The geochemistry of Tertiary igneous rocks from Arran, western Scotland, provides a probe for the structure of the crust in the region of the Highland Boundary Fault (HBF). New Nd isotope data, coupled with other geochemical evidence, point to variable contamination of primitive mantle-derived magmas during magmatic differentiation in the crust. Two different isotopic contamination trends are seen. The northern granite was generated by contamination of basic differentiates by crust resembling exposed Dalradian units. Data for the central granite, and several other minor intrusions from south of the HBF, trend towards the reported isotopic signatures of granulite-facies xenoliths from the Midland Valley. However, quartz porphyry intrusions in the south of the island are compositionally similar to the nothern granite, and were probably intruded southwards across the HBF in dykes. Hence the isotopic signatures of the Tertiary intrusions reflect the different character of the crustal units on either side of the fault.