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

Abstract

The age and formation of zircons in Neoproterozoie migmatites from the northern Grampian Highlands are assessed using field observations, imaging and U-Pb isotope dilution techniques. Acicular zircons within monzogranitie leucosomes provide evidence for neocrystalline growth during localized anatexis. U-Pb zircon data show that formation of the regional gneissose fabric, melt generation and new zircon growth occurred at 840 ±11 Ma contemporaneous with peak metamorphism. These data and previously dated shear fabrics at c. 800 Ma, confirm that a Neoproterozoic tectonometamorphic event affected at least part of the metasedimentary sequence southeast of the Great Glen Fault. Data from detrital zircons indicate a Palaeoproterozoic/Grenville province, rather than Archaean, as the most likely source for the sedimentary protolith. In the absence of identifiable orogenic discontinuities within the Scottish Highlands, we question current correlations with Laurentian passive margin sequences, and the inferences that the rocks of the Grampian Block only experienced a simple Caledonian (Palaeozoic) orogenic history.