87Sr/86Sr)0 = 0.7005 ± 0.0002. Mineral separates of both the Dore Lake Complex and the Chibougamau Pluton do not lie on the respective isochrons and do not form isochrons of their own. The samples from the Roy Group form a poorly-defined isochron of 2206 ± 85 m.y. with (87Sr/86Sr)0 = 0.7018 ± 0.0003. The Chibougamau Greenstone Belt has apparently been affected by the Kenoran orogeny, after which the rocks seemingly have remained closed with respect to rubidium and strontium. The initial 87Sr/86Sr ratios of these units probably represent the isotopic composition of strontium in the upper mantle about 2.5 b.y. ago.

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