(a) The eastern facing skyline of the Rum Cuillin (Glen Dibidil) viewed from the Sound of Rum. The hill on the left is Ainshval, the central peak is Trollaval and the hill to the right is Beinn nan Stac. Ainshval and Beinn nan Stac form part of the Southern Mountains Zone (SMZ). Rhyodacite ignimbrite sheets and sedimentary breccias make up the back wall of Nameless Coire and the Ainsval Ridge. Layered ultrabasic units in the Eastern Layered Intrusion (ELI) form the distinctive peak of Trollaval (far centre). Faulted Lewisian gneisses and Torridonian sandstones crop out along the foreshore to the right (see also Fig. 3). (b) Hallival and Askival, viewed from the Sound of Rum from the northeast. Torridonian sandstones crop out in the lower foreground. Note the distinct layering in the ultrabasic rocks and gabbro on the western face of Hallival.
(a) Panorama and geological outline of the lower southwest side of Glen Dibidil and the Sgurr nan Gillean–Ainshval Ridge, SMZ, viewed from the southeast side of Beinn nan Stac (see also Fig. 1a). L, Lewisian gneiss. Torridon Group: T1, Fiachanis Gritty Sandstone Member; T2, Laimhrig Shale Member; T3, Allt Mor na h-Uamha Member; T4, Sgor Mhor and Scresort sandstone members. Palaeocene: B, Bedded breccia, sandstone and tuff; Re, extrusive rhyodacite; Ri, intrusive rhyodacite; MRF Main Ring Fault. (modified after Holohan et al., in Emeleus and Troll, 2008). (b) Sketch showing the relationships between the rhyodacitic ignimbrite feeder, the caldera in-fill and the shattered caldera-floor rocks (after Holohan et al., 2009)
(a) Panoramic view of the Northern Marginal Zone (NMZ) in the Coire Dubh area viewed from Meal Breac (from the northwest). Rocks of the NMZ form the low foreground (Coire Dubh Breccia) and the pale crags and outcrops on Cnapan Breaca (centre) are rhyodacite ash-flow deposits. The base of the crags on Cnapan Breaca marks the position of bedded tuffs and fine-grained sandstone. The easily weathered marginal gabbro of the ELI (Layered Suite) forms the grassy area on the right-hand flank of Cnapan Breaca. The terraced slopes leading up to Hallival mark the positions of troctolite (‘allivalite’) in layered units in the ELI. (b) Large angular block (~1 m × 2 m) of reddish Torridonian sandstone in finer Coire Dubh-type mesobreccia, SMZ. Hammer shaft is ~45 cm long.
(a) Coire Dubh Breccia with a chaotic assemblage of angular and sub-angular clasts mainly derived from the Torridonian Fiachanis Gritty Sandstone Member. Tape measure (bottom left) ~8 cm across. (b) Plan view of shallow dipping Coire Dubh Breccia beds showing a coarser clast assemblage to the left than on the right, implying pulses of sediment accumulation within the caldera. Photograph is ~40 cm across.
(a,b) Photomicrographs of fine-grained portions of Coire Dubh Breccia showing dominant quartz and potassium feldspar in a fine-grained matrix (crossed polarizers). (c) Basaltic chilled margin of a sheet intrusion against Coire-Dubh breccia (plane polarized light). Note the plagioclase microphenocryst alignment in the intrusion and the larger breccia clast in the top of the image.
(a) Attenuated fiamme in porphyritic rhyodacite ash-flow deposits, southwest end of Meall Breac, NMZ. The foliation defined by the fiamme points towards the centre of the caldera, implying flow from the caldera margin towards its interior. (b) Xenolith of Lewisian gneiss in rhyodacite (~3 cm across), Meall Breac.
(a,b) Photomicrographs of porphyritic rhyodacite with magma mixing features. Fragments of plagioclase and broken and corroded β-quartz phenocrysts in a felsic matrix rich in small mafic inclusions (a, crossed polarizers) and larger vesicular ones (b, plane polarized light), Cnapan Breaca. (c–f) Photomicrographs of magma mixing textures in a rhyodacite plug north of Cnapan Breaca (crossed polarizers). Note the resorbed plagioclase in (c) and the strong reaction rim on plagioclase inside a basic enclave in (e). An example of emulsion texture, due to mixing between a rhyolite (top left, with crystals) and a basalt (bottom right), is shown in (f).
