Rum Central Complex, Hebridean Igneous Province
From: Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.
The Rum Central Complex was emplaced into sandstones of the Neoproterozoic Torridon Group which, together with Lewisian gneisses, form the ridge (or horst) that separates the Sea of the Hebrides and the Inner Hebrides Mesozoic sedimentary basins (Binns et al., 1974; Fyfe et al., 1993; P914123). There were at least three distinct phases in the growth and decay of the Rum Central Complex (Emeleus, 1997). Phase 1 was dominated by silicic magmatism and the development of an arcuate system of faults, termed the Main Ring Fault (P914142). Basic and ultrabasic magmas were intruded mainly during Phase 2, when numerous minor intrusions, and gabbros and layered ultrabasic rocks were emplaced. Phase 3 was marked by deep subaerial erosion and unroofing of the central complex, with intermittent burial of the developing topography by basaltic lavas of the Skye Lava Field (p. 67), and by the coarse detritus eroded from the central complex and its surroundings.
The basic and ultrabasic intrusions of Phase 2 were emplaced at about 60.5 Ma (Hamilton et al., 1998; P914126). All of the intrusive and extrusive rocks are reversely magnetised and it is probable that emplacement of the central complex occurred over about 1 million years, during magnetic anomaly C26R. The central complex is the site of a pronounced positive Bouguer gravity anomaly.
The Northern Marginal Zone, the Southern Mountains Zone and the Western Granite were all formed during Phase 1 (Dunham, 1968; Emeleus, 1997; P914142). Initial activity involved uplift within the Main Ring Fault, causing Lewisian gneisses and basal members of the Torridon Group to be elevated by as much as 2 km inside the fault system. The uplift was accompanied by tilting, deformation and doming of the Torridon Group rocks that surrounded and overlay the central complex. At this time the two major masses of Torridonian sandstone that form Mullach Ard and Welshman's Rock (P914142), each now underlain by low-angle faults inclined at about 35° to the east and north-north-east, respectively, are thought to have slid off the developing dome. The Torridonian rocks south of the Main Ring Fault in southern Rum most likely comprise a similarly displaced block (SB, P914142).
Subsidence within the Main Ring Fault next led to caldera formation, during which quartz–feldspar-phyric rhyodacitic magmas were intruded near to and along the faults, leading to explosive, surface eruptions. These eruptions formed the ignimbrites and crystal-vitric tuffs exposed in Dibidil, and on Cnapan Breaca and Meall Breac (P914143). The porphyritic rhyodacite ignimbrites commonly show a distinctive streaky banding resulting from the welding of flattened and attenuated fiamme (P580477). In addition to quartz and plagioclase, they also contain small, generally altered phenocrysts of augite, pigeonite, rare orthopyroxene and iron-titanium oxides. The ignimbrites overlie or, less commonly, are interbedded with coarse-grained volcaniclastic deposits in the form of largely unbedded breccias. The breccias consist of angular to subrounded blocks and megablocks of Torridonian sandstone; clasts of gneiss, basalt and dolerite also occur but are rare, as are scoria fragments. Examples of megablocks of Torridonian sandstone, tens of metres across, occur in Dibidil. Finer grained, bedded tuffaceous deposits containing sandstone clasts, thin layers of crystal tuff and impersistent beds of coarse, gritty sandstone are interbedded with and immediately underlie the ignimbrites. Dykes and veins of porphyritic rhyodacite cut the breccias, as do rare dykes of breccia and tuff which contain lobate inclusions of rhyodacite, plagioclase crystals derived from rhyodacite and rounded fragments of baked sandstone.
