A body of gneissic granite west of Shawinigan, Quebec, yields a Rb–Sr whole-rock isochron of 1258 ± 24 m.y. with an initial 87Sr/86Sr ratio of 0.7034 ± 0.0012. The St-Didace pluton yields a Rb–Sr whole-rock isochron of 1163 ± 51 m.y. with an initial 87Sr/86Sr ratio of 0.7026 ± 0.0006. These isochrons are thought to reflect the original crystallization of these rock units. From these isochrons and the stratigraphic and crystallization histories of these rocks, it may be concluded that: (1) the Grenville Group in this area was deposited prior to 1258 m.y. ago, (2) the first folding recognizable in this immediate area occurred about 1163 m.y. ago, (3) the second recognized folding probably took place about 1124 m.y. ago, and (4) both these deformations probably represent pulses in a common tectonic event, the Grenville Orogenic Event.