The Plutonic History of the Maas Area, Co. Donegal

Summary The paper describes the plutonic geology of part of the envelope of the Donegal granite . In the Maas area a thick and varied series of Dalradian rocks is displayed in a relatively simple anticline, the southern limb of which is buckled into smaller folds. After the folding, which was accompanied by regional metamorphism, there followed three successive plutonic events, each differing in the nature of the introduced material, the manner of its emplacement and the associated metamorphic effects. Migmatization in the highest levels of the structure is associated with mobilization and the production of sillimanite-hornfelses. The later intrusion of the composite granodiorite pluton of Ardara was clearly accompanied by the shouldering and pushing aside of the country rocks by a hot plastic body. Early fold structures were tightened and dislocated during the formation of a zoned aureole characterized by sillimanite-garnet and andalusite-staurolite associations. Alongside what was probably the feeding channel of the pluton, migmatization and mobilization of a special kind, concentrated in action, have resulted in flow tectonics; involved in these operations are metasediments of varying properties, amphibolites of appinitic types and many kinds of migmatitic derivatives. Lastly, these rocks are truncated by the pegmatitic fringe of part of the main body of the Donegal granite and as this consolidated it recorded an intense shear deformation, possibly produced by its own upward pressure. I. Introduction The investigation of the plutonic geology of the Maas district is of considerable importance in the general study of North-West Donegal. There is here