The layered ultrabasic rocks of south-west Rhum, Inner Hebrides

Abstract

The ultrabasic rocks of the Tertiary volcanic centre of Rhum are best exposed in the mountains of Hallival and Askival, in the eastern part of the island, where they form a series of alternating peridotite and allivalite layers. Harker (1908) attributed the formation of these to a series of sill-like injections of already differentiated magma, but Brown (1956) has convincingly demonstrated that these rocks are the basal accumulates of a basic magma, and has suggested that the repetition of units comprising peridotite overlain by allivalite is the result of successive pulses of magma entering the magma chamber. The present study was initiated in the Harris area, which covers the south-western part of the ultrabasic tract, because it was believed that the rocks exposed there represented a different level in the original intrusion, and that they would provide further evidence of the mode of formation. A new terminology (Wager, Brown & Wadsworth 1960) has been used to describe the rocks of the layered intrusion. The general term `cumulate' is applied to rocks formed by bottom accumulation of crystals (cumulus crystals), and a more specific designation of composition is made by using prefixes based on the nature of the mineral assemblage. For a precise description of a given rock type, the cumulus minerals are listed in order of decreasing abundance (i.e. olivine-feldspar-pyroxene cumulate) and for a general implication of composition, the terms ultrabasic or eucritic are used. More subtle distinctions are made on the basis of differences in the cooling history of the inter-cumulus liquid, trapped in the pile of accumulated crystals. In the Harris area of south-west Rhum, a thickness of approximately 4500 ft. of layered cumulates is exposed, and has been subdivided into four petrologically distinct groups. The lowest of these is the Harris Bay series of eucritic cumulates (olivine-feldspar-pyroxene cumulates), which are at least 400 ft. thick, the base being below sea level. These are conformably overlain by a thin Transition series (150 ft.), comprising cumulates intermediate in composition between those of the underlying Harris Bay series, and the overlying Ard Mheall series. The latter consists of approximately 1200 ft. of ultrabasic cumulates (olivine, and olivine-feldspar cumulates) which exhibit prominent rhythmic layering. The Dornabac series is nowhere seen to overlie the Ard Mheall series, but is separated from it by a zone of igneous breccia. This consists of blocks of cumulates which are believed to have been derived from the Ard Mheall series by faulting within the magma chamber, and to have accumulated at the base of the fault scarp contemporaneously with settling olivine and feldspar crystals which form the matrix of the breccia. The Dornabac series overlies this breccia, and consists of approximately 400 ft. of slumped olivine-feldspar and feldspar-olivine cumulates. It is overlain by the Ruinsival series, which has been divided into a lower part (about 1500 ft. thick) and an upper part (about 1000 ft. thick), both parts consisting of a thick series of olivine-rich cumulates below a thinner series of feldspathic cumulates. The sequence in the Ruinsival series is obscured by poor exposure in the lower part, and by the complications of igneous breccia, later intrusions of eucrite, and later faulting affecting the upper part. Thus, as in the Hallival-Askival area, there is a major pattern of layering in south-west Rhum, although the units there are not all complete, and are much thicker than in eastern Rhum. The four units are as follows: D, the upper part of the Ruinsival series; C, the lower part of the Ruinsival series; B, Dornabac series; Ard Mheall series; A, Harris Bay series. A study of the variation in the composition of the cumulus minerals (in particular olivine) throughout this sequence confirms Brown's hypothesis of successive replenishment of the magma chamber to account for the major layering. There is generally an upward gradation of olivine composition from Fo$_{88}$, in the olivine-rich cumulates of the lower part of a unit, to Fo$_{86}$ in the overlying feldspathic cumulates, followed by an abrupt reversal to more magnesian olivine at the base of the succeeding unit. This reversal is less abrupt between units A and B, and the intervening Transition series appears to represent an intermediate stage, probably resulting from the mixture of fresh magma and differentiated magma of the previous pulse, following incomplete removal of the latter. Small-scale rhythmic layering is developed throughout the sequence. In part, this consists of a simple alternation of the mineral proportions, but in the Harris Bay series, Transitional series, and Ard Mheall series there are also numerous layers, varying from less than an inch to many feet in thickness, of a remarkable rock termed harrisitic cumulate. It is characterized by relatively large olivines of rather embayed or intricate shape, sometimes exhibiting well-developed parallel growth structures, and tending to have a preferred orientation with (010) vertical. These features are believed to indicate that the olivines crystallized in situ, growing upwards from previously settled grains at the top of the crystal pile, during periods of exceptional tranquillity when there was little or no crystal settling. It is suggested that the rhythmic layering was the result of a frequently repeated process, comprising supercooling of the magma, spontaneous nucleation, followed by crystal settling, and then a period of quiescence while the necessary degree of supercooling was developed again. Each shower of crystals would have been sorted by the action of gravity, and if the interval between showers was long enough, upward growth from the floor would have occurred. A comparison of the layered series in eastern and western Rhum indicates that the latter represents a distinctly lower level within the original intrusion. It is clear from...