A SHRIMP ion microprobe study of inherited and magmatic zircons from four Scottish Caledonian granites

Middle Proterozoic age components were found in inherited zircons from the Ben Vuirich, Glen Kyllachy, Inchbae and Vagastie Bridge granites. Late Proterozoic (900-1100 m.y.) components were identified in zircons from the first two granites. SHRIMP isotopic and U, Th and Pb analyses were made on successive shells of zoned zircon surrounding inherited cores from the Glen Kyllachy granite to monitor chemical changes during magmatic zircon growth and showed that zircon shells have characteristic but significantly different Th, U and Pb contents. Magmatic zircon from the Vagastie Bridge granite also forms as clearly defined oscillatory zoned shells around unzoned nuclei of inherited zircon. Zircons from the Ben Vuirich granite occur as euhedral, magmatic crystals, or as rounded, subhedral inherited grains; a SHRIMP age of 597 +- 11 m.y. for euhedral magmatic zircon from this granite is identical, within the uncertainty, to the conventional multigrain zircon age of 590 +- 2 m.y. reported by Rogers et al. and confirms that sedimentation of the Dalradian sequence took place in the Precambrian.