The Scourie dike swarm of the Lewisian complex, northwestern Scotland, comprises a dominant suite of quartz tholeiites and lesser volumes of norites, olivine gabbros, and bronzite picrites that were emplaced at depths of >14km in two episodes at 2.0 and 2.4 Ga.Unaltered quartz tholeiite dikes have low and homogeneous whole-rock δ18O values (2.04‰ ±0.14‰ relative to Standard Mean Ocean Water) and concordant mineral δ18O values (plagioclase = 3.1‰ to 4.1‰, clinopyroxene = 1.5‰ to 1.9‰, magnetite = -0.3‰ to 0.6‰). The dikes intrude granulite facies gneisses that have heterogeneous δ18O whole-rock values (5.8‰ to 9.7‰) and show no evidence of 18O depletion. Low-δ18O intrusive igneous rocks are generally believed to have exchanged with high-temperature meteoric or oceanic fluids after emplacement. The heterogeneous high δ18O values of the country rocks suggest that the Lewisian complex has not been pervasively infiltrated by such fluids. It is also unlikely that fluids were channeled along the dikes because: (1) the gneisses immediately adjacent to the dikes have unaltered mineralogies and stable-isotope ratios; (2) the dikes preserve unaltered igneous textures and mineralogies; (3) the dike minerals do not show open-system oxygen-isotope resetting; and (4) surface-derived fluids generally do not penetrate to the depths at which the dikes were intruded. It is more likely that these dikes were emplaced as primary low-δ18O magmas derived from the subcontinental lithosphere. The geochemistry of the Scourie dikes suggests that they have not assimilated large volumes of continental crust, and the best explanation of the data is that the dikes were derived from a low-δ18O mantle source that possibly contained the remains of subducted, hydrothermally altered oceanic crust.