These deposits, termed the Coire Dubh Breccias, are interpreted as caldera-fill materials (Troll et al., 2000). The coarse, poorly sorted breccias are interpreted as having formed when the over-steepened caldera walls collapsed onto the caldera floor, and the thin beds of gritty sandstone represent materials washed out of the coarser deposits. The porphyritic rhyodacites formed as ignimbrites, erupted from feeders situated on the Main Ring Fault, their eruption being preceded by several ash-fall eruptions. On Meall Breac and in Dibidil, the steep-sided, intrusive rhyodacite bodies most likely pass laterally into ignimbrites (Donaldson et al., 2001). In Coire Dubh and in Dibidil, certain of the intrusive porphyritic rhyodacites contain numerous irregular, rounded masses of basic material, indicating that there was thorough mixing of co-existing silicic and basic magmas beneath the central complex during Phase 1, and suggesting that some of the eruptions were of mixed magma. (Troll et al., 2004) Subsurface explosive disintegration of the partially crystalline rhyodacite magma also shattered the sandstone surroundings, the mixture of fragments forming the rather rare tuff dykes as well as the more extensive areas of explosion breccia exposed south of Dibidil.
Breccias of a different kind occur in the Northern Marginal Zone. These rocks, termed the Am Màm Breccias, comprise blocks and megablocks of gabbro, blocks of thermally altered gneiss and sandstone, and rare feldspathic peridotite fragments. The matrix is a medium-grained, hybrid rock of quartz-dioritic or granodioritic composition. The breccia is intruded by the porphyritic rhyodacite but its relationship to the other breccias described above is uncertain; it is regarded as an early member of Phase 1. The clast content of the Am Màm Breccias is important since it shows that plutonic basic and ultrabasic rocks had formed at a very early period in the evolution of the central complex, long before the rocks of Phase 2, described below.
On the north-east side of Dibidil and on the eastern slopes of Beinn nan Stac, slivers of Lower Jurassic (Broadford Beds) limestone, siltstone and sandstone, and of altered basaltic lava derived from the Eigg Lava Formation, owe their preservation to subsidence within the Main Ring Fault. However, the final event of Phase1 involved further uplift on the Main Ring Fault system, when Torridon Group sandstones were reverse-faulted over the Mesozoic sedimentary rocks and Paleocene lavas on the eastern slopes of Beinn nan Stac in eastern Rum. In western Rum, granite was similarly faulted over sandstones of the Torridon Group near Bloodstone Hill. This late uplift is tentatively attributed to the forcible emplacement of the silicic magma that formed the Western Granite.
The Western Granite and similar smaller granite bodies near the Long Loch and Papadil postdate the porphyritic rhyodacites. The well-jointed Western Granite is magnificently exposed in cliffs and raised-beach platforms in south-west Rum, although inland exposure is poor. The granite is microporphyritic and of similar composition to the porphyritic rhyodacites. It contains phenocrysts of plagioclase together with ferroaugite and ferropigeonite, both of which are generally partially altered to amphibole. A small area of granophyric granite containing microphenocrysts of fayalite and ferroaugite, as well as plagioclase, crops out on the south-east slopes of Sròn an t-Saighdeir.
Basaltic and picritic magmatism predominated throughout Phase 2. Initially, numerous basaltic dykes were intruded, accompanied by many inclined sheets (cone-sheets), which are present throughout the Northern Marginal Zone and Southern Mountains Zone. The cone-sheets focus on a point at depth beneath upper Glen Harris; unlike the cone-sheets of the Cuillin Centre on Skye, those on Rum are fine grained, possibly reflecting emplacement at a shallower level. The principal trend of the basaltic dykes is north-west, and they merge with the regional Muck Dyke Swarm (Chapter 7). Additionally, there are many dykes that trend north—south or north-north-east so that, overall, the dykes define a radial pattern, as was noted by Harker (1908). In south-west Rum, several generations of approximately north-west-trending dykes intrude the Western Granite but few extend into the layered intrusions.
The greatest development of basic and ultrabasic rocks is in the layered intrusions, which crop out over much of the central and southern parts of the island (P914142; P580478). These intrusions postdate the cone-sheets and the majority of the dykes. Many of the layered structures are identical with, and of similar origin to, those in the Cuillin Centre of Skye (see above). In eastern Rum, the layered rocks are intruded by numerous semiconcordant sheets of gabbro, for example in Atlantic Corrie and on Askival where they accentuate the bedded appearance of the exposures (Volker and Upton, 1990). The layered rocks are also cut by plugs of gabbro and feldspathic peridotite, as are earlier members of the central complex and the country rocks (see also below).