(a,b,c,d) Representative portions of the Am Màm Breccia with characteristic coarse gabbro blocks on the north side of Am Màm hill (a,c) and close to the northeast shore of Loch Gainmhich (b). The dacitic matrix of the Am Màm Breccia contains a variety of lobate basic enclaves, sedimentary (Torridonian) as well as metamorphic (Lewisian) xenoliths. (d) Portion of the Am Màm intrusion breccia with a basic basaltic enclave containing a xenolith of bedded Torridonian sandstone (inside left-hand dashed line) and a larger fragment of Lewisian feldspathic gneiss (dashed line lower right-hand corner) north of Meall Breac. (e–h) (all crossed polarizers): Photomicrographs of Am Màm breccia and inclusions. Quartz, augite and plagioclase in Am Màm Breccia plus a large rounded K-feldspar crystal that is probably foreign to this rock are shown in (e). Augite and plagioclase xenocrysts and a disintegrating coarser-grained mafic xenolith in quartz-microdiorite matrix are shown in (f). Thermally altered Lewisian xenolith in Am Màm intrusion breccia, with partial melting and recrystallization textures is shown in (g). Coarse gabbro from a megablock near Loch Gainmich (h). Nicoll et al. (2009) suggest these gabbros are amongst the ‘deepest’ rocks of the Rum pluton and were brought to this level by the Am Màm dacite magma that was probably forced into the active MRF during caldera collapse (Figs 9b,d modified after Nicoll et al., 2009).
(a) Sr87/Sr86vs. Ce (ppm) for representative Rum samples (after Meyer et al. 2009). Ce is an incompatible element and is highly enriched in crustal rocks. The increasing Ce concentrations with increasing Sr isotope ratios is indicative of strong crustal influences on the felsic members of the Rum igneous suite. (b) Sr87/Sr86vs. Nd143/Nd144 isotope ratios for representative Rum samples. Picrite (M.9) is isotopically the most primitive sample while dykes and gabbros are displaced towards crustal compositions (Lewisian amphibolite- and Lewisian granulite-facies rocks). The acidic igneous rocks from Rum (rhyodacites, dacites, microgranites) spread along a mixing line between Lewisian amphibolite and granulite implying that either both types of crust were involved or that the main contaminant (the Lewisian amphibolites) had experienced some loss of melt during a previous heating event. (c) Comparison of the Rum Nd and Sr isotope ratios with data from other North Atlantic Volcanic Rifted Margins ODP Leg 152, SE Greenland margin and ODP Leg 104, Vøring Plateau. Note the similarities of the Rum suite with available SE-Greenland data (Fig. 10 modified after Meyer et al., 2009).
Marginal relationships of the Ultrabasic Intrusion, Layered Suite: (a,b) Tongues of harrisitic gabbro in bytownite gabbro (Western Layered Intrusion (WLI)) extending into a heated and ductile granite zone to the left, west of mausoleum, Harris Bay. Hammer for scale is ~30 cm. (c,d) Zone of intrusion breccia at the contact of the Western Granite with later bytownite gabbro of the WLI, East end of Harris Bay. The line of dark blocks is a broken up dyke in the felsic granite matrix formed when granite was partially melted and mobilized by heating from the WLI gabbro. (e) Layered gabbro in the WLI intruded by remobilized (‘rheomorphic’) acidic veins derived from partial melting of the adjoining Western Granite. The acidic magma has been channelled along planes of weakness provided by the layered structures. East side of Harris Bay, hammer shaft ~30 cm. (f) Back-veining breccia on Cnapan Breaca where marginal gabbro remobilized earlier acidic rhyodacite ignimbrites. The rheomorphic magma is being sucked into the cooling and hence increasingly jointed gabbro rock, leading locally to severe disintegration of the intruded gabbros. (g) Gabbro of the Central Intrusion (dark rock above person) intruding bedded sandstone of the Sgorr Mhor Sandstone Member (right of photograph). Dark beds in the sandstone are relatively rich in heavy minerals. Note how the beds sag towards the steep contact, southeast of Loch Papadil. (h) Detail of the intrusion breccia zone at the gabbro – sandstone contact of (g). The light-coloured acidic matrix has come from the partial melting of the adjacent sandstone. Scale: hammer shaft ~30 cm.
