Glaucophanic amphibole occurs within recrystallized basic volcanic rocks derived from the Gwna Group at Penrhyn Nefyn in the Lleyn Peninsula. The glaucophanic rocks are restricted to the ‘Penmynydd Zone of Metamorphism’ which at Nefyn is interpreted as a narrow blastomylonite belt produced by the shearing of the Gwna Group against a body of tonalite. The blue amphibole coexists with blue-green amphibole and grades into actinolite greenschist away from the focus of shearing. Fragments of Gwna Group (dated as Lower Cambrian) and blastomylonitic ‘Penmynydd’ lithologies lie in the Cambro–Ordovician sediments of the Welsh Basin. Therefore, the metamorphism of parts of the Gwna Group during the ‘Penmynydd’ (subduction zone?) shearing is interpreted as having occurred in Cambrian times.