SILURIAN TURBIDITES USED TO RECONSTRUCT A VOLCANIC TERRAIN AND ITS MESOPROTEROZOIC BASEMENT IN THE IRISH CALEDONIDES

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