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Journal of the Geological Society; 1995; v. 152; issue.2; p. 269-278;
DOI: 10.1144/gsjgs.152.2.0269
© 1995 Geological Society of London

Article

Silurian turbidites used to reconstruct a volcanic terrain and its Mesoproterozoic basement in the Irish Caledonides

J. F. MENUGE1, D. M. WILLIAMS2 & P. D. O'CONNOR2

1 Department of Geology, University College Dublin, Belfeld, Dublin 4, Ireland
2 Department of Geology, University College Galway, Galway, Ireland

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 {varepsilon} 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.

Key Words: Ireland • provenance • Caledonides • isotopes • geochemistry




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