The results of the 40Ar/39Ar step heating study of a folded and cleaved metasiltstone from Malmanare Point, Gerrans Bay, S Cornwall are presented. The rock comes from the mélange of the Roseland area, NE of the Lizard. The age spectrum obtained has two distinct sectors: (i) an older age component giving apparent ages of 521–490 Ma and (ii) a younger component with an apparent age of 424–410 Ma. The petrography suggests that the former is derived from the more resistant parts of large detrital muscovite flakes, while the latter comes from smaller metamorphic micas aligned along the prominent cleavage and also, in part, from original detrital grains partially or totally overprinted during the metamorphic event. The sediment was probably mid- to late-Ordovician in age. The source rocks appear to have been very largely of late Cambrian/early Arenig age. Folding, cleavage and low-grade regional metamorphism occurred as a result of late Caledonian orogenesis between 424–410 Ma. There is no evidence of any Variscan overprint on the age spectrum. Very minor, low energy argon losses appear to have taken place in mid-Mesozoic times (possibly related to regional mineralization) but, apart from this, the absence of any major overprint younger than 410 Ma indicates that since the end of the Silurian the rock has suffered only high-level, cold, non-penetrative deformation.