An Rb–Sr whole-rock isochron age of 465 ± 10 Ma (95% confidence level) has been obtained for slates from the Ingleton Group sampled in the Ingleton area. This result is thought to be close to the age of metamorphism, which reached high anchizone grade, though it is impossible to rule out a diagenetic interpretation. The co-linearity of the data points implies a high degree of isotopic homogenization of Sr isotopes within the sample set caused by migration of aqueous fluids expelled from the clay minerals during either the smectite-illite transition or low-grade metamorphism. A previously published isochron age of 494 ± 18 Ma was derived from samples collected in the Horton area. They are coarser-grained and less metamorphosed (low anchizone to high diagenetic grade) than the Ingleton samples, and their older age may reflect their less complete isotopic homogenization. The new Rb–Sr age is consistent with a possible Arenig age of deposition. If interpreted as a metamorphic age, it implies that the Ingleton Group was metamorphosed much earlier than the Devonian (Acadian) metamorphism of the overlying Windermere Supergroup. Whether of metamorphic or diagenetic origin, the survival of the Rb–Sr isochron through the Acadian metamorphism implies that the latter event did not cause significant movement of aqueous fluids through the fine-grained components that were analysed.