Summary and pre- and post-Palaeocene geology of Rum

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From: Emeleus, C. H. and Troll, V. R. A geological excursion guide to Rum: the Paleocene igneous rocks of the Isle of Rum, Inner Hebrides. Edinburgh : Edinburgh Geological Society in association with NMS Enterprises Limited, 2008.

Introduction

The Paleocene Rum Central Complex (c. 60 Ma; Figure 2) is situated on a ridge composed of Archaean Lewisian gneisses and sandstones belonging to the Late-Proterozoic Torridon Group. The ridge is bounded to the east and west by basins filled with Mesozoic sedimentary rocks and Paleocene basalt lavas. On Rum, Torridon Group sandstones form the country rocks to the Paleocene central complex. Numerous north-west- to north-trending, predominantly basaltic dykes of Paleocene age traverse these sandstones, which are overlain in north-west Rum by Triassic sandstones and Paleocene lavas and conglomerates. Relict masses of Paleocene basaltic lavas, Jurassic sedimentary rocks, Torridon Group sandstones, and Archaean gneisses crop out within the central complex. The geological succession is summarised in Table 1 (page 8).

The Rum Central Complex developed in two distinct stages. During Stage 1, central uplift on a major arcuate fault system (the Main Ring Fault, MRF) was accompanied by felsic and mixed felsic/mafic magmatism and the formation of a caldera which filled with silicic ash flows, tuffs, and breccias formed by collapse of the uplifted dome and the unstable caldera walls. The country rocks were strongly domed over the central complex, probably accompanied by contemporaneous lateral displacement of large masses of sandstone, while uplift within the ring faults brought masses of Lewisian gneiss and the basal members of the Torridon Group close to the present erosional levels, with later subsidence resulting in the preservation of Jurassic sedimentary rocks and Paleocene basaltic lavas. Slightly later, several microgranites were intruded, including the Western Granite.

Table 1: Sequence of faulting, folding and intrusion in the Rum Central Complex

Table :	Sequence of faulting, folding and intrusion in the Rum Central Complex
1	Pre-Palaeogene: tilt to west of Triassic and Torridonian strata; faulting in these successions; early movement on the Long Loch Fault?
2	Doming of the Torridonian strata around the central complex accompanied initial uplift, with formation of the Welshman’s Rock and Mullach Ard faults as country rocks slid off the dome. Fault blocks broke up and behaved independently, the Welshman’s Rock block rotating c.90°.
3	Initial uplift on the Main Ring Fault (MRF): Lewisian and basal Torridonian uplifted by as much as 2 km, also tilting of elevated block to the east.
4	Subsidence on the MRF: eruption of rhyodacite ash flows, intrusion of rhyodacite along the MRF, intrusion of tuffisites, collapse of caldera walls to form breccias and megabreccias, intrusion of the Am Màm Breccias; subsidence brings Broadford Beds and Eigg Lava Formation flows down c.1 km within the MRF.
5	Emplacement of the Western Granite (may have been associated with movement that formed the inner component of the MRF).
6	Final uplift on the MRF (inner component).
7	Emplacement of radial dykes, regional north-west-trending dykes, and cone sheets.
8	Formation of a Loch Scresort–Glen Shellesder Fault?
9	Emplacement of the Eastern and Western layered intrusions.
10	Emplacement of the Central Intrusion – re-activation of the Long Loch Fault?
11	Small radial faults within the Central Intrusion and Eastern layered intrusion.
12	Accumulation of the Canna Lava Formation (Skye Lava Group), with concomitant erosion of the Rum Central Complex.
13	Long Loch Fault (final movement); faults in Canna Lava Formation.

Stage 2 commenced with the intrusion of a set of basaltic cone-sheets and numerous basaltic dykes, many of which trend north-west to north-north-west and belong to the Rum Dykeswarm. Emplacement of the Rum Layered Centre (feldspathic peridotites, troctolites and gabbros) followed. On Hallival and Askival, in eastern Rum, these mafic and ultrabasic rocks form prominent, gently-dipping layers (generally termed ‘Units’) and comprise the Eastern Layered Intrusion (formerly ‘Series’). Layered rocks also occur in south-west Rum where they form the Western Layered Intrusion. The Central Intrusion separates the Western and Eastern layered intrusions. This comprises a north–south belt of igneous breccias composed of blocks and megablocks of bytownite troctolite and feldspathic peridotite enclosed in matrices of similar compositions. The Central Intrusion is regarded as the feeder system for the Layered Centre. It is located along a major north–south fracture, the Long Loch Fault. Numerous sheets and plugs of gabbro and feldspathic peridotite intrude the layered rocks, and they are also found as plugs throughout the country rocks. A few dykes, including rare picrites, also intrude the Layered Centre.

After Stage 2: a major volcanic edifice was likely built over Rum during stages 1 and 2, but subsequent (and probably also contemporaneous) erosion rapidly reduced this to a hilly landscape. Evidence for this comes from north-west Rum where the Western Granite and sandstones of the Torridon Group are overlain by predominantly basaltic lava flows and intercalated fluviatile conglomerates, belonging to the Canna Lava Formation (c. 60 Ma). The flows and conglomerates have buried and preserved a hilly landscape dissected by steep-sided valleys that drained central Rum. The interlava conglomerates contain abundant clasts of red sandstone and gneiss, together with rhyodacite, microgranite, troctolite and gabbro, all clearly derived from the central complex. Clasts derived from Rum have also been identified in conglomerates belonging to the Canna Lava Formation on Canna and Sanday (Emeleus, 1973) and in conglomerates interbedded with lavas belonging to the Skye Lava Group in south-west Skye. Since the Skye lavas pre-date the earliest gabbros of the Paleocene Cuillin Centre on Skye (59 Ma), the Rum Central Complex (60.5 Ma) was clearly extinct and thoroughly dissected before intrusion of the earliest members of the Skye Central Complex.

