The rift-related magmas of the Proterozoic Gardar Igneous Province were emplaced across the contact between the South Greenland Archaean craton and the Palaeoproterozoic Ketilidian mobile belt. It has been suggested that the geochemistry of Gardar intrusive rocks in the two areas varies across the craton margin and that this reflects a lithospheric control. However, comparison of the geochemical and isotopic signatures of basic and ultrabasic dykes from across the area shows that there is no systematic variation related to the age of the country rock. All the Gardar basic rocks are inferred to have been derived from the mantle, with relatively little crustal contamination. We suggest that the lithospheric mantle beneath the Gardar Province was enriched by slab-derived fluids during the Ketilidian orogeny (c. 1800 Ma). Subsequent melting of this mantle source was promoted during Gardar rifting when volatile-rich, small-degree melts from the asthenosphere were introduced into the lithospheric mantle, forming enriched metasomites. Ultrabasic lamprophyre dykes in the Gardar Province represent melts derived largely from these metasomites, whereas basaltic magmas were formed by larger-scale melting of the lithospheric mantle, inheriting a subduction-related signature. There is no evidence that the Gardar magmas were derived from a highly enriched lithospheric keel that had existed since craton formation.