The layered structures in the ultrabasic rocks and bytownite gabbros (the latter were formerly known as 'eucrites') vary in scale from tens of metres to millimetres (P580478; P580479). They are defined principally by varying proportions of anorthite-rich plagioclase, forsteritic olivine and subsidiary chrome-spinel and diopsidic clinopyroxene, although in some instances, textural features define the layers. Predominance of olivine gives rise to dark, easily weathered layers of feldspathic peridotite, whereas the resistant layers of light-coloured bytownite troctolite (or 'allivalite', so-called after the mountain Hallival on Rum; Harker, 1908) contain abundant plagioclase. In addition to their bedded appearance, many of the layered rocks contain structures closely resembling those found in clastic sedimentary rocks. These include size-graded and density-graded bedding, cross-bedding, slump structures, flame structures, drop-stones with associated deformation of small-scale layering, and coarse breccias (Wadsworth, 1992; P580480; P580481; P580482). Flat-lying zones of shearing and dislocation, as evidenced by deformation of small-scale structures, indicate that there has been much internal movement during accumulation of the layered rocks prior to their lithification. In south-west Rum, layering in bytownite gabbros at Harris and in feldspathic peridotites on Ard Mheall is accentuated by the presence of elongate olivine crystals which appear to have grown upwards from a succession of planar surfaces in the magma chamber. Rocks with this distinctive structure (P580483) were termed 'harrisites' by Harker (1908).
The microscopic textures, particularly in the ultrabasic rocks, suggest an origin by sedimentation of crystals from cooling magmas, with crystal sorting by density and, to a lesser extent, by size, followed by 'cementation' by later-crystallising phases. Feldspathic peridotite contains abundant, rounded to well-formed olivine and small chrome-spinel crystals enclosed by anhedral plagioclase and pyroxene, whereas the bytownite troctolites contain tabular plagioclase with pronounced parallel orientation or lamination, interstitial pyroxene and only rare olivine. The term 'cumulates' was coined to describe these and other layered rocks with distinctive textures presumed to reflect the accumulation of crystals precipitated from a magma (Wager et al., 1960; Wager and Brown, 1968). However, many of the textures now preserved are not wholly primary. It is now recognised that the rocks have achieved a high degree of textural equilibration: during prolonged cooling and consolidation their crystal shapes were modified by resorption and reprecipitation, processes that were aided by hot residual fluids percolating through the pile of layered rocks (e.g. Hunter, 1996). Composition was also modified during recrystallisation, as is evident from detailed studies of mineral compositions across the boundaries of layers on Hallival (Dunham and Wadsworth, 1978; Tait, 1985) and where late-stage veins cut the layered rocks (Butcher, 1985). Some of the larger-scale structures are attributable to replacement, for example where olivine-rich rocks cut across apparently undisturbed layered troctolites, as on the north-west shoulder of Hallival, and where olivine-pyroxene-rich 'finger' structures cut across undisturbed small-scale layering in bytownite troctolites (Butcher et al., 1985; compare with P580476).
