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1 Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USA (e-mail: sdsamson{at}syr.edu)
2 Department of Geology, Oxford Brookes University, Oxford 0X3 OBP, 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 207Pb/206Pb dates of large, multi-grain fractions of zircon, and thus rules out this intrusion as the earliest marker of the onset of Cadomian magmatism in the North Armorican Massif. A 611 Ma crystallization age for the Perelle quartz diorite is consistent with previous detailed fabric development studies that suggested magmatic fabrics within the pluton are largely overprinted by similarly oriented solid-state deformation fabrics resulting from syntectonic emplacement during Cadomian orogenesis. The U-Pb age is approximately 8–15 Ma older than 40Ar/39Ar hornblende dates for the pluton, consistent with the emplacement of the intrusion followed by slow cooling in a region of moderate to high ambient 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 ultra-low blank U-Pb analytical techniques documents the feasibility of determining precise emplacement ages of other Neoproterozoic granitoids that contain a significant percentage of xenocrystic zircon.
Key Words: Cadomian Orogeny • English Channel Islands • U-Pb • syntectonic processes • quartz diorites
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