The Blackstones Bank igneous complex; geochemistry and crustal context of a submerged Tertiary igneous centre in the Scottish Hebrides

The Blackstones Bank, approximately 60 km WSW of Mull, is a submarine plutonic complex. Geochemical and isotopic analysis of gabbros, microgabbros and basic dykes shows that the magmas interacted strongly with crustal rocks during their emplacement. The Blackstone rocks have crustal signatures resembling the Proterozoic basement and cover rocks of W Islay, rather than evidence of any interaction with Archaean basement (despite Blackstones Bank lying to the W of the line of the Great Glen fault). It is therefore inferred that early Proterozoic crust extends to the W of the Great Glen fault at this point on the Scottish continental shelf. When combined with published evidence from the Rockall bank and the occurrence of similar isotopic signatures in Tertiary igneous rocks. E of the Loch Gruinard fault, the new data constrain the location of an Archaean-Proterozoic crustal suture to a WNW trajectory cutting across the continental shelf of NW Britain.