Central Intrusion, road to Harris and west and south of Loch Long - an excursion

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Geological map of Excursion 4A, Central Rum, covering the northern end of the Central Intrusion and adjoining layered areas of the Eastern Layered Intrusion and Western Layered Intrusion Extract from BGS 1:50,000 map of Rum © NERC Full map and key.
Peridotite blocks (‘dropstones’) with deformed layering (impact structures?) in layered troctolite of the Central Intrusion. Scale: pen 13 cm. Locality 4.3.
Layered troctolite in the Central Intrusion. Locality 4.4, ‘whale-back’ ridge west of Long Loch.
Detail of the peridotite ‘cobble’ avalanche deposit as in Figure Locality 4.4. Central Intrusion. Scale: pen c.15 cm.
Graded and slumped layered troctolite cut by thin basalt sheets at the east side of the ‘whaleback’ ridge. 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 (see arrows).n Locality 4.5 Central Intrusion, ‘whaleback’ west of Long Loch. Rock-face is c.2–5m high.
Large triangular block of layered troctolite and peridotite enclosed in disturbed troctolite draped over the top. Locality 4.6, about 120 m south-south-east of Long Loch. Central Intrusion. Scale: hammer shaft at left centre 30 cm.
Pronounced, narrow trench eroded along the course of the Long Loch Fault. (Near Locality 4.6.) Looking south. Central Intrusion, south of the Long Loch.
Portion of ultrabasic intrusion breccia (conduit fill) in the Central Intrusion, Abhainn Rangail. Scale: hammer shaft 30 cm. Locality 4.18.
Breccia of layered troctolite blocks in a feldspathic peridotite matrix (debris avalanche type), Central Intrusion, Harris track near Locality 4.7.
Radiating rays of plagioclase in feldspathic peridotite (‘poikilo-macro-spherulitic structures’) in Central Intrusion east of Loch MacIver (Loch an Dornabac). [NM3569 9772], Locality 4.7. Scale: pen 13 cm. INSET: Close up of a poikilo-macro-spherulite.
Rhythmic harrisite and ‘sedimentary’ layers in feldspathic peridotite. Sedimentary layers contain recycled harrisite crystals. Scale: hammer shaft c.15 cm. Locality 4.11, Western Layered Intrusion, east of Ard Mheall. INSET: Fragments of dark harrisite olivines that have been ‘reworked’ by sedimentary processes (see arrows).
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 (2) and intruded laterally into earlier cumulates (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 and their fragmentation to form breccia zones (5). Slides of coherent blocks of cumulate across partly liquefied cumulate led to spectacular slump mélanges (6). (Emeleus et al. [1996]. After Emeleus and Bell [2005].) (© NERC)

Excursion 4A 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

Highlights (Excursion 4A) Exposures in the Central Intrusion close to the road to Harris and to the west and south of the Long Loch include excellent examples of layered troctolite, apparent erosion of troctolite by ‘debris flows’ crowded with peridotite ‘cobbles’ and ’pebbles’, and blocks of layered troctolite and feldspathic peridotite ranging in size from a metre to many tens of metres in breccias with highly disturbed feldspathic peridotite matrices. Further south, unusual feldspar and olivine growth structures occur in the Central Intrusion and also in the Western Layered Intrusion, which may be visited in the latter part of the day. The excursion may be extended (Excursion 4B) by continuing to Harris Bay (a long walk), where layering in gabbros extends to within a few metres of the contact with microgranite and where the complicated contact between the Western Layered Intrusion and the Western Granite is excellently exposed in sea cliffs.

Follow the Kilmory road from Kinloch for 3 km then take the south fork towards Harris (See image).

Excursion 4A

Locality 4.1 Harris road near Kilmory fork – quarry shows sandstone baked by dolerite plug [NG 3696 0016]

A small roadstone quarry on the south side of the road exposes a dolerite plug with adjoining bleached, fractured and baked sandstone. The alteration is typical of that encountered next to the dolerite and gabbro plugs found throughout northern Rum. From this locality the distinctive, localised grey scree derived from fragmented sandstone next to a substantial gabbro plug may be seen on the western slopes of Mullach Mòr, about 2 km to the north-north-east (at [NG 376 012]). Another 400 m along the road a contact between peridotite and baked sandstone is exposed in a stream bed above a bridge [NG 3672 0000], but the degree of alteration appears much less than that found adjacent to gabbro or dolerite plugs.

Locality 4.2 Bridge south of Long Loch – evidence for Long Loch Fault in riverside exposures [NM 3639 9944]

The Long Loch Fault follows the course of the Kilmory River at the bridge, and fractured, sheared and crushed peridotite, microgranite, sandstone and gneiss are exposed in the river bed hereabouts. The latest movement on the fault post-dates the central complex, but the fault is thought to have had significant pre-central complex movement. Thermally altered gneiss (pyroxene hornfels) crops out in sparse exposures in a low, east-facing scarp about 100 m west of the bridge. Continue along the road to a shallow east–west valley through peridotite [NM 3617 9919].

Locality 4.3 North of Harris road – layering in troctolite deformed by ‘dropstones’ [NM 3615 9929]

Blocks of troctolite are enclosed in feldspathic peridotite about 150 m to the north of the road. Distortion of the layering around the blocks resembles that seen where dropstones have impacted into bedded sedimentary rocks (See image).

South of the road a low, ‘whaleback’ ridge extends north–south for about 500 m on the west side of the Long Loch. Westward-dipping (30–40º) layered troctolites are magnificently exposed on flat, glaciated surfaces along much of the length of the ridge. They are interpreted to be within a ‘megablock’ that spalled off the adjoining Eastern Layered Intrusion, subsiding into magma rising along feeders located on the early Long Loch Fault.

