Abstract

Smd zircon fractions from the deformed alkaline igneous complex on Inishtrahd, County Donegal, Ireland yield near-concordant U-Pb ages of 1779 * 3 Ma interpreted as dating the igneous crystallization of the gneiss's precursor. Sm-Nd depleted mantle model ages range from 1912 Ma to 1978 Ms. The similarity of the model ages and the crystallization age indicates that the protoliths were derived from a depleted mantle source essentially uncontaminated by older crust. The isotopic data confirm previous tentative correlations of the Inishtrahull rocks with those of Islay in southwestern Scotland. Archaean crust is apparently absent from this region, consistent with recent models placing a southern limit on the Archaean along the line of the Great Glen fault zone.

Pre-Caledonian orthogneisses in Scotland and Ireland, south of the Great Glen fault zone, have generally been correlated with the late Archaean Lewisian Complex of northwestern Scotland (e.g. Wilkinson 1907; McCallien 1930; Bowes & Hopgood 1975; Dixon et al. 1990) although Aftalion & Max (1987) concluded that the Lewisian was absent from the Annagh Division gneisses of north Mayo, Ireland. There are now several lines of evidence to suggest that Archaean crust may be absent southeast of the Great Glen fault zone (e.g. Fitches et al. 1990; Daly 1990; Halliday et al. 1985).

A large area of pre-Caledonian basement, here termed the Rhinns Complex, is thought to be present within the fault-bounded Colonsay-West Islay block (Fig. 1; Bentley et al. 1988; Fitches et al. 1990; Marcantonio et al. 1988)), which extends from Colonsay in

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