The hot picritic magmas (1100° to 1300°C; Chapter 10), parental to the ultrabasic and basic rocks were emplaced into rhyodacites, sandstones, granites and gneisses. Since these host rocks generally have relatively low melting points (less than 1000°C), they underwent complete or partial melting and remobilisation (rheomorphism). The silicic melts back-veined and shattered the chilled margins of the basic and ultrabasic rocks, giving rise to the spectacular zones of intrusion breccia generally found at the margins of the layered intrusions. Intrusion breccias are common throughout the central complexes of the Hebridean Igneous Province, where felsic and later mafic rocks are juxtaposed; on Rum some of the most accessible and varied examples of this phenomenon occur at Harris Bay (P580484). At Harris, and elsewhere on Rum, there is also evidence that rheomorphic silicic melts, together with basic and rare ultrabasic blocks from the intrusion breccias, may have become detached from their sources and intruded for quite considerable distances into the layered suite. Such an origin could provide an explanation for the Am Màm Breccias of the Northern Marginal Zone (see above), although these would have been related to a much earlier period of intrusion of hot basic and ultrabasic magmas. Elsewhere, rocks in contact with the layered intrusions are thermally metamorphosed. At Allt nam Bà, in eastern Rum, impure limestones of Early Jurassic age are altered to calc-silicate hornfels containing tilleyite, spurrite and other high-temperature (sanidinite-facies) minerals, and there are also small amounts of peralkaline hybrid rock (Hughes, 1960). East of Hallival, xenoliths of fine-grained pyroxene-plagioclase-olivine rock in the Eastern Layered Intrusion are probably fragments of metamorphosed country-rock basalt; in these rocks, small, diffuse areas a few centimetres or so in diameter contain fassiaite and hydrogarnets and may represent metamorphosed amygdales (Faithfull, 1985).
Three major intrusions of basic and ultrabasic rock, the Eastern Layered Intrusion, the Western Layered Intrusion and the Central Intrusion, form the core of the Rum Central Complex (P914142). The Eastern Layered Intrusion (Brown, 1956; Wager and Brown, 1968) contains the classic layered sequences of feldspathic peridotites and bytownite troctolites that are at least 700 m thick. The Western Layered Intrusion (Wadsworth, 1961) consists of layered bytownite gabbro at Harris, overlain by a thick succession of layered feldspathic peridotite on Ard Mheall. In both areas, the layering is commonly accentuated by layers containing harrisitic olivines (compare with P580483). The layered succession is about 500 m thick. The Central Intrusion (McClurg, 1982; Volker and Upton, 1990) intrudes both of the Eastern and Western layered intrusions and truncates layered structures in each. The Central Intrusion is characterised by narrow zones of complex ultrabasic breccias enclosing areas of normally layered troctolites and peridotites. The breccias consist essentially of blocks and megablocks derived from earlier parts of the layered intrusions, including blocks which themselves exhibit complex internal relationships (Emeleus, 1997, P580482). The Central Intrusion is considered to have been the feeder zone for all of the layered intrusions (Emeleus et al., 1996b; P914144). Successive batches of ascending ultrabasic and magnesium-rich basaltic magmas were focussed into an approximately north—south zone of weakness on the proto-Long Loch Fault (p. 150), spreading laterally when they encountered the density trap at the Lewisian gneiss—Torridonian sandstone boundary and crystallising as layered rocks. Later batches intruded and brecciated layered rocks formed earlier and, in turn, spread laterally over the earlier, denser, and now largely crystalline material to build the Eastern and Western layered intrusions.
Over 40 small plugs of feldspathic peridotite, gabbro and dolerite intrude the central complex and the surrounding country rocks. Where the plugs intrude the Torridon Group and Triassic sandstones, the sedimentary rocks are generally bleached, closely jointed and thermally metamorphosed, commonly with the formation of tridymite (preserved as quartz paramorphs); rarely, there is also evidence of melting. The altered feldspathic sandstones are commonly spotted, with pale rounded patches (a few millimetres in diameter) that have a spherulitic microstructure. Thin, linear zones of bleaching and, less commonly of brecciation, traverse the sedimentary rocks north of the central complex. Termed fissure breccias, the altered rocks are thermally metamorphosed in a similar manner to the sandstones adjoining the peridotite and gabbro plugs. However, the only spatially associated igneous rocks are rare stringers and veinlets of fine-grained basalt.
During phases 1 and 2, a considerable volcanic edifice must have built up over the Rum Central Complex. At the end of Phase 2, with the cessation of all igneous activity within the central complex, this massif was subjected to intense erosion. At the same time, lavas of the Canna Lava Formation, derived from sources outwith Rum, were ponded in the developing valley and canyon system on its flanks, where they became interbedded with the coarse detritus being stripped off the Rum massif. These features, which comprise Phase 3, are discussed more fully in Chapter 6.