Recent reconstructions of the geologic evolution of Atlantic Canada based on plate-tectonic models have aroused new interest in the timing of orogenic events on both sides of the Atlantic. K–Ar apparent ages ranging from 332–403 m.y. are reported for 15 whole-rock slate and hornfels samples from the Halifax Formation of the Meguma Group. Four biotite concentrates from spatially associated granitic intrusions yield K–Ar ages of 364–369 m.y. A minimum age of about 390 m.y. for the regional metamorphic event is deduced from the whole-rock slate data. Granitic intrusion at approximately 367 m.y. profoundly affected the argon clock in many Halifax slates and resulted in its partial resetting even in slates which do not display mineralogic or textural effects of superimposed thermal metamorphism. The 390 m.y. minimum age for regional metamorphism supports suggestions by other workers that the currently widely-accepted 395 m.y. date for the Devonian–Silurian boundary may need revision.