U-PB ZIRCON GEOCHRONOLOGICAL EVIDENCE FOR NEOPROTEROZOIC EVENTS IN THE GLENFINNAN GROUP (MOINE SUPERGROUP) - THE FORMATION OF THE ARDGOUR GRANITE GNEISS, NORTH-WEST SCOTLAND

The age and Precambrian history of the Moine Supergroup within the Cal edonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granit e formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its en closed segregation pegmatites is 873 +/- 7 Ma. This demonstrates the r eality of a Neoproterozoic episode of high-grade metamorphism in the G lenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints o n Neoproterozoic palaeogeographic reconstructions of the North Atlanti c region, indicating that the Moine rocks cannot be used as a link bet ween the Grenvillian belt of North America and the Sveconorwegian orog en in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obt ained from detrital, inherited zircons and reflect the provenance of t he Glenfinnan Group Moine sediments which must, therefore, have been d eposited be tween c. 1100 and 870 Ma. Potential sources are found as r elatively minor, tectonically bounded basement inliers within the Brit ish Caledonides, although more widespread source areas occur outside B ritain in both Laurentia and Baltica. The most important feature of th e provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement c omponent of the western foreland to the Caledonides in Britain, was no t a major contributor to the Glenfinnan Group basin.