HYDRATION DURING UPLIFT IS RECORDED BY RESET RB-SR WHOLE-ROCK AGES

The Garn Fadryn sub-volcanic intrusion of N Wales gives a U-Pb zircon age of 453.3 +/- 2.2 Ma. This is the first reliable emplacement age fr om the Welsh Caradoc volcanic province proving that the Rb-Sr whole ro ck age of 392 +/- 32 Ma does not date intrusion but reflects resetting during early Devonian low-grade metamorphism. Resetting in this setti ng is chemically controlled and is a function of disturbance during wa ter rock interaction; it is not analogous to the predominantly thermal ly controlled resetting of K-Ar systems. Post-volcanism Rb-Sr whole ro ck secondary ages in this region have a mean age of 399 +/- 9 Ma and r ecord a specific ''event'' which is the time of hydration. It is propo sed that this event was triggered by uplift which opened fractures, in creased permeability, facilitated formation of secondary hydrated mine rals and reset the Rb-Sr system. This model for resetting should have widespread applicability to other volcanic provinces affected by low-g rade metamorphism.