Moine geology of East Glenelg and Loch Duich, East Glenelg - an excursion

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From: Strachan, Rob, Friend, Clark, Alsop, Ian, Miller, Suzanne (Editors). A Geological excursion guide to the Moine geology of the Northern Highlands of Scotland.: Edinburgh Geological Society, Glasgow Geological Society in association with NMS Enterprises, 2010.

By Craig Storey

Fig. 7.1 Geological map of Glenelg and Loch Duich, showing the locations of (Fig. 7.2), (Fig. 7.4) and (Fig. 7.5).
Fig. 7.2 Geological map north of Glen More covering Localities 7.1A-E (modified from Sutton & Watson, 1959, plate 9).
Fig. 7.3 Thin section of typical eclogite from Locality 7.1E.Grt = garnet; Rut = rutile; Omph = omphacite. Field of view = 3 mm.
Fig. 7.4 Geological map south of Glenmore covering Locality 7.2A-C (modified from Ramsay, 1958, plates 37 & 38).

Excursion 7 East Glenelg and Loch Duich is composed of the following articles:[edit]

Excursion 7 East Glenelg and Loch Duich, East Glenelg - an excursion[edit]

Locality 7.1 North of Glen More [NG 8397 2039] to [NG 860 234][edit]

North of Glen More (Fig. 7.2). A range of characteristic lithologies within the eastern Glenelg inlier, including spectacularly preserved eclogite facies assemblages and deformation fabrics.

From Glenelg village, drive east up Glen More and after ~3km park opposite a small farm (Iomairaghradain) that is on the south side of the road close to the Lamont Holiday Homes at Creag Mhor. Parking here is limited to two cars or one minibus, but passing places are closely spaced along the road so further vehicles could be parked close by. Allow a whole day for this locality.

A path leads up on the north side of the road through a gate and skirts the east side of Cnoc Mór which is Locality 7.1A [NG 836 203]. To the right (east) of this path, on the hillside, black-weathering marble contains numerous disharmonic folds. Head westwards uphill towards the summit where eclogite and retrograde amphibolite are widely exposed. Coarse-grained dark green omphacite and dark red garnet are preserved in patches, often transected by mm-scale veinlets where the rock has been converted chiefly to pargasitic amphibole. The best preserved eclogite occurs where the amphibolite veins are less intense. Where the mafic rock has a strong fabric, wholesale retrogression has generally occurred and often the only remnant of the high-pressure paragenesis is in the form of kelyphitic (symplectic) amphibole and plagioclase rims partially replacing garnet.

Head WNW towards Creag Dubh [NG 826 207], approximately 1km from Cnoc Mór. On the highest point of Creag Dubh, Locality 7.1B, are exposures of interlayered felsic and mafic rock. The mafic layers occasionally contain relict garnet surrounded by kelyphites (symplectites), but generally are retrogressed completely to amphibolite and epidote-amphibolite. In places, mafic layers are very quartz-rich and omphacite is sporadically preserved. This may indicate dehydration reactions that occurred during eclogite facies metamorphism, particularly in the surrounding felsic gneisses. Garnet is mostly only preserved in the felsic gneisses as relicts or pseudomorphic replacements by amphibole and/or biotite. However, SW of the summit, at [NG 8255 2055], a coarse-grained felsic layer contains omphacite, garnet and kyanite, recording eclogite facies conditions of 20kbar and 750°C (Storey et al., 2005). These rocks are strictly high-pressure granulites due to the presence of plagioclase, although their peak metamorphism occurred at eclogite facies. The basement here is moderately strained, but not mylonitised. However, the relative ages of the protoliths of the mafic and felsic gneisses are difficult to establish on the basis of field evidence alone. The author’s unpublished work indicates that the protoliths of the felsic gneisses are Late Archaean in age, whereas geochemical evidence, from Hf isotopes in zircon from the eclogites, suggests that their protolith may have formed close to 2.0 Ga (Brewer et al., 2003). It therefore seems likely that the protoliths of the eclogites intruded the pre-existing felsic basement.

