The Pre-Carboniferous rocks of the Slieve League promontory, Co. Donegal

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Abstract
Summary: The metamorphic rocks of the Slieve League promontory, in South-West Donegal, are shown to form a conformable succession consisting of the Killybegs Pelitic and Calcareous Group (oldest), the Glencolumbkille Limestone Group, a boulder-bed with granite boulders and the Slieve Tooey Quartzitic Group. Both pre- and post-metamorphic intrusions are also described; in places there is an unconformable cover of Lower Carboniferous sediments. Structurally, the Quartzitic Group forms several synclines in the older schists. Overfolding and thrusting have taken place towards the north; parts of the folds have been bent sideways through angles of up to 90°. The effects of post-Lower Carboniferous folding and faulting are also discussed; the faults are mainly of the strike-slip type with sinistral displacement. Most of the schists fall within the biotite zone of metamorphism, but garnet occurs in the west of the promontory. The garnets are largely replaced by chlorite, suggesting retrograde metamorphism. A connexion is established between the schists of the promontory and those of Lough Derg, described in an earlier paper. The present work supports the view that the Lough Derg rocks are of Moinian age. The schists succession in southern Donegal is correlated with the Moinian and Dalradian succession of the Scottish Highlands, and particularly with that of Islay.

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