THE STRUCTURAL SETTING AND U-PB GEOCHRONOLOGY OF KNOYDARTIAN PEGMATITES IN W INVERNESS-SHIRE - EVIDENCE FOR NEOPROTEROZOIC TECTONOTHERMAL EVENTS IN THE MOINE OF NW SCOTLAND

U-Pb monazite data for the Ardnish and Sgurr Breac pegmatites in the S W Moine block give Knoydartian ages of 827 +/- 2 and 784 +/- 1 Ma resp ectively. Structural and metamorphic studies of the pegmatites and the local Moinian metasediments suggest that pegmatite generation was a c onsequence of localized high-temperature shearing and metamorphism wit hin the pelitic horizons. Petrographic evidence from the sheared pelit es is interpreted as indicating that the local garnet-grade metamorphi sm was contemporaneous with the shearing and pegmatite generation, and was, therefore, Knoydartian in age. Chemical zoning profiles in garne ts are consistent with their growth during prograde regional metamorph ic increases in P and T. The c. 45 Ma difference between the pegmatite ages implies that the Knoydartian tectonothermal event was diachronou s, being either a prolonged or episodic event. The metamorphic and geo chronological data are consistent with the presence of Neoproterozoic orogenic events in the SW Moine.