The migmatitic Older Granodiorite of Thorr district, Co. Donegal

Abstract

Summary The Donegal granite is emplaced in highly folded Dalradian strata. The meta-sediments comprise pelitic and semi-pelitic schists, siliceous granulites, calcareous rocks and quartzites; these rocks are intruded by numerous basic sills of pre-metamorphic age. In the district of Thorr, on the northern margin of the granite, these strata are highly metamorphosed and sillimanite is the characteristic mineral of the pelitic types. This metamorphism is superimposed on one of low grade and is connected with the presence of heterogeneous rocks of tonalitic character; these are collectively called the " Older Granodiorite " and they contain many large enclaves of meta-sediment. Between such separated masses there is a concordance of structure, and the stratigraphical relationships are preserved. The presence of such a " ghost stratigraphy ", which indicates a considerable measure of replacement, has to be equated with the evidence of composition, which requires incorporation of country rock, and, again, with the evidence of sharp contacts and flow which suggests intrusion. A hypothesis requiring the partial mobilization of country rock granitized in place is contrasted with that of piecemeal stoping and marginal assimilation. The Older Granodiorite is intruded by a series of microgranite sheets and dykes with associated aplites and pegmatites. These form the early phases of the relatively xenolith-free, sodi-potassic Newer Granites, which are of magmatic origin and constitute the greater part of the Donegal mass. A wide belt of shearing enters the south-eastern part of the Thorr district and all the rocks suffer varying degrees of cataclasis. This may be a protoclastic deformation connected with the forceful intrusion of the Newer Granites. Late events in the geological history include the formation of a perfect joint system. The prominent north-north-westerly joints occasionally become faults, and they commonly afford the paths for numerous Tertiary dykes.