The Lettergesh Formation, Co. Galway, Ireland, comprises a 1.5 km thick sequence of turbidites deposited c. 430 Ma ago during closure of the Iapetus Ocean. Petrographic and geochemical data show that the turbidites consist overwhelmingly of detritus derived from a young volcanic terrain. Palaeocurrent data indicate that the detritus was derived from the north. Modelling of the chemical compositions of the turbidites indicates provenance from a high-K calc-alkaline, subduction-related volcanic province built on continental crust. A subordinate ultrabasic component may represent detritus from either an ophiolite or tectonically emplaced serpentinite. The turbidites are characterized by low variability of initial ɛ Nd (−6.5 to −3.2) and highly variable initial 87Sr/86Sr ratios. Modelling of the isotopic data suggests a mean crustal residence age of 1440–1600 Ma for the continental foundation of the volcanic terrain. Such a model age is consistent with a mixture of crust having TDM ages of c. 1.9 Ga and c. 1.3 Ga, both of which are known from the Annagh Gneiss Complex of NW Ireland.