New whole rock Rb–Sr isochron data indicate that the Archean greenstones of the Yellowknife area are 2625 ± 160 m.y. old (λ87Rb = 1.39 × 10−11 yr−1). The thick, complexly folded sequence of volcanics and overlying sediments is intruded by two syntectonic to late tectonic batholiths (Rb–Sr isochron ages 2610 ± 58 m.y. and 2590 ± 90 m.y.) and a large irregular mass of muscovite granite (Prosperous Lake granite), which yields a mineral isochron age of 2575 ± 25 m.y.Low initial 87Sr/86Sr ratios (0.702–0.703) suggest that the late tectonic batholiths were unlikely to have formed by granitization of a great thickness of sediments beneath the greenstones (as has been suggested). Since the initial ratios of the volcanics and intrusive rocks are similar, the basalt–dacite association is probably the early extrusive expression of the subcrustal process that leads to the formation of quartz diorite–granodiorite batholiths. Appreciable sialic contamination need not be invoked. On the other hand, the Prosperous Lake granite ((87Sr/86Sr)0 = 0.712) may be the product of partial anatexis of the newly derived sialic crustal segment.

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