Provenance and age constraints of the South Stack Group, Anglesey, UK: U-PbSIMS detrital zircon data

U-Th-Pb Secondary Ion Mass Spectrometry (SIMS) data from detrital zircons extracted from the South Stack Group, Anglesey, UK, indicate that: (1) the maximum depositional age of the Holyhead Formation (South Stack Group, Monian Supergroup) is 501 +/- 10 Ma; (2) the Monian Supergroup was deposited between c. 500 and 475 Ma and is part of the Cambrian-Lower Ordovician succession found in southern Britain and Ireland; (3) Avalonia was a major sediment source (age maxima at 543-552 and 604-627 Ma); (4) Amazonia probably also provided zircons (common Neoarchaean-Mesoproterozoic grains) weakening suggestions that Avalonia had rifted off Gondwana by Cambrian times.