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1 Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QF, UK (e-mail: G.Rogers{at}surrc.gla.ac.uk)
2 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
3 Geology & Cartography Division, School of Construction and Earth Sciences, Oxford Brookes University, Oxford OX3 0BP, UK
5 Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK
U–Pb monazite data for the Ardnish and Sgurr Breac pegmatites in the SW Moine block give Knoydartian ages of 827±2 and 784±1 Ma respectively. Structural and metamorphic studies of the pegmatites and the local Moinian metasediments suggest that pegmatite generation was a consequence of localized high-temperature shearing and metamorphism within the pelitic horizons. Petrographic evidence from the sheared pelites is interpreted as indicating that the local garnet-grade metamorphism was contemporaneous with the shearing and pegmatite generation, and was, therefore, Knoydartian in age. Chemical zoning profiles in garnets are consistent with their growth during prograde regional metamor-phic increases in P and T. The c. 45 Ma difference between the pegmatite ages implies that the Knoydartian tectonothermal event was diachronous, being either a prolonged or episodic event. The metamorphic and geochronological data are consistent with the presence of Neoproterozoic orogenic events in the SW Moine.
Key Words: Scotland, U • Pb, Moinian, pegmatite
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