Moine geology of North Sutherland. Loch Cormaic and Borgie peat cuts - 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 Rob Strachan, Bob Holdsworth, Clark Friend, Ian Burns and Ian Alsop

Fig. 13.1 Simplified geological map of north Sutherland together with the localities for the excursion.
Fig. 13.6 Geological map of Localities 13.6 and 13.7 (modified from British Geological Survey, 1997).
Fig. 13.7 D3 folds deforming interbanded psammite and garnetiferous semi-pelite at Locality 13.6A.

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

Excursion 13 North Sutherland. Loch Cormaic and Borgie peat cuts[edit]

Locality 13.6 Loch Cormaic [NC 6234 5858] to [NC 6291 5766][edit]

Loch Cormaic (Fig. 13.6). Complex D2 and D3 folds, Loch Cormaic Metagabbro.

At Strathtongue on the A836, take the minor road to Dalcharn and park at the end of the track by a gate [NC 6222 5864]. There is space for two minibuses or four to five cars. Allocate 2-3 hours for this locality. Go through the gate and ascend the small knoll on the hillside to the SE. At 6A [NC 6234 5858], banded psammites with concordant quartzo-feldspathic segregations (the onset of melting?) are deformed by numerous complex D2 folds. These display highly curvilinear axes, resulting in closed ‘eye’ structures and opposing vergence patterns. Most of the fold hinges plunge to the SE, parallel to a strong mineral and extension lineation (L2 ). Coaxial, asymmetric D3 folds (Fig. 13.7) locally refold D2 structures.

Descend and follow the track southeastwards to Loch Cormaic. Note the presence by the track just to the north of the loch [NC 6259 5841] of a tiny outlier of red conglomerate of uncertain age. If water levels allow, continue on the track on the east shore of the loch. At 6B [NC 6283 5803] by the lochside, banded psammites with some semi-pelite bands are deformed by a mesoscopic D2 isoclinal fold pair with ‘Z’ geometry and a penetrative, axial-planar S2 schistosity. Although a strong pre-D2 fabric is commonly present, in places the strain is lower to reveal probable cross-bedding. Hillside exposures above the SE end of the loch (e.g. [NC 6288 5771]) contain examples of cross-bedding within psammites, here deformed by SE-plunging D3 folds.

Craggy exposures to the southeast are of the Loch Cormaic Metagabbro (a member of the Loch a’ Mhoid Metagabbro Suite of Moorhouse & Moorhouse, 1979). It is worthwhile first of all examining the coarse, little-deformed metagabbro that is characteristic of the internal part of the body. At 6C [NC 6291 5766] coarse, randomly oriented hornblende and plagioclase grains define a relict ophitic texture. The boundaries between plagioclase and hornblende clusters are mantled by metamorphic garnet. This may indicate that the mafic domains, now amphibole aggregates, once represented igneous clinopyroxene. Relict layering on the dcm scale is indicated by the alternation of mafic-rich and mafic-poor bands. Near the eastern margin of the body [NC 6292 5758] the metagabbro is noticeably leucocratic and garnetiferous. The igneous and meta-igneous fabrics are commonly reworked within shear zones defined by anastomosing, often curviplanar bands of hornblende and/or actinolite schist. These shear zones typically carry a strong mineral and extension lineation that plunges to the SE, parallel to L2 in the host Moine rocks. A top-to-the-northwest sense of shear parallel to this lineation can be deduced from the sense of fabric curvature on the margins of some of the southeasterly-dipping shear zones. The margins of the metagabbro body are invariably highly strained, and the intrusion seems likely to have acted in a highly competent manner with respect to its metasedimentary host during deformation. The contact with the adjacent Moine psammites is nowhere exposed, but its location can be narrowed down to within 5-6m.

The field relations are consistent with a pre-D2 age of intrusion for the metagabbro and it therefore forms an important marker in the regional deformation chronology. U-Pb SHRIMP dating of zircons from a leucocratic part of the Loch Cormaic metagabbro has yielded a late Neoproterozoic age that is interpreted to date intrusion and crystallization of the igneous protolith (Strachan & Kinny, unpublished data). In a regional context, the Loch Cormaic body, and by implication other members of the Loch a’ Mhoid Suite, were probably intruded during the period of late Neoproterozoic rifting that resulted in the break-up of Rodinia and formation of the Iapetus Ocean (Kinny et al., 2003a).

Locality 13.7 Borgie peat cuts [NC 6380 5770][edit]

Borgie peat cuts (Fig. 13.6). Relict garnet-pyroxene gneisses within the Borgie basement inlier.

One kilometre east of Strathtongue on the A836, turn south onto an untarred track at [NC 6320 5912]. There is parking for three to four cars at the end of the track. Allocate 1½ hours for this locality. Walk south-eastwards across the moor to reach the low-lying outcrops around 7A [NC 6380 5770]. These expose gneisses of the Borgie basement inlier. Lenses of mafic gneiss, up to several hundred metres long, contain largely unretrogressed metamorphic assemblages dominated by garnet and clinopyroxene, indicative of at least upper amphibolite facies conditions. These are cut by narrow shear zones defined by hornblende schist, and at their margins pass imperceptibly into host hornblende gneisses typical of large tracts of the Borgie inlier. These carry an east- to SE-dipping foliation and a SE-plunging mineral and extension lineation that is parallel to, and correlated with, L2 in the Moine. Similar relict garnet-pyroxene mineral assemblages are preserved within the Naver basement inlier in Central Sutherland (Locality 10.6, Excursion 10), and they have been compared with the Scourian granulites of the Caledonian foreland (Moorhouse, 1976). However, in the absence of isotopic data, at the time of writing nothing precludes a significantly younger age.

Return to the cars and pay a brief visit (10-15 minutes) to the small quarry adjacent to the A836 at 7B [NC 6413 5958]. This exposes typical banded hornblendic gneisses of the Borgie inlier, intruded by a mafic sheet, now converted to hornblende schist. This has been correlated with the Scourie Dyke Suite of the Caledonian foreland (Moorhouse et al., 1988), but given its state of deformation it could equally be a member of the Ben Hope amphibolite suite or a highly sheared member of the Loch a’ Mhoid Metagabbro Suite. U-Pb SHRIMP dating of zircons from a gneiