Jf. Menuge et al., SILURIAN TURBIDITES USED TO RECONSTRUCT A VOLCANIC TERRAIN AND ITS MESOPROTEROZOIC BASEMENT IN THE IRISH CALEDONIDES, Journal of the Geological Society, 152, 1995, pp. 269-278
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.