Pb–Sr–Nd isotope and major and trace element data are reported for a suite of samples from the Siluro–Ordovician Loch Borralan pluton, NW Scotland. Rock types ranging from pyroxenite through highly potassic melasyenites to quartz syenite were primarily generated by extensive fractional crystallization from mantle derived magmas. All rock types show pronounced Nb-depletion indicating shoshonitic, subduction-related, affinities for the magmas. Sr–Nd–Pb isotope compositions of the undersaturated rocks vary little and indicate derivation from a time-integrated mildly LREE-enriched mantle source very similar to that of the Silurian Lorne shoshonitic lavas further south. This mantle source is very unlikely to be lithosphere stabilized in Archaean times. The slightly younger quartz syenite intrusion contains 10–20% Archaean Pb and Nd indicating that its oversaturated and mildly peraluminous character is a function of assimilation of local Lewisian crust.