Retrace the route back towards Locality 7.1A until you find the path that led up from the road. Continue uphill on the path which finishes as the hillside flattens out; exposures ahead around [NG 838 209] continue NW to the southern termination of a prominent ridge at [NG 8365 2105]. Follow the ridge in a NNE direction examining the excellent exposure along the way. This is Locality 7.1C. These rocks are a different type of eclogite, termed by Sanders (1988) ‘streaky eclogite’. The rock is mafic with white quartzo-feldspathic streaks that vary from mm-scale isolated threads up to cm-dcm-scale networks of veins that form locally up to half of the rock mass. They typically are intensely rodded with a dominant L>S D1 fabric. Intervening eclogite patches also have a tectonic fabric defined by aligned omphacite grains and tabular garnet. Kyanite, sometimes preserved within the streaks at the microscopic scale, is also aligned and occasionally forms asymmetric fish, indicating non-coaxial shearing. Sanders (1988) demonstrated that the streaks formed during eclogite facies metamorphism, and Storey et al. (2005) estimated peak conditions at around 20 kbar and 750°C.

Along the length of this ridge, up to around [NG 834 223] (area marked on OS maps as Cruachan Meadhon), the rodding lineation plunges shallowly towards between 010° and 040°, which has been attributed to possible transcurrent shearing during eclogite facies metamorphism (Sanders, 1988).

Continue NE from Cruachan Meadhon towards the east side of Lochan na Beinne Faide. At [NG 86251 23497], in low cliff exposures on the SW side of the loch, browny-grey weathered ultrabasic rocks occur at Locality 7.1D. Rawson et al. (2001) described both garnet-bearing olivine websterites and their garnet-absent equivalents, websterite. The olivine websterites contain two pyroxenes, olivine, garnet, amphibole and minor magnetite and spinel. Websterites are essentially identical in mineralogy but lack garnet and, generally, the olivine has been replaced by serpentine and the pyroxenes by amphibole as a retrograde reaction. Similar rocks are reported by Rawson et al. (2001) from the north side of a small loch approximately 0.5km SE of Lochan na Beinne Faide [NG 866 233]. The garnet-bearing olivine websterite gave a pressure-temperature estimate of ~20 kbar and 730°C (Rawson et al., 2001). The relationship of the ultrabasic rocks to the other lithologies is unclear and it cannot be determined whether they represent tectonically emplaced alpine-type peridotites or whether they crystallized from a basaltic magma as a result of crystal accumulation. The agreement of the pressure-temperature estimate with that of the surrounding high-pressure granulites (felsic gneisses) and the streaky eclogites implies that they are cofacial, and thus achieved peak conditions at ~1082 Ma (during the Grenvillian orogeny), but their earlier history is enigmatic. If they are related to the basic protoliths of the eclogites, then they may have formed at ~2.0 Ga. On the north side of Lochan na Beinne Faide lies a large mass of exposure elongated in a NE-SW direction for approximately 2km, known as Beinn Fhada, Locality 7.1E. This mainly comprises a distinctive type of garnet and pyroxene-bearing felsic gneiss, which is trondhjemitic in bulk-rock composition. In fresh outcrops, omphacite and garnet co-exist with plagioclase. These rocks are strictly high-pressure granulites, although the presence of omphacite indicates eclogite facies conditions (Sanders, 1979). These rocks are considered identical to the more limited exposure of felsic high-pressure granulite at Creag Dubh, Locality 7.1B. Concordant mm- dcm-scale eclogite layers are interbanded with the felsic gneisses. It is within this exposure, within a thick eclogite layer at ([NG 860 234]; (Fig. 7.3)), that Sanders et al. (1984) obtained a garnet-clinopyroxene-whole rock Sm-Nd age of 1082 ± 24 Ma, interpreted to be close to the peak of eclogite facies metamorphism.