Photomicrographs (crossed polarizers). (a) Quartz paramorphs after tridymite fringing relict quartz grains in partially fused Torridonian feldspathic sandstone adjoining the gabbro plug, north side of Kinloch Glen. (b) Spherulitic feldspar and tridymite paramorphs in a matrix of partially fused Torridonian feldspathic sandstone. The radial spherulitic texture developed particularly in partly melted sandstone next to a gabbro plug, Allt Bealach Mhic Neill, south side of Kinloch Glen.
Classic cumulate textures in ultrabasic rocks of the Layered Suite. (a) Modally layered troctolite in ~5 m high section on the southwest face of Hallival. The layering is defined by alternating feldspar- and olivine-rich portions, ELI. (b) Modally layered troctolite with small-scale slump structures (centre of image) and peridotite replacing troctolite (near base of 2 m high section), SE Hallival, ELI. (c) Fine-scale layering in troctolite. WLI, near Loch an Dornabac. (d) Layered troctolite overlying peridotite ‘cobble avalanche’ deposit. Central Intrusion. (e) Deformed, steeply dipping layered troctolite containing peridotite blocks, Central Intrusion. (f) Coarse breccia of layered troctolite blocks in a peridotite matrix, Central Intrusion.
Details of layering in the Central Intrusion. (a) Intricate, fine-scale layering in peridotite. Scale: hammer shaft ~30 cm. (b) Peridotite block (‘dropstone’) in troctolite. The underlying layering has been strongly deformed. Scale: central block is ~20 cm across. (c,d,e) Details of peridotite ‘cobble avalanche’, as in 13(d). Scale: pen ~15 cm.
Photomicrographs of ‘classic’ cumulate(s) from the ELI (all crossed polarizers). (a) Strongly laminated troctolite (allivalite) with cumulus plagioclase > olivine > clinopyroxene. Unit 10, north of Hallival. (b) Troctolitic cumulate with cumulus calcic plagioclase enclosed poikilitically by clinopyroxene. Unit 11, northwest face of Hallival. (c) Feldspathic peridotite. Typical olivine cumulate with olivine enclosed poikilitically in plagioclase. Unit 8, northeast of Hallival.
(a) Peridotite fingers penetrating undisturbed, overlying layered troctolite. Central Intrusion, east of Minishal. Scale: hammer shaft ~30 cm. (b) Detail of finger structures from Fig. 16a recording corrosion/replacement of layered troctolite by peridotite. Note how the brown peridotite at the base of the section cuts across the layering and sends irregular finger-like projections into the overlying troctolite. (c,d) Photomicrographs of a chromitite seam at the boundary of Units 11 and 12, northwest of Hallival, (crossed polarizers). Note the sharp boundary of the chromitite seam with the adjoining chromite-poor peridotite (above) compared with the more gradational boundary against anorthositic troctolite (see below the seam). Numerous small, rounded inclusions in chrome-spinel crystals may contain biotite and/or amphibole.
(a) A 2 m high face north of Hallival shows the Unit 9 Wavy Horizon, separating layered troctolite from overlying dark, pyroxene-rich gabbro, ELI. The position of traverse g is indicated (modified after Holness, 2005). (b) Dihedral angles measured in three traverses through the Unit 9 troctolite and gabbro overlying the Unit 9 peridotite. The dihedral angles have been reset in troctolite adjoining the intrusive peridotite and at the troctolite/gabbro boundary, implying the underlying peridotite to be a later intrusive sill. The horizontal bar shows the comparatively small dihedral angle variation in Unit 10, Traverse ‘a’ (Holness, 2005, fig.1; = Traverse ‘A’ in Holness et al., 2012)) where the basal peridotite is an integral part of the unit and not a later intrusion (based on traverses e,g,h, in Holness, 2005, fig. 1)
The Unit 9 Wavy Horizon, ELI. (a) Undulatory contact between troctolite and overlying dark, pyroxene-rich gabbro. The lamination and weak layering in the troctolite is approximately parallel to the layering and lamination in the overlying gabbro yet plumes/fingers of one have punctuated the other. Locality close to Fig. 13. Scale: hammer shaft ~35 cm. (b–e) Photomicrographs of Unit 9 lithologies (all crossed polarizers). (b) Peridotite with large poikilitic clinopyroxene. (c) Allivalite (troctolite) with feldspar lamination. (d,e) Pyroxene-rich gabbro immediately above troctolite at the Wavy Horizon. The clinopyroxene is strongly laminated.
