The Garn Fadryn sub-volcanic intrusion of N Wales gives a U–Pb zircon age of 4533 ± 2.2 Ma. This is the first reliable emplacement age from the Welsh Caradoc volcanic province proving that the Rb–Sr whole rock age of 392 ± 32 Ma does not date intrusion but reflects resetting during early Devonian low-grade metamorphism. Resetting in this setting is chemically controlled and is a function of disturbance during water rock interaction; it is not analogous to the predominantly thermally controlled resetting of K–Ar systems. Post-volcanism Rb–Sr whole rock secondary ages in this region have a mean age of 399 ± 9 Ma and record a specific ‘event’ which is the time of hydration. It is proposed that this event was triggered by uplift which opened fractures, increased permeability, facilitated formation of secondary hydrated minerals and reset the Rb-Sr system. This model for resetting should have widespread applicability to other volcanic provinces affected by low-grade metamorphism.