Ordovician volcanic rocks and associated sub-volcanic intrusions in North Wales give Devonian, metamorphic Rb-Sr whole-rock ages. Isotopic resetting is not confined to acid volcanic rocks. Three intrusions preserve Ordovician ages. The closed system resetting is not directly related to metamorphic grade, but appears to correlate with the extent to which the secondary metamorphic mineral assemblage is developed. Rocks giving Devonian ages have a well developed secondary mineralogy whereas those retaining Ordovician ages are less extensively altered. The metamorphic ages combine to give a Lower Devonian mean age and weighted 2a error of 399 f 9 Ma. Metamorphism is therefore synchronous with Acadian deformation in the North Wales area.

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