The Rhum Layered Complex, Inner Hebrides, Scotland

Abstract

The lower Eocene Rhum Central complex has a core of ultra-basic rocks and gabbros which formed at a late stage in the centre’s evolution. Three major components are recognised: the Eastern Layered Series of alternating layers (up to 80m thick) of feldspathic perioditite and allivalite, totalling c. 600m in thickness; the Western Layered Series of layered gabbros conformably overlain by layered feldspathic peridotites; and a later intrusive Central Series consisting of feldspathic peridotites and subordinate gabbros, dunites and layered allivalites. The Central Series is noteworthy for its abundant development of ultrabasic breccias. Emplacement of the ultrabasic and gabbroic rocks was initially controlled by the Main Ring Fault but the Long Loch Fault zone exercised control during intrusion of the Central Series. The Eastern and Western Layered Series are considered to be parts of a once-continuous steep-sided intrusion with a domed roof that extended only slightly above the level of the Rhum mountains (c.800m altitude).

The Rhum parental magmas were of olivine-rich picritic or ultrabasic composition although basaltic magmas were also present. The field relations strongly suggest that the magmas crystallized at the levels at which ultrabasic rocks and gabbros now occur. Layering in the Eastern Layered Series is attributed to injection of pulses of picritic magma which ponded beneath more evolved, lighter and cooled residual magma and above allivalitic cumulates. Crystallization of each new batch gave a layer of peridotite and the residual liquid mixed with earlier, evolved residual magma from previous batches to form allivalite, building up the layered succession of sixteen major peridotite/allivalite units. Gravity studies on Rhum show that the mafic rocks are the top of a dense body at least 15 km in vertical extent.

Field, petrographic and mineral/chemical evidence point to a least small-scale replacement of allivalite by peridotite and there is the possibility that replacement occurred on a large scale: migration of intercumulus magma from layer to layer is considered largely responsible for the replacement. Emplacement of the mafic magmas into acidic country rocks cause anatexis and the formation of marginal intrusion breccias. Melted country rocks probably also mixed with the magmas resulting in the contamination recognised in some of the gabbros and allivalites.