On the age of the Donegal Granite

Abstract
Synposis: Argon–40/argon-39 step heating analysis of a muscovite concentrate from the pegmatitic facies of the main Donegal granite indicates an age of 400 ± 2 Ma (IUGS standard constants) for this important intrusive event in British Caledonian history. This suggests that the previously accepted Rb-Sr regression line age of 498 ± 5 Ma ( λ 87 Rb = 1.39 × 10 −11 y −1 ) may be in error.