There is a gap in the geological record from the Paleocene until the Pleistocene Epoch, when the island was almost completely covered by the Main Late Devensian ice sheet sourced in mainland Scotland. At a later stage, during the Loch Lomond Stadial, it supported a local ice cap with several valley glaciers. The ice had gone by about 11,500 BP and there is evidence that Man arrived fairly soon thereafter; at Kinloch a recently excavated site yielded implements made from the bloodstone found in the lavas of north-west Rum. Remains from this site have been dated at about 8,500 BP.

Pre-Paleocene Geology

Lewisian Gneiss Complex

Archaean gneisses crop out along and within the Main Ring Fault (Figure 2; Tilley 1944; Bailey, 1945, 1956). They include interbanded felsic and mafic varieties and amphibolites after original mafic dyke or sheet intrusions. The outcrops are generally fault-bounded or cut by later intrusions but at a few localities gneiss is unconformably overlain by coarse-grained sandstone at the base of the local Torridonian succession; for example, in Sandy Corrie [NM 374 940], and near the Priomh-lochs [NM 370 986]. The gneisses have been thermally metamorphosed to varying degrees and felsic varieties may show signs of partial melting (e.g. Holness and Isherwood, 2003).

Torridon Group

The group is part of the more extensive Torridonian succession found on the mainland and is represented on Rum by a succession of sandstones, siltstones and, locally, sedimentary breccias totalling at least 2500 m in thickness, and several of the mainland formations are recognised (See image). The rocks are largely unmetamorphosed, except in the vicinity of the central complex and adjacent to plugs and other minor intrusions (e.g. Holness and Isherwood, 2003). The group is best developed in the north of Rum where the beds dip consistently west to west-north-west at 10° to 30°, giving rise to the pronounced terrace featuring seen, for example, on Monadh Dubh (See image); however, where affected by doming in the vicinity of the Main Ring Fault, the dips are commonly steep (Excursions 1 and 2).

Medium- to fine-grained feldspathic sandstones of the Applecross Formation form most of the Torridon Group succession on Rum. This formation lacks good marker horizons but members of other formations have distinctive lithologies that have proved to be of considerable use in elucidating the structure of Rum. They are the dark-coloured, fine-grained siltstones of the Laimhrig Shale Member (TCDL), the coarse-grained gritty sandstones of the Fiachanis Gritty Sandstone Member (TCDF) and the fine-grained sandstones and siltstones of the topmost Sgorr Mhòr Sandstone Member (TCSM), characterised by the presence of dark grey to black beds rich in heavy minerals (principally magnetite, but also zircon, garnet, sphene and rare green tourmaline) (See image). The rocks of the Torridon Group on Rum are considered to have been laid down within a major fluvial braidplain (Nicholson, 1992, 1993).

Mesozoic strata

Sedimentary breccias, gritty sandstones and calcareous sandstones and siltstones of the Triassic Monadh Dubh Sandstone Formation crop out in small outliers in north-west Rum. Cornstones (caliches) are present and are particularly conspicuous at the angular unconformity with the Torridon Group rocks (Excursion 6). Rare ostracods and ill-preserved plant remains occur in the uppermost beds (Bailey, 1945; Steel, 1974, 1977; Emeleus, 1997). The Triassic rocks of Rum are probably the feather-edge of the Mesozoic Minch Basin (Binns et al., 1974; Fyfe et al., 1993).

Coarse-grained grey marble, calc-silicate hornfelses, quartzite and baked mudstones crop out south of Allt nam Bà and on the northern slopes of Dibidil. Poorly preserved fossils of Early Jurassic age have been recovered from these rocks which are correlated with the Broadford Beds of Skye (Smith, 1985). These beds are preserved in fault-bounded slices on the Main Ring Fault. At Allt nam Bà, where they are in contact with Marginal Gabbro of the central complex, the hornfelsed rocks contain the calc-silicate minerals spurrite, tilleyite and harkerite, indicating high-grade sanidinite-facies thermal metamorphism (Excursion 7; Hughes, 1960b; Emeleus, 1997).

Paleocene

Pre-Stage 1

Basaltic lavas belonging to the Eigg Lava Formation probably covered much of Rum prior to initiation of Stage 1 of the central complex. These lavas are now restricted to faulted slivers of basalt within the Main Ring Fault in eastern Rum. Additionally, locally abundant xenoliths of basic granulite-facies hornfels (‘beerbachites’) up to 10 m long occur in ultrabasic rocks in the Eastern Layered Intrusion (Stage 2). They are considered to be foundered blocks derived from lavas that roofed the central complex (Excursion 3). Clots and veins in the xenoliths contain grossular, calcic plagioclase and iron-rich pyroxene (ferri-fassaite), possibly derived from the metamorphism of lava amygdales (Faithfull, 1985).

It is likely that some of the numerous north-west- to west-north-west-trending basaltic dykes that intrude the Torridonian beds pre-date the central complex since these and less common sheets are affected by movements on the Main Ring Fault. However, conclusive proof is elusive and the majority of the dykes on Rum probably belong to the early part of Stage 2 (see below).

References

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