Moine geology of North Sutherland. Torrisdale Bay - an excursion

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 Rob Strachan, Bob Holdsworth, Clark Friend, Ian Burns and Ian Alsop

Fig. 13.8 Geological map of Locality 13.8 (from Burns, 1994). Many granite sheets have been omitted for clarity. NT = Naver Thrust; TT = Torrisdale Thrust.

Excursion 13 Moine geology of North Sutherland is composed of the following articles:

• Excursion 13 North Sutherland - introduction
• The Melness area. Locality 13.1
• South and east side of Tongue Bay. Localities 13.2 to 13.5
• Loch Cormaic and Borgie peat cuts. Localities 13.6 to 13.7
• Torrisdale Bay. Locality 13.8
• Craig Ruadh to Glaisgeo. Localities 13.9 to 13.11
• Swordly Bay, Kirtomy Bay and Cnoc Mor. Localities 13.12 to 13.14
• Port Mor to Portskerra. Localities 13.15 to 13.18

Locality 13.8 Torrisdale Bay [NC 6875 6089] to [NC 6896 6169]

Torrisdale Bay (Fig. 13.8). The ductile Naver Thrust separating Moine Nappe psammites from high-grade meta-basic rocks and migmatitic gneisses of the Naver Nappe with Lewisianoid basement gneisses along the thrust; Caledonian granitic bodies emplaced during thrusting; development of the Torrisdale Steep Belt.

Turn off the A836 just west of Borgie Bridge [NC 6675 5872], drive northwards for nearly 3km and park by the roadside at [NC 6807 6111]. There is sufficient space for a coach; allow 3 hours for this locality. Walk down to the river, across the bridge, follow the path towards the raised beach over a second bridge, and then climb up the gorse-covered bluff in a gully with a small stream to emerge on a large flat grassy area by a wall. Head ESE to the lowest outcrops on the west-facing slope.

At 8A [NC 6875 6089] platy, high strain psammites of the Moine Nappe contain concordant, thin quartz veins and are deformed by asymmetric, minor D₂ isoclinal folds. These psammites lie in the immediate footwall to the Naver Thrust that occurs in unexposed ground along the line of the next gully uphill. The foliation dips rather more steeply eastwards than is the case at most of the localities thus far, and it carries a mineral and extension lineation that plunges gently to moderately to the SSE. The trend of the lineation does not correspond to the direction of tectonic transport during ductile thrusting. Instead, the steepening, composite fabric and associated low-angle lineation are associated with development of the Torrisdale Steep Belt (D₄) that reworks all pre-existing structures, including major ductile thrusts (Burns, 1994; Holdsworth et al., 2001a). Cross the gully containing the Naver Thrust and traverse southeastwards uphill into the next gully and then southeastwards across the next ridge to [NC 6884 6081] to view west-facing exposures of hornblendic basement gneisses that are interpreted to rest as an allochthonous sheet on the Naver Thrust. These gneisses display well-developed tight-to-isoclinal folds and ‘eye’ structures of probable D₂ age that are interpreted as cross-sections through the noses of sheath folds. The crags above these expose high strain, banded psammitic gneisses of the Naver Nappe, containing migmatitic layering and numerous deformed granitic and quartz veins. Gneissic layering is deformed by isoclinal D₂ and asymmetric D₃ folds. Walk downslope northwards, along the base of the slope, to the first outcrops by the sand dunes.

At 8B [NC 6874 6128] are banded, migmatitic psammitic gneisses of the Naver Nappe. Two generations of granitic rocks are apparent. A series of early, concordant, pink migmatitic leucosomes are attenuated and flattened parallel to the composite gneissic banding; the grey melanosomes that fringe melt layers are dominated by residual quartz. The migmatitic layering is cut by discordant pegmatitic veins of the Torrisdale Vein Complex (Holdsworth et al., 2001a). U-Pb SHRIMP dating of zircons from the early migmatitic phase sampled here yielded an age of 467 ± 10 Ma (Kinny et al., 1999). This is thought to date to mid-Ordovician (Grampian) high-grade metamorphism and migmatization of the Naver Nappe. An L₄ mineral and extension lineation plunges gently to moderately to the SSE.

Traverse northeastwards upslope across psammitic gneisses, including a prominent garnetiferous semi-pelite, across the (unexposed) Torrisdale Thrust and into an interbanded assemblage of strongly foliated amphibolites, hornblendic gneisses (locally with garnet) and augen granites – the Druim Chuibhe Orthogneiss Complex (Burns, 1994; (Fig. 13.8)). Geochemical data indicates that these lithologies are not part of the pre-Moine basement, and they are thus viewed as most likely constituting a series of possibly contemporaneous intrusions that were emplaced into the Moine rocks of the Naver Nappe. At 8C [NC 6886 6159], the amphibolites contain brown-weathering ovoid cores, 1-2m across, that correspond to a relict, anhydrous garnet-pyroxene metamorphic assemblage from which a P-T estimate of 650-700°C and 11-12kb has been estimated (Friend et al., 2000). These cores are cut by dark, hornblendic rehydration veinlets, and the margins of the cores pass imperceptibly into the host foliated amphibolites. A few similar smaller cores that are more highly retrogressed can be found along strike to the south. These high-pressure granulite-facies rocks were formed on a very different P-T trajectory to that associated with thrust-related (Scandian) deformation and metamorphism. These unusual assemblages are interpreted to be a relic of burial during early crustal thickening, perhaps during the Grampian orogenic event (Friend et al., 2000). The composite gneissic fabrics are deformed by at least two sets of folds: an early set of attenuated isoclines, probably of D₂ age and later asymmetric D₃ structures. Granitic pegmatite sheets of the Torrisdale Vein Complex, ranging in thickness from a few cm to 3-4m, cross-cut the ductile planar fabrics; these sheets are commonly either folded or boudinaged during dextral shear associated with the development of the SSE-plunging L₄ lineation. Those that were apparently intruded clockwise of banding were folded, whilst those that were intruded anticlockwise of banding were boudinaged. A dextral sense of shear parallel to L₄ is apparent from asymmetrically sheared porphyroclasts and granite veins.

Traverse further upslope across extensive exposures that display a variety of intrusive relationships between the granites and host orthogneisses. The percentage of mafic material within the orthogneisses decreases; a common lithology present is a grey, rather homogenous gneiss with feldspar porphyroclasts, and interpreted as a strongly mylonitized granitoid. Refolding of D₂ isoclines and ‘eye’ structures by asymmetric D₃ folds is apparent at [NC 6896 6169]. The eastern boundary of the orthogneisses with interbanded Moine semi-pelitic and psammitic gneisses is diffuse and apparently intersheeted. Once the presence of Moine rocks is established, it is worthwhile pausing to view the prominent granite sheets of the Torrisdale Vein Complex seen in the cliffs on the northwest and east sides of Torrisdale Bay. These granites, as well as those encountered at outcrop at this locality, are all thought to correlate broadly with the Strath Vagastie Granite (see Locality 10.5, Excursion 10) and similarly to have been emplaced during Silurian (Scandian) displacement along the Naver Thrust. Hornblende and muscovite 40Ar/39Ar ages obtained from basement and Moine lithologies across the Torrisdale Bay section described here all fall within the range c.420-415 Ma, corresponding to the time of cooling through closure temperatures, most probably shortly after the formation of the Torrisdale Steep Belt and intrusion of the Torrisdale Vein Complex.

References

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