The structural setting and U–Pb geochronology of Knoydartian pegmatites in W Inverness-shire: evidence for Neoproterozoic tectonothermal events in the Moineof NW Scotland

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Abstract
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.