The Ardara granitic diapir of County Donegal, Ireland

Summary The composite diapir of Ardara, County Donegal, foreibly displaced contact-altered Dalradian rocks of its envelope by folding, thrusting and shearing. The intrusion consists of a granodiorite core surrounded by a mantle of course tonalite. The outer part of the core is made up, in the north, of contaminated quartz-diorite and, in the south, of less contaminated granodiorite. These rocks, and the tonalite, are well foliated. The central part of the core is composed of uncontaminated and unfoliated biotite-granodiorite The emplacement of the body is believed to have begun with the movement of granodiorite magma along a pre-existing thrust-plane. The magma came in contact with basic rocks of an older complex and mobilized part of this complex, sweeping it upwards as a plastic skin which consolidated to form the tonalite mantle. Magma contaminated with basic material accumulated within this mantle to form the quartz-diorite and the slightly contaminated southern granodiorite. The outer part of the body cooled and developed radial fractures. A new upwelling of essentially uncontaminated granodioritic magma pierced the still molten core of the pluton, causing remobilization and further distortion of the envelope. Radial fractures, later followed by tangential fractures, were again formed. At a late stage, the solidifying pluton rose vertically, with resultant shearing of its envelope.