A Laurentian provenance for the Dalradian rocks of north Mayo, Ireland, and evidence for an original basement-cover contact with the underlying Annagh Gneiss Complex

Abstract

Metasediments of the early Dalradian Grampian Group (Erris Group) and probable equivalents (Inishkea Division) structurally overlie Palaeoproterozoic to Neoproterozoic Annagh Gneiss Complex orthogneisses in NW Mayo, Ireland. Sm-Nd isotopic data suggest a Palaeoproterozoic source for the metasediments. K-feldspar and granitoid clasts from the Doonamo Formation, Erris Group, record U-Pb zircon ages of c. 1740 Ma and c. 980 Ma, respectively. These ages are within error of the c. 1730-1750 Ma Mullet gneisses and c. 990 Ma Grenvillian migmatitic leucosomes in the underlying Annagh Gneiss Complex. U-Pb detrital zircon data reveal that the Erris Group was deposited after c. 955 Ma, with predominant input from c. 1640, c. 1500 and c. 990 Ma interpreted Laurentian sources (Labradorian, Pinwarian and Grenvillian terranes, respectively). Limited detrital zircon data from the Inishkea Division yield similarly aged detritus and a tentative maximum depositional age of c. 1005 Ma. Correlation of both sequences with the Scottish Dalradian Grampian Group is considered valid based on detrital zircon U-Pb and whole-rock Sm-Nd data. The clast ages support the hypothesis that the Dalradian unconformably overlies the Annagh Gneiss Complex in north Mayo, whereas the detrital zircon data imply more distal Laurentian sources. Dalradian deposition is thereby tied to the margins of Laurentia. © 2010 Geological Society of London.