Abstract

Deformed and recrystallized, amphibolite facies, trondhjemitic to granodioritic orthogneiss along the north margin of the Archean Lake of the Woods greenstone belt has an Rb–Sr isochron age of 2950 ± 150 Ma and an initial 87Sr/86Sr ratio of 0.7028 ± 0.0014. Preserved primary textures and complex internal intrusive relationships document the original plutonic nature of the orthogneiss. Based on isotopic age and degree of deformation and recrystallization, the orthogneiss is interpreted to be basement to the adjacent metavolcanic sequence of the greenstone belt. The contact between the orthogneiss and greenstone belt is the locus of deformation, but may be an unconformity. It is also the boundary between the English River subprovince on the north and the Wabigoon subprovince on the south.Similar orthogneiss ranging in age from 2800 to 3800 Ma is widespread in the Canadian Shield. It is basement to the 2650–2750 Ma Archean volcanism, at least in some areas, but is not necessarily basement to the older (2800–3000 Ma) volcanism. The orthogneisses are remnants of a once much more extensive sialic terrain, but the contemporaneity and extent of this sialic terrain are uncertain.

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