The Blackstones Bank, located about 60 km WSW of the Isle of Mull in Western Scotland, is a submarine plutonic complex in the British Tertiary Igneous Province. Geochemical and isotopic analysis of gabbros, microgabbros and basic dykes shows that the magmas interacted strongly with crustal rocks during their emplacement. The isotopic signature of the contaminated Tertiary intrusions shows no evidence of any interaction with Archaean basement, despite the location of the Blackstones complex to the west of the Great Glen fault. Instead, the Blackstones rocks have crustal signatures resembling the Proterozoic basement and cover rocks of western Islay. It is therefore inferred that Early Proterozoic crust extends to the west of the Great Glen fault at this point on the Scottish continental shelf. In addition, the occurrence of similar isotopic signatures in Tertiary igneous rocks east of the Loch Gruinard fault confirms that Early Proterozoic basement extends under the Grampian block of mainland Scotland. When combined with published evidence from the Rockall bank, the new data constrain the location of an Archaean–Proterozoic crustal suture with a WNW trajectory which cuts across the continental shelf of northwest Britain.