Whole-rock Sm–Nd model ages of 1601 Ma (lower Laggan Formation) and 1674 Ma (upper Blackrock Formation) suggest a late Palaeoproterozoic, early Mesoproterozoic source for the metasediments of the Bowmore Sandstone Group, central Islay, SW Scotland. In situ U–Pb detrital zircon (SIMS) analyses show that the group was deposited after 1003 Ma ± 19 Ma in a basin dominated by Mesoproterozoic detritus, with some Palaeoproterozoic material and minimal Archaean input. The data are consistent with Laurentian sources, mostly Grenville (possibly including reworked sedimentary material from the southwestern Grenville Province), but with some input from Ketilidian, Labradorian, Pinwarian and Archaean terranes. Previous correlations with the Torridonian and the upper Dalradian are no longer tenable given the paucity of Archaean detritus in the Bowmore Sandstone Group. However, the data are comparable to parts of the Dalradian Grampian Group, in particular, the Glen Spean Subgroup (Upper Grampian Group, Strathtummel succession). It is proposed that the Bowmore Sandstone Group was deposited in an eastern Grampian Group basin, similar to the Strathtummel and Cromdale basins, which are also dominated by Grenville detritus.