Moine geology in South and Central Sutherland. Central Sutherland - 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, Maarten Krabbendam, Graham Leslie and Jack Soper
Excursion 10 South and Central Sutherland is composed of the following articles:
- Excursion 10 South and Central Sutherland - introduction
- South Sutherland. Localities 10.1 - 10.4
- Central Sutherland. Localities 10.5 - 10.6
Excursion 10 South and Central Sutherland. Central Sutherland
Locality 10.5 Vagastie Bridge [NC 5324 2712]
Park adjacent to the A836 in the large parking place immediately south of Vagastie Bridge [NC 5324 2712] (Fig. 10.1). There is sufficient space for a coach if necessary. Allocate ½-1 hour for this locality. The Vagastie Bridge granite represents one of the larger members of a series of deformed igneous intrusions that occur in Central Sutherland (Read, 1931), the Vagastie suite (Soper, 1971; Soper & Brown, 1971). The granite is not a single body, but a series of sheets that intrude Morar Group psammites. A detailed description of the petrology and field relationships of these granitic sheets is presented by Holdsworth & Strachan (1988) and only the most important points are covered here.
Walk to the bridge and examine the outcrops of a pink, coarse-grained augen granite by the banks of the stream on the east side of the bridge. A strong tectonic foliation defined by recrystallized feldspar augen dips gently to the ESE and carries a SE/SSE-trending mineral and extension lineation. NW-dipping shear bands are occasionally present and demonstrate a general top-to-the-NW sense of displacement parallel to the lineation. A sample of the augen granite collected a few metres NE of the bridge yielded a U-Pb zircon age of 424 ± 9 Ma, interpreted as dating intrusion of the igneous protolith (Kinny et al., 2003b). It therefore follows that the deformation that affects the granite must have occurred either during or after granite intrusion.
Cross the road and descend to the outcrops in the stream immediately beneath the bridge to examine the relations between the granite and its host Moine psammites. The psammites are deformed by a metre-scale, open-to-close, SW-overturning D2 fold pair, the upper hinge and short limb of which are cut by the base of the lowest granite sheet (Fig. 10.6). Contacts between the granite and its psammitic host are sharp and well-defined. Three important observations can be made: (1) the granite sheet is not folded; (2) it carries a foliation that is oblique to its margins and sub-parallel to D2 axial planes and S2 in the psammites; and (3) the SE/SSE-trending lineation within the granite is sub-parallel to the dominant (L2) lineation within the psammites. Granite-Moine contacts elsewhere display the same structural relations (e.g. [NC 5329 2726]) (Fig. 10.6).
Given that the granite sheets are not folded by D2, but nonetheless carry S2 and L2, the simplest interpretation is that granite intrusion occurred during D2 (Holdsworth & Strachan, 1988). Intrusion must have post-dated folding, but predated the later stages of fabric development. The U-Pb zircon age obtained for the Vagastie Bridge Granite thus demonstrates that D2 and correlative structures in this part of Central Sutherland occurred during the Scandian phase of the Caledonian orogeny (Kinny et al. 2003b).
If driving north from Vagastie Bridge, turn right at Altnaharra onto the B873 signposted to Syre and Bettyhill. Drive eastwards for c.13km and park by the roadside either side of the bridge at [NC 6405 3867]. There is space for four to five cars; allocate 6-7 hours for this locality, which involves c.14km of walking over hillsides and rough ground.
Follow the Allt Gruama Beg NW as far as Loch Morlach. Walk around the north shore of the loch to the small stream that enters the loch at [NC 6305 3925] and head northwards across moorland to Cnoc Liath. Lowlying exposures at 6A [NC 6288 4048] (Fig. 10.7) are massive to coarsely-foliated garnet-pyroxene gneisses of the Naver basement inlier. Although the igneous protoliths of this inlier are undated at the time of writing, a late Archaean age seems likely by comparison with the Borgie inlier just a few kilometres to the north (Friend et al., 2008). The high-grade metamorphic event that resulted in formation of the garnet-pyroxene gneisses has been correlated with the Scourian event in the Caledonian foreland (Moorhouse, 1976), but a much younger age cannot be ruled out (see also Locality 13.7, Excursion 13). Late retrogressive shear zones, defined by narrow bands of hornblende schist, cut these high-grade gneisses. Other exposures within a radius of 100m are of banded hornblende gneisses with concordant amphibolite sheets, more typical of large tracts of the Naver inlier. Gneissic banding dips to the ESE and carries a down-dip mineral lineation commonly defined by aligned hornblende. The hornblende gneisses were probably derived, at least in part, from retrogression of the garnet-pyroxene gneisses. Additional outcrops of relic garnet-pyroxene gneisses are present a little further to the north on a small hillock at [NC 6276 4070].
