Rb-Sr isochron data from whole-rock samples of dolerite and metadolerite indicate emplacement of dikes 2,826 ± 58 m.y. ago, during formation of the Bighorn gneiss and granitic complexes. Whole-rock samples of other dolerite dikes and mineral separates mark another intrusive event at 2,200 ± 35 m.y. B.P., well after the regional metamorphism. Two episodes of mafic dike emplacement are consistent with field relations but are not in agreement with recent K-Ar age determinations.