AbstractThe St Malo region in north‐west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis atc.540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range fromc.550 toc.300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation.40Ar/39Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting.Eleven muscovite concentrates and one whole‐rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record40Ar/39Ar plateau ages ofc.527 and 521 Ma. An internally discordant40Ar/39Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain‐Deux Evailles‐Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north‐western sector of the St Malo region exhibit internally discordant40Ar/39Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south‐eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole‐rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan‐Bobital Granite Complex, yields a40Ar/39Ar plateau age ofc.307 Ma.The40Ar/39Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to und