Field evidence shows that the Huronian section of metasediments in Ontario is younger than the Archean and older than the Nipissing diabase. Previous radiometric investigations show the age of the latest Archean granitic intrusions to be about 2500 m.y. and the Nipissing diabase to be 2155 ± 80 m.y. In this paper we present whole-rock Rb–Sr data which (1) confirm the age of the diabase in a single sill at Gowganda (2162 ± 27 m.y.), and (2) show the age of the Gowganda Formation (which the sill intruded) to be 2288 ± 87 m.y. In marked contrast metasediments and metavolcanics in the Huronian section southwest of Sudbury show younger ages ranging between 1800 and 2200 m.y. This is interpreted in terms of known orogenic disturbance of the Huronian rocks south and west of Sudbury and its absence in the Cobalt–Gowganda area to the northeast. Both minerals and whole rocks have responded by showing low and discordant ages in the disturbed area. The higher and accordant ages in the undisturbed environment agree with the geologic succession and are believed to be the true ages of these rocks.

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