(a) Extensive slumping in the Unit 13 troctolite, on the north side of Hallival, ELI. Scale: hammer shaft ~30 cm. (b) Deformed layering in the Unit 14 troctolite with structures that share similarities to slump structures in sedimentary rocks. South face of Askival.
(a) Harrisitic structures in peridotite and gabbro of the Western Layered Intrusion. Surface showing the weathering out of elongate olivine crystals in typical harrisite for comparison with the smooth surface of the underlying layered peridotite (below hammer). Episodic growth of harrisites is indicated. (b,d) Meshwork of harrisitic olivine crystals. Harris Bay. (c) Elongate, skeletal olivine crystals (~1 m long) WLI. (e) Fragments of olivine crystals in normal peridotite WLI. These have been broken off from the underlying harrisite layer (see Fig. 21a,b). (f) Spherulitic growth of feldspar in troctolite (‘poikilo-macrospherulitic’ structures, Donaldson et al., 1973), Central Intrusion. Scale: pen ~15 cm.
(a,b) Successive stages in the formation of harrisitic structures (after O'Driscoll et al., 2007). Harrisitic olivine growth (a) is repeatedly interrupted by replenishments to produce both in situ and broken varieties (b).
(a,b) Photomicrographs of picrite dyke ‘M.9’ (see Upton et al., 2002) that intruded peridotite in the WLI (crossed polarizers). Quarry at summit of Harris road, east of Ard Nev. Olivine can be clouded (large crystal at top-left in (b)) and often appears black in hand specimen, probably a high-temperature oxidation effect. In (b) the groundmass contains colourful olivine together with fine-grained needle-like plagioclase and small clinopyroxene crystals. M.9 is one of the most primitive (basic) compositions known from Rum.
(a) Polished surface of downward-projecting chrome-spinel-rich cones developed on a chromitite seam (the Main Seam) at the contact of Unit 8 peridotite with underlying Unit 7 anorthositic troctolite. The cone structures transgress the flat-lying feldspar lamination in the troctolite and a package of chromitite appears to be detached from the right-hand cone, or, represents an elongation outside the area of view. Head wall of Coire Dubh. (b) A sketch of the relationships at the Unit 7/8 boundary shown in (a). A chrome-spinel-poor zone overlies the main seam and is in turn overlain by chrome-spinel-enriched peridotite. A subsidiary chromitite seam is underlain by laminated troctolite and separated from the overlying main seam by up to 20 cm of well laminated anorthositic troctolite that contains the detaching tip of a cone and is traversed by a near-vertical clinopyroxene veinlet. (c) Flat-lying, elongate cumulus olivine crystal apparently truncated at the Main Seam (crossed polarizers; crystal 4.5 mm long; after O'Driscoll et al., 2010).
(a,b,c) Ar/Ar age determination of early acidic rocks (after Troll et al., 2008). (a) Isochron derived from analysis of 20 single crystals separated from the early Rum rhyodacite. Note the high mean square weighted deviation (MSWD) indicating more than one age population is present amongst the sampled crystals. (b) Frequency distribution of crystal ages. The main peak at 60.33 is probably the eruption age of the early rhyodacite. Two older groups of crystals appear to be present, represented by the two shoulders to the right of the main peak. (c) Simplified age framework for Rum. If older ‘xenocrysts’ in the rhyodacite are discarded, a duration for the lifespan of the Rum volcano can be derived. The Rum volcano was probably short lived (<1 m.y.), as were other volcanoes in the province (e.g. Hamilton et al., 1998).
