Abstract

The Scourian complex of northwest Scotland underwent granulite-facies metamorphism at 2.7 Ga. Granulite-facies orthogneisses have whole-rock δ18O values similar to those of igneous rocks of comparable compositions (ultrabasic gneisses 5.5–5.9‰, basic gneisses 6.4–6.8‰, tonalitic gneisses 9.5–9.7‰), and steep 18O gradients of up to 3‰in 10cm are recorded at two lithological contacts. These data suggest that large volumes of pervasive metamorphic fluids have not infiltrated the rocks, but do not preclude localized fluid-rock interaction. Scourian paragneisses have lower δ18O values (8.5–9.9‰) than typical metasediments, and may have undergone fluid-hosted isotopic exchange with the surrounding orthogneisses. Felsic sheets within tonalitic and basic gneisses have similar δ18O values to their hosts: these data, combined with petrological and geochemical information, suggest that the sheets were formed by local anatexis during granulite-facies metamorphism. Fluid infiltration during the 2.5 Ga amphibolite-facies Inverian retrogression caused homogenization of δ18O values of the Scourian gneisses. Unaltered tholeiitic Scourie dykes have low δ18O values of 2.0‰. It is unlikely that heated, surface-derived fluids infiltrated the Scourian complex, and these dykes were probably emplaced as low-δ 18O magmas.

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