Granitoid emplacement and deformation: a case study of the thorr pluton, Ireland, with contrasting examples from Scotland

Abstract

The importance of pre-existing country rock structure in controlling the siting and geometry of intrusive bodies has been overlooked by recent workers, with most studies tending to concentrate on the effect of tectonic processes. This thesis describes three Caledonian plutons emplaced at intermediate crustal levels, the Thorr Granite, Co. Donegal, the Ratagain Complex, Inverness-shire and the Loch Loyal Syenite Complex, Sutherland. None of these plutons appear to have been emplaced in association with large tectonic strains. In all three cases, data collected from both the country rocks and the plutons demonstrates that regional tectonics alone cannot account for the observed shape, fabric evolution and mode of emplacement. Instead these features are more directly controlled by the interaction of the pre-existing structural architecture in the country rocks with fault and shear zones. Thus, whilst tectonic forces act as the catalyst in initiating the creation of space into which magma can be emplaced, the pre-existing structural architecture will control the ultimate form of the pluton. The plutons described in this thesis can, therefore, be considered to represent an intermediate stage between the sheeted intrusions that are emplaced in association with active shear zones, for example the Main Donegal Granite, and high level, passively emplaced plutons, such as the Rosses Granite. The evolution of deformation fabrics within the three plutons is described in detail. In all cases magmatic state deformation fabrics are predominant. However, most of these magmatic state fabrics are aligned parallel to the intrusion margins; this is particularly true in the case of the Thorr Pluton. Such a geometry could be accounted for by invoking the presence of large shear strains at the time of fabric formation. However, in the absence of evidence for large shear strains in the country rocks, it is proposed that such fabric geometries may be produced as a result of localised coaxial strain component in response to the body forces, or 'buoyancy head', exerted by the magma acting across the intrusion walls. Finally, in studying the kinematics of intrusion of the Thorr Pluton, a new technique for determining shear sense in rocks deformed in the magmatic state has been applied. The data collected from the application of this technique was found to corroborate the shear sense data collected from the envelope rocks. In this instance, the technique was a valuable aid to kinematic analysis, and ultimately to deducing an emplacement model for the Thorr Pluton.