U-Pb zircon geochronology of migmatization in the northern Central Highlands: evidence for pre-Caledonian (Neoproterozoic) tectonometamorphism in theGrampian block, Scotland

The age and formation of zircons in Neoproterozoic migmatites from the nort hern Grampian Highlands are assessed using field observations, imaging and U-Pb isotope dilution techniques. Acicular zircons within monzogranitic leu cosomes provide evidence for neocrystalline growth during localized anatexi s. U-Pb zircon data show that formation of the regional gneissose fabric, m elt generation and new zircon growth occurred at 840 +/- 11 Ma contemporane ous with peak metamorphism. These data and previously dated shear fabrics a t c. 800 Ma, confirm that a Neoproterozoic tectonometamorphic event affecte d at least part of the metasedimentary sequence southeast of the Great Glen Fault. Data from detrital zircons indicate a Palaeoproterozoic/GrenvilIe p rovince, rather than Archaean, as the most likely source for the sedimentar y protolith. In the absence of identifiable orogenic discontinuities within the Scottish Highlands, we question current correlations with Laurentian p assive margin sequences, and the inferences that the rocks of the Grampian Block only experienced a simple Caledonian (Palaeozoic) orogenic history.