Oxygen-isotope geochemistry of the Scourian complex, northwest Scotland

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 ¹⁸0 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 simillar 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 amphibolitefacies 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-δ¹⁸O magmas.