Paleoproterozoic basement occurs on the islands of Islay and Colonsay in western Scotland and on Inishtrahull off the north coast of Ireland. The basement is composed of a deformed alkalic igneous association, collectively referred to as the Rhinns complex. Isotopic data indicate that the complex represents new addition of mantle-derived material to the crust at ca. 1.8 Ga. The Rhinns complex may link with rocks of similar age in North America, Greenland, and Scandinavia, which formed part of the southern margin of the Laurentia-Baltica supercontinent.