Hornblende from a diorite stock within the Kellys Mountain plutonic complex within the Avalon composite terrane, and hornblende, muscovite, and biotite from amphibolite and gneiss within the contact aureole record similar 40Ar/39Ar plateau ages of 493–498 Ma. These data indicate relatively rapid cooling through the contrasting temperatures appropriate for intracrystalline retention of argon in the different minerals. This is consistent with the relatively high level of intrusion of the pluton (3–10 km) suggested previously from the study of mineral assemblages developed in the contact aureole. These relationships suggest that intrusion of the Kellys Mountain complex took place at approximately 500 Ma, indicating an age close to the Cambro-Ordovician boundary. The complex may have formed in the same within-plate rifting regime as the Middle Cambrian Bourinot Group volcanic sequences exposed within the Avalon composite terrane.