THE MAIN DONEGAL GRANITE

Abstract

The Main Donegal Granite is described in its regional setting and details are given of the envelope of Dalradian metasediments and the relation to the earlier members of the granite complex. A separation is made into the Main Granite Proper and the structureless Trawenagh Bay Granite, covering areas of 140 and 20 square miles respectively. The Main Granite Proper is a biotite-granite showing a medium-grained, grey, apparently marginal type in the south-east of its outcrop, and a coarser, lighter-coloured variety in the north and west. Pegmatite forms a roof facies in the south-west. Flow structures are shown by mineral orientation and a perfect banding ; the latter is produced by granites of varying grain and biotite content, the finer-grained biotite-richer type being of earlier consolidation and possibly constituting a roof phase. Banding is everywhere steep and along the NE.–SW. length of the Main Granite Proper. Within the Granite, perfectly aligned trains of free-swimming rafts of country rocks, seen to be derived from roofs or walls, extend for some dozen or more miles in the Granite and agree perfectly with its flow pattern. The contact-zones of the Main Granite Proper show marked sheeting by granite and, in these zones, high-grade contact schists are produced with staurolite and garnet—deformation, and heating have acted together. The marginal parts of the Granite and the adjacent contact-zones show an intense shoar structure, lineation and mullioning, the planar structures running vertically NE.–SW. and the linear elements plunging gently north-eastwards in the north-eastern part of the outcrop and south-westwards in the south-western. The production of these structures was contemporaneous with that of the contact schists and is related to the emplacement of the Main Granito Proper. The phenomena shown by the Main Granite Proper and by its roof and walls are consistent with the almost horizontal flow, from north-east to south-west, of viscous magma in a long narrow body beneath a pronged roof and between steep walls. A broad divergence of raft-trains as they are followed into the granite shows that it cannot be of replacement origin, nor can the room problem be solved by major stoping. The authors propose that lateral magmatic wedging is the mechanism of emplacement this great granite body.