The late Precambrian Cadomian Orogen exposed in the North Armorican Massif (northwest France) is a collage of displaced terranes that, in part, developed during amalgamation of continental-arc and marginal-basin complexes. 40Ar/39Ar mineral ages reported here place new constraints on the timing of Cadomian tectonothermal activity in the southern part of the St Brieuc terrane. In the baie de Saint-Brieuc area Brioverian supracrustal units were deformed, metamorphosed, and intruded by calc-alkaline plutonic complexes. Metamorphic hornblende from a metabasic amphibolite sheet within Brioverian rocks records an isotope correlation age of 568.4 ± 2.6 Ma (interpreted to date postmetamorphic cooling through appropriate argon closure temperatures). Similar isotope correlation cooling ages are recorded by metamorphic hornblende within both an amphibolite sheet intrusive into the Penthièvre complex (567.5 ± 1.2 Ma) and the La Croix Gibat amphibolite (574.8 ± 2.1 Ma). Igneous hornblende from the late tectonic to posttectonic St Quay quartz diorite and muscovite from Brioverian metasedimentary rocks in the contact aureole record isotope correlation ages of ca. 565–570 Ma. These and a ca. 568 Ma isotope correlation age determined for hornblende from the foliated Fort de la Latte quartz diorite are interpreted to date postmagmatic cooling.The 40Ar/39Ar ages indicate that Cadomian tectonothermal activity within southern parts of the St Brieuc terrane occurred prior to ca. 570 Ma. This is markedly older than the ca. 540 Ma date previously suggested for peak Cadomian metamorphism and granite emplacement in the adjacent St Malo terrane and is consistent with palinspastic separation of the contrasting Cadomian elements until at least the latest Precambrian. A pre-570 Ma age for Cadomian tectonothermal activity in the St Brieuc terrane suggests correlation with similar-aged orogenic activity in other circum-Atlantic, late Precambrian Gondwanan marginal terranes (including southern portions of the Iberian massif and various sectors of the West African orogens).