Summary of the early history[edit]

It is thought that the majority of the basement gneisses formed from trondhjemitic and granitic protoliths in the Late Archaean, and thus there are similarities with gneisses of the Lewisian Gneiss Complex within the Caledonian foreland and also with the gneisses of the western Glenelg inlier (see also Friend et al., 2008). The major difference is the preponderance of eclogite and paragneisses, which do not have a direct comparison, although possibly similar metasediments occur within the Gairloch region of the Lewisian outcrop. It is considered likely that the metasediments and the majority of the eclogite protoliths formed as part of a volcano-sedimentary sequence that accumulated upon pre-existing trondhjemitic gneisses possibly at around 2.0 Ga. The eastern Glenelg inlier was subsequently buried to depths of around 70 km and metamorphosed within the eclogite facies during the ~1.1-1.0 Ga Grenvillian orogeny, most likely as a result of the deep subduction of continental crust in a collision zone.

Locality 7.2 Between Glen More and Glean Beag [NG 823 193] TO [NG 8223 1873][edit]

Between Glen More and Glen Beag (Fig. 7.4). A traverse from the uppermost part of the western Glenelg inlier across the intervening ‘Moine strip’ and into the shear zone which defines the structural base of the eastern Glenelg inlier.

Either park in Glenelg village or it is possible to drive up a small partially made road that turns sharply to the left off the main road through the village as you head southwards; the junction is between the two entrances to the Glenelg Inn car park. The road follows the edge of the south side of Glen More towards a farm at Cósag [NG 823 193]. The farm is gated at the end of the road, but approximately 200m before the gate there is a disused croft and it is possible to park here on the side of the track. Space is limited to two cars or one minibus. Allow 3-4 hours for this locality.

Walk back towards Glenelg and, where the road turns a first right bend, cross the barbed wire fence and a small stream and walk uphill to the SW. After about 0.5km, around Locality 7.2A [NG 817 187] , low hummocky outcrops expose highly strained gneisses of the western Glenelg inlier. It is recommended that some of the localities in Excursion 6 are visited first in order to be able to recognize these lithologies in their lower strain state.

Here, these gneisses commonly contain a strong ductile D2 L-S fabric, with a mineral stretching lineation plunging moderately down-dip to the east. In exposures where banded tectonised mafic and felsic layers occur, surfaces parallel to the lineation show low-angle C’ shear bands that indicate a top- to-the-west sense of shear. Upright D3 dcm-scale extensional shear bands disrupt the earlier fabric and are associated with a mineral elongation lineation that plunges fairly steeply towards the SE.

To the east, around Locality 7.2B [NG 818 187] low angular outcrops expose Moine psammites assigned to the Morar Group with variably deformed feldspathic sedimentary clasts and thin pegmatitic segregations. The fabric is mylonitic and contains a strong mineral stretching lineation formed mainly by muscovite. Low-angle truncations within the mylonitic foliation are thought by Ramsay (pers. comm.) to be relicts of cross-bedding and therefore that the Moine has a modified unconformable relationship with the western basement inlier. However, the high state of strain makes this conclusion difficult to accept and, with a critical eye and looking on variously oriented surfaces, it is more likely in the view of the present author that these low-angle truncations originated as rootless, detached isoclinal folds within the mylonitic foliation. There are two mineral stretching lineations within the Moine rocks: the earliest plunges moderately towards the east (L2), whereas a later lineation, often associated with upright extensional shear bands, formed during D3. Occasional kinematic evidence from D2 fabrics can be found, particularly where pegmatitic layers are disrupted, and these indicate a top-to-the-west sense of shear.

To the east, around Locality 7.2C [NG 819 188] are exposed strongly banded ultramylonites of the eastern Glenelg inlier. Felsic and mafic components are interlayered on the mm scale, the latter commonly containing highly rounded porphyroclasts of garnet and dark green-brown amphibole. The ultramylonitic foliation is often disrupted by disharmonic folds with curvilinear hinges and sometimes eye structures can be observed. A strong mineral stretching lineation plunges moderately towards the east and the fold hinges are generally sub-parallel to this lineation. These are sheath folds formed during shearing. The state of strain is so high that it is impossible to gain kinematic information. The rocks remain mylonitic towards the east for several hundred metres, forming part of the Barnhill Shear Zone which separates the Moine rocks and the eastern Glenelg inlier (Storey, 2002). At the summit of Sgiath Bheinn, around Locality 7.2D [NG 8223 1873], the rocks are locally in a lower state of strain and it is possible to see disrupted migmatitic textures within the trondhjemitic gneisses.

References[edit]

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