Two lamprophyre suites are used to constrain sub-continental lithospheric mantle domains in Late Caledonian Northern Britain (at 400 Ma). A Northern Highlands and a Southern domain are resolved. The first has low (ɛNd (−6.4 to − 12.8); the latter has higher (+3.9 to −3.4). The boundary between them is coincident with the surface expression of the Great Glen Fault. The two mantle domains tightly bracket the fault. The lamprophyre magmas were generated at depths of at least 100 km. At the end of the Caledonian Orogeny the Great Glen Fault was a major vertical discontinuity that transected the sub-continental lithospheric mantle.