A precise late Neoproterozoic U-Pb zircon age for the syntectonic Perelle quartz diorite, Guernsey, Channel Islands, UK

A precise U-Pb zircon age of 611.4+/-(2.0)(1.1), Ma has been determined for the foliated quartz diorite at Perelle Bay, Guernsey, UK Channel Islands. The age is based on the analysis of single zircons, half of a single grain, and groups of two and three zircons. The age is nearly 100 Ma younger than that suggested by other workers, based on Pb-207/Pb-206 dates of large, mu lti-grain fractions of zircon, and thus rules out this intrusion as the ear liest marker of the onset of Cadomian magmatism in the North Armorican Mass if. A 611 Ma crystallization age for the Perelle quartz diorite is consiste nt with previous detailed fabric development studies that suggested magmati c fabrics within the pluton are largely overprinted by similarly oriented s olid-state deformation fabrics resulting From syntectonic emplacement durin g Cadomian orogenesis. The U-Pb age is approximately 8-15 Ma older than Ar- 40/Ar-39 hornblende dates for the pluton, consistent with the emplacement o f the intrusion followed by slow cooling in a region of moderate to high am bient crustal temperatures. These new data emphasize the fact that periods of pluton emplacement are considerably briefer than periods of deformation and thus syntectonic intrusions do not date regional deformational events, but only mark events within a period of regional tectonism. The ability to analyse single zircons, and selected portions of single zircons, using ultr a-low blank U-Pb analytical techniques documents the feasibility of determi ning precise emplacement ages of other Neoproterozoic granitoids that conta in a significant percentage of xenocrystic zircon.