The layered ultrabasic rocks of Sgurr Dubh, Isle of Skye

Synopsis The Sgurr Dubh ultrabasic rocks show many features indicative of bottom accumulation of cumulus crystals from a basaltic or picritic magma. Dunites are common towards the base of the layered sequence, intercumulus feldspar and clinopyroxene becoming more plentiful as the sequence is ascended. Chromite bands are common within the dunitic rocks. In the higher structural levels, cumulus feldspar crystals appear with the olivines, and layered structures, comprising alternating feldspar-rich and olivine-rich bands, here become common. Optical studies of the component minerals, together with four analyses of selected rocks, indicate a small, albeit significant, cryptic variation throughout the mass—olivine from Fa 13 to Fa 16.5 , feldspar from An 85 to An 80. Diffusion within intercumulus material is considered the most probable explanation for the local formation of dunites. Comparison with the layered ultrabasic rocks of Rhum indicates that the Skye ultrabasic rocks crystallized at somewhat higher temperature, and that a longer temperature interval ensued between the crystallization of the olivines and the feldspars, a factor considered important in explaining the formation of dunites within the Skye rocks. Auto-brecciation and segregation-veins are common in parts of the mass, and there is evidence that marginal movements have occurred. The ultrabasic rocks are considered to have formed by crystal accumulation affected by various physical processes; they appear also to represent a magmatic disconformity within the sequence of basic igneous intrusions of the Cuillin.