A 1,800-million-year-old Proterozoic gneiss terrane in Islay with implications for the crustal structure and evolution of Britain

Abstract

A basement gneiss terrane is exposed in the western part of Islay, an island south of Mull in Scotland. This gneiss terrane has been inferred previously^1–5 to be part of the Lewisian complex (that is, Archean in age), although no direct radiometric information is available. Bentley et al.⁶ questioned the Lewisian age for the basement and suggested that the Islay terrane is allochthonous. In this study we employ four different isotope techniques (Rb–Sr, Sm–Nd, Pb–Pb and U–Pb) to determine the crystallization and crustal extraction ages of the Islay gneisses. The isotopic evidence shows that the Islay terrane is early Proterozoic in age and that it is juvenile mantle-derived material rather than a reworking of Archean crust during the Proterozoic. There are, in consequence, two major implications for the crustal evolution of Northern Britain. First, the Grampian terrane, an area directly adjacent to the newly defined Proterozoic Islay block, is probably underlain by Proterozoic basement. Second, Northern Britain can be included in the reconstruction of the Laurentian Shield.