The Canna Lava Formation in western Rum. (a) Pale-coloured outcrops and scree of the Western Granite on Ard Nev (left distance) and Orval (right distance). On Orval, basaltic hawaiite lavas of the Orval Member (Canna Lava Formation) form an unconformable cap to the granite. Peridotite of the Central Intrusion crops out in the foreground. (b) Western Granite crags (right) on Orval, overlain by later lavas of the Canna Lava Formation. Further lavas form the middle-distance hill, Fionchra, where they overlie Torridonian sandstone. The pale crags and talus in the foreground are in the Western Granite. The gabbro Cuillin hills of the Skye Central Complex are seen in the far distance. (c) Inter-lava fluviatile conglomerate containing clasts derived from the Rum Central Complex and surroundings, Canna Lava Formation, south side of Fionchra. Scale: hammer ~30 cm. (d) Pillow-lava textures in hyaloclastites at the northeast side of Fionchra, marking the base of the upper Fionchra Formation. Scale as in (c). Based on Emeleus (1997), with the permission of the British Geological Survey and Emeleus and Troll (2008).
Schematic representation of possible events leading to the formation of the Central Intrusion. Periodic replenishments of picritic magma (1) rejuvenated the magma chamber causing sliding and slumping and (2) intruded laterally into earlier cumulates leading to partial melting and cumulate recycling (3). Magma fountaining into the chamber (4a) flows off the roof and down the sides as crystal-laden, gravity-driven currents (4b), dislodging crystal mushes as they move, then spread across the floor, reworking cumulate debris and depositing this material and primary crystals on the floor (4c). Movement on faults was accompanied by magma injection, thermal erosion of earlier rocks, fragmentation to form breccia zones (5a), and dilation by intrusive brecciation of wall cumulates (5b). Slides of coherent blocks of cumulate across partly liquefied cumulate led to spectacular slump mélanges (6). Based on Emeleus and Bell (2005), with the permission of the British Geological Survey.
Pre-Palaeogene Lewisian gneisses (Amphibolite-facies) overlain by >2.5 km of mid-Proterozoic Torridon Group sandstones and siltstones, a thin covering of lower Jurassic Broadford Beds (limestone, sandstone and siltstone) and Paleocene basaltic lavas of the Eigg Lava Formation, all intruded by dykes of the Muck swarm and the Rum sub-swarm. The Long Loch Fault was probably active well before the Palaeogene.
Palaeogene Rum Central Igneous Complex: Stage 1 Initiation of the MRF accompanied by major central uplift, with subsequent subsidence leading to caldera formation, the collapse of caldera walls and the formation of debris avalanche deposits, with concomitant intrusion and effusion of acidic and mixed acid/basic magmas and emplacement of the Am Mam intrusive breccia. Severe distortion of country rocks adjoining the MRF and collapse of major masses of country rocks off the rising dome, as at Welshmans Rock, etc. Emplacement of the Western Granite. Possible further central uplift on the MRF.
Palaeogene Rum Central Igneous Complex: Stage 2 Change to basaltic and ultrabasic magmatism, heralded by the intrusion of basaltic cone-sheets and dykes of the Rum sub-swarm, followed by emplacement of ultrabasic rocks and gabbros of the Central Intrusion, with construction of the Layered Suite (the Eastern and Western layered intrusions) through successive sheet-like injections of ultramafic and basaltic magmas from the Central Intrusion but ultimately supplied by feeders on the Long Loch Fault. Intrusion of numerous gabbro and peridotite plugs pre- and postdating the MRF. Further central uplift probably accompanied emplacement of Stage 2 as the Central Complex developed beneath a cover of earlier uplifted rocks (Stage 1 and pre-Palaeocene) and (contemporaneous?) lavas.
Post Central Complex Immediately following the formation of the Central Complex, a succession of deep valleys developed during vigorous erosion of the Rum Volcano, becoming rapidly filled with coarse clastic debris (conglomerates, breccias) derived from the Central Complex and surroundings; these are interbedded with predominantly basaltic lavas belonging to the Canna Lava Formation (Palaeocene Skye Lava Group) which ponded against the flanks of the volcano. Some north to northwest-trending basalt dykes post-date the Canna Lava Formation but regional dykes were probably intruded throughout the Paleocene and a subsequent heating event (45 Ma) indicates that activity may have been prolonged. Significant right-lateral movement on the Long Loch Fault postdates the Central Complex and Rum underwent further deep erosion by local glaciers and by ice from the Scottish mainland during the Pleistocene.