On the petrology of the ultrabasic and basic plutonic rocks of the Isle of Rum

Abstract

The Isle of Rum contains one of the most interesting assemblages of plutonic rocks of Tertiary age in the British Isles. The pioneer work of Macculloch and the later exploratory efforts of Judd and Geikie were followed by an admirable account of the rocks by Harker, given with his usual lucidity and precision. In 1938 I spent a fortnight on this island, and this paper—the outcome of my visit—aims merely at supplementing Harker's petrographical description by certain quantitative data in respect of the ultrabasic and basic rocks found there and of recording certain observations relating to the igneous tectonics.The earliest phase of igneous activity on Rum was the eruption of doleritic and mugearitic lavas. As shown by Bailey the eruptive phase of igneous activity was followed or accompanied by the formation of a ring-fault and two subordinate vents iufilted with explosive breccia and intruded by felsite. The space inside the ring-fault was then intruded by a banded peridotite-allivalite complex and later by eucrite in the form of a semicircular ring-dike and associated sheets. Finally, a granophyre boss was intruded along the western side of the ultrabasic-basic complex.