Locality 4.4 ‘Whaleback’ west of Long Loch – depositional and erosional sedimentary structures in troctolite [NM 3620 9908]

Troctolite seen on the glaciated slabs contain slump, scour, ‘flame’, and graded-bedding structures resembling those found in clastic sedimentary rocks (See image). The upper surface of the layered structures appears to have been eroded by a ‘debris avalanche’ of peridotite laden with rounded pebble- and cobble-sized peridotite fragments which also show some size-grading (See image).

Locality 4.5 ‘Whaleback’ – layering in troctolite, transgressed by underlying peridotite [NM 3623 9893]

Particularly good three-dimensional examples of the layering occur in a low cliff on the east side of the ridge (See image). At this locality, the troctolite is underlain by brown peridotite which has finger-like protrusions extending upwards into the layered rocks (cf. Locality 6.2). Towards the south end of the ridge, the peridotite that underlies layered troctolite at Locality 4.5 cross-cuts and locally replaces troctolite.

Continue to the south end of the Long Loch [NM 3627 9810] (but note that the stream at [NG 3628 9853] may be difficult to cross after heavy rain), cross the dam and continue a further 200 m to the south-south-east.

Locality 4.6 South of Long Loch – layered troctolite/peridotite blocks in deformed layered peridotite [NM 3635 9792]

The layered peridotites exposed in small cliffs and scarps are extremely deformed around large blocks of troctolite and layered peridotite that have collapsed into them. In some instances the blocks, which range from metres to tens of metres in size, are themselves layered, and individual blocks may preserve a record of magmatic sedimentation, slumping, corrosion and replacement (See image). The blocks are thought to have avalanched off the edge of the Eastern Layered Intrusion during emplacement of the Central Intrusion, deforming the poorly consolidated feldspathic peridotite cumulates. About 150 m to the south, across a small stream, steeply dipping layered troctolite exposed in an elongate (north–south) knoll is another example of one of these megablocks.

Cross the 50 m-wide, steep-sided valley marking the course of the Long Loch Fault (See image) and walk uphill for 300 m in a westerly direction, until the outlet of a lochan is reached at [NM 3588 9782]. This traverse crosses part of the Central Intrusion characterised by approximately north–south zones of peridotite breccias (Donaldson, 1975), commonly crowded with blocks of peridotite and troctolite, some of which are layered (Figure 40). This traverse crosses part of the complex feeder zone to the Layered Centre of the Rum Central Complex (See image).

Locality 4.7 Loch an Dornabac – poikilo-macro-spherulitic structures in peridotite of Central Intrusion [NM 3569 9769]

These structures are on a shelf about 200 m to the south-west of the lochan. Here, spectacular bunches of radiating, bifurcating rays of plagioclase crystals occur in feldspathic peridotite (See image). The feldspars are up to 40 cm in length and enclose innumerable small olivine crystals. These are the type examples of poikilo-macro-spherulitic structures (Donaldson et al., 1973) and have grown in situ, possibly from a hydrous feldspathic peridotite magma. About 50 m to the east-north-east, olivine lamination in peridotite is deformed around peridotite inclusions.

Locality 4.8 North of Loch an Dornabac – small-scale layering in peridotite of Western Layered Intrusion [NM 3576 9800]

South-east-dipping, finely layered peridotite belonging to the Ard Mheall member (See image) at the top of the Western Layered Intrusion forms the cliffs 300 m north-north-east of Locality 4.6. The contact with the Central Intrusion is exposed a short distance to the north on the north-north-west-trending ridge.

To regain the Harris road (at about [NM 3527 9810]), walk 500 m west-north-west across wet peaty ground with scattered peridotite exposures. At this point, either (A) return to Kinloch along the road (c. 5 km) or (B) continue south through the Western Layered Intrusion towards its outer contact with the Western Granite at Harris.

Locality 4.9 Road east of Ard Nev – dense, black picrite dyke intruding peridotite [NM 353 984]

Following option ‘A’, walking towards Kinloch, there are several shallow abandoned quarries in crumbling peridotite on the east side of the track. Loose blocks of a dense, matt-black picrite come from a badly weathered picrite dyke. The highly magnesian picrite contains an abundance of forsteritic olivines (Fo93); (McClurg, 1982; Upton et al., 2002).

Locality 4.10 Roadside north-east of Ard Nev – baked microgranite and (nearby) altered dolerite, near peridotite [NM 3540 9870]

Farther along the road, baked microgranite (the Western Granite) is exposed in contact with peridotite. Off the road some 70 m to the west a thick, north-west-trending dolerite dyke intrudes the microgranite. This dyke, which forms a low ridge, has also been altered by the nearby peridotite. Continue along the road to Kinloch.

If proceeding towards Harris (option ‘B’, Figure 44) and (See image), follow the road south for about 200 m, then contour around the hillside for a further 200 m, passing derelict eagle cages on the way.

Locality 4.11 East of Ard Mheall – harrisitic structures in peridotite, some showing erosional features [NM 3502 9772]

Excellent harrisitic structures (Harker, 1908; Donaldson, 1974, 1976; O’Driscoll et al., 2007a) occur in layered peridotites of the Ard Mheall member (Western Layered Intrusion), exposed in a series of low, west-facing crags (See image). Elongate olivine crystals in the peridotite appear to have grown upwards for 5–20 cm from a substrate of granular olivine. In places, dark, platy olivine crystals are arranged in an imbricate manner, suggesting they have been broken from the tips of harrisitic structures and redistributed by magmatic currents (See image), (See image). Walk downhill to the road and continue south for 2 km. Exposures are few and the ground is largely drift covered.


References

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