Traverse NE across numerous other low-lying outcrops of hornblende gneisses. The Morar Group rocks that structurally underlie the Naver basement inlier are exposed at 6B [NC 6356 4197] (Fig. 10.7) on a series of north-facing crags. These lithologies are banded, fine-grained psammites with thin semi-pelite bands. Note the lack of any evidence for high-grade metamorphism and migmatization, in contrast to those to be seen later above the Naver Thrust. The banding is interpreted as tectonically modified bedding; sedimentary structures are absent, but detrital feldspar grains are common. A mesoscopic, reclined D2 fold with S-geometry plunges to the SSE, parallel to a mineral and extension lineation defined by aligned micas and quartz-feldspar aggregates. Walk eastwards along strike, noting several metre-scale, foliated and boudinaged sub-concordant granite sheets, possibly members of the same suite as those seen at Vagastie Bridge.
Head eastwards across the moorland to the two lochs labelled ‘Loch Bad na Fheoir’ on the 1:25,000 scale OS map (Fig. 10.7). On the east side of the smaller of the two lochs at 6C [NC 6481 4191] are exposed very flaggy Morar Group psammites and semi-pelites. The high-strain fabric dips gently to the SE and carries a down-dip mineral and extension lineation that is inferred to lie parallel to the direction of tectonic transport along the overlying D2 Naver Thrust. Walk around to the SE side of the larger loch to [NC 6495 4184] where similar high-strain psammites are cut by thin, discordant pegmatites. A sharp contact separates these psammites from overlying biotite schists that are interpreted as highly retrogressed basement gneisses near the northern termination of the Naver inlier (Fig. 10.7). A series of low-lying exposures to the south (e.g. [NC 6495 4181]) show rather less-retrogressed, banded hornblende gneisses.
Walk upslope to 6D [NC 6501 4178] (Fig. 10.7), crossing the unexposed Naver Thrust, and traversing into extensive outcrops of banded psammitic, migmatitic gneisses of the Naver Nappe. These are deformed by highly attenuated isoclinal D2 folds of banding and open, asymmetric D3 folds, both plunging sub-parallel to the SSE-trending mineral and extension lineation. The gneisses are intruded by numerous concordant and boudinaged granitic pegmatites. Migmatization in Central Sutherland is thought to have occurred during the Ordovician Grampian phase of the Caledonian orogeny (c.470-460 Ma; Kinny et al., 1999). Traverse to the top of Cnoc Bad an Fheòir through similar lithologies, and then head east from the summit of the hill for c.1km to the next set of exposures on the west side of an unnamed hill.
A prominent set of outcrops at 6E [NC 6602 4166] (Fig. 10.7) comprise flaggy garnet-biotite schists that overlie the Torrisdale Thrust, a tectonic break within the Naver Nappe (Fig. 10.7). The thrust is not exposed here, but near the summit of Ben Klibreck to the SW is associated with interleaving of basement orthogneisses with Moine metasedimentary gneisses of the Naver Nappe (Strachan & Holdsworth, 1988). The garnet-biotite schists are intruded by a series of pegmatites that vary from early, foliated types to late, discordant and undeformed sheets. Traverse upslope through SE-dipping, banded psammitic and semi-pelitic gneisses with numerous concordant sheets and pods of foliated amphibolite, and abundant pegmatite. A SSE-plunging mineral and extension lineation is present locally. Head southwards from the top of the hill to reach the Allt Dail a’Thuraich.
Follow the stream southwards to 6F [NC 6537 3925] (Fig. 10.7) where coarse migmatitic pelites with abundant concordant pods and layers of coarse pegmatitic material are exposed in the stream bed. Follow the stream southwards to the main road, and then walk westwards for c.1km back to the vehicles.
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