The Rum Layered Suite

Abstract

Abstract Palaeocene igneous activity in the Rum Central Complex culminated in the formation of the ultrabasic and gabbroic rocks of the Layered Suite. Its three components, the Eastern Layered Series, the Western Layered Series and the Central Series, represent a continuum in time during which replenishments of picritic (MgO 15-20 wt.%) and basaltic magmas ponded in thin sill-like bodies at the Lewisian gneiss - Torridonian sandstone unconformity, each contributing incrementally to a layered cumulate sequence. The magmas were probably guided during ascent by the long-lived Long Loch Fault. Peridotite (olivine - chrome-spinel) cumulates formed from picritic magma. The residual (basaltic) magma mixed with resident residual magma from earlier batches, and with small amounts of siliceous rheomorphic melts from country rocks, forming (isotopically contaminated) allivalitic (= troctolitic), plagioclase - olivine cumulates or, less commonly, gabbroic (plagioclase - olivine - clinopyroxene) cumulates. Residual basaltic magma was probably also intruded as gabbroic sheets and plugs, and extruded as lavas. Widespread slump and shear structures indicate mechanical instability of unconsolidated cumulate mushes, especially in the allivalites. Ultrabasic breccias are common in the Central Series, and are attributed to (i) disruption of earlier cumulates as new batches of magma rose along an elongate, north-south feeder zone and (ii) collapse of cumulates into this zone during episodic magma withdrawal. Equilibrated textures, lack of compositional zoning in olivine and pyroxene, offsets between compositional and modal variation at unit and other lithological boundaries, the occurrence of finger structures and other replacement features, and the compositional modification of ultrabasic rocks adjoining late-stage gabbroic veins, all attest to the pervasive influence of migrating intercumulus liquids during crystallization and consolidation of the cumulates.