Recent research developments on the Isle of Rum, NW Scotland

Abstract

The appearance in 1997 of the British Geological Survey's memoir on Rum was followed by a period of intense research, leading to upwards of 35 papers, books and other articles. The scope of these publications, and the research progress over the last 15 years since publication of the memoir, is reviewed here. Igneous activity on Rum was short lived, possibly only ca. 500 ka, and, at about 60.5 Ma. The Rum central complex thus pre-dates the nearby Skye central complex. The earliest, acidic and mixed acidic/basic magmatism on Rum involved both shallow intrusions and ignimbrite eruptions into a collapsing caldera bound by the Main Ring Fault, a structure which probably also exercised a structural influence on subsequent mafic and ultrabasic magmatism. Subsequent emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Detailed textural studies on the gabbroic and ultrabasic rocks allow distinction between intrusive peridotites and peridotite that formed as part of the classic layered units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movements of trapped magma and magma derived from later intrusions, may produce textures and structures hitherto regarded as primary features of cumulate rocks. Rare picritic dykes provide an indication of likely parent magma for the mafic and ultrabasic rocks, but these and other magmatic rocks on Rum have all undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite crust but, notably, not Moine rocks as at Ardnamurchan. Sulphides in the chromite seams and ultrabasic rocks show possible influences from assimilated Jurassic sediments. From recent apatite fission track studies it seems likely that Rum, in common with other Palaeogene centres, underwent a brief, but significantly younger (Mesozoic) heating event.