U-Pb zircon ages for late Precambrian igneous rocks in South Wales

U-Pb zircon data are presented for two Precambrian calc-alkaline intrusive bodies associated with large volumes of felsic volcanic rocks in SW Wales. The Johnston complex was probably emplaced 643+5,-28 Ma ago, while the St. Davids granophyre was intruded between the limits 650 and 570 Ma ago, a later Pb loss rendering the precise magmatic age in this case uncertain. The St. Davids granophyre result indicates major Precambrian volcanic/intrusive activity in this area of southern Britain, and the emplacement of the Johnston complex may be part of these events. Initial 87Sr/86Sr ratios for the St. Davids granophyre fall between 0.7037 and 0.7053 and probably suggest an ultimate derivation of the magma from the mantle. The variation of initial ratios could be explained by factors operating during the genesis or ascent of the magma, or by contamination during late- to post-magmatic hydrothermal activity. © 1979 The Geological Society.