New U–Pb isotope data from the Precambrian (Sarn Igneous Complex) of North Wales establishes that calc-alkaline plutonic rocks of the same age occur on either side of the Menai Strait fault system. Zircon fractions from the Sarn Igneous Complex gabbro give a near concordant U–Pb age of 615 ± 2 Ma which is interpreted as the first accurate age of igneous crystallization and supersedes previous dating attempts using the Rb–Sr method. The new U–Pb age is the same, within error, as recent zircon dates obtained from the Arfon Group ignimbrites along strike from the Sarn Igneous Complex, and from the Coedana Complex granite on Anglesey. These data emphasize the importance of what appears to have been the major phase of calc-alkaline magmatism at 630–600 Ma recognizable along the Avalonian arc in Britain, Newfoundland and elsewhere. Despite the similar ages and tectonic setting of the Coedana and Sarn plutonic rocks, however, they remain petrogenetically and geochemically distinct and are not directly linked as part of the same igneous complex. Instead the Coedana Complex granite is interpreted as having been generated in the Avalonian arc during a widespread magmatic event that is also recorded in Britain not only by the Sarn Igneous Complex and Arfon Group, but by volcanic and plutonic rocks in central England. Investigation of granite clasts in the Gwna Group melange (Monian Supergroup) on Anglesey failed to produce zircons adequate for dating purposes but revealed Rb–Sr. whole-rock geochemical and petrographic characteristics similar to the nearby Coedana Complex and suggest linkage between fault-bounded 'Monian' terranes on Anglesey. In the light of these new data rocks on either side of the Menai Strait terrane boundary are interpreted as belonging to the same arc system that was dismembered, dispersed and laterally duplicated by transcurrent faulting after late Precambrian magmatism.

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