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Geological Society, London, Special Publications; 1991; v. 57; p. 199-213;
DOI: 10.1144/GSL.SP.1991.057.01.16
© 1991 Geological Society of London

A local source for the Ordovician Derryveeny Formation, western Ireland: implications for the Connemara Dalradian

John R. Graham1, John P. Wrafter2, Stephen Daly2 & Julian F. Menuge2

1 Department of Geology, Trinity College, Dublin 2, Ireland
2 Department of Geology, University College, Dublin 4, Ireland

Facies and palaeocurrent data indicate that the conglomeratic Ordovician Derryveeny Formation is the deposit of an alluvial fan whose apex was located a few kilometres east of the present outcrop. Clasts in the conglomerates include migmatite, schist, gneiss, granite, acid porphyry, spilite and vein quartz. Sillimanite bearing migmatite clasts closely resemble rocks of the neighbouring Lough Kilbride Formation, a unit of the Connemara Dalradian (Lower Proterozoic) basement from which they are presumed to be derived. This local source for the conglomerates is also suggested by the similarity of the Nd-model ages (c. 2.15 Ga) from the metamorphic clasts and from the Lough Kilbride Formation. Moreover peraluminous granite clasts give the same model ages, suggesting the granites are intracrustal melts from the same source. Interpretation of the Connemara Dalradian as a suspect terrane requires that it docked before Upper Ordovician times. Differences between the clast assemblage in the Derryveeny Formation and the presently exposed Lough Kilbride Formation are related to stripping of the original cover, emphasizing the value of clastic detritus in the study of the uplift history of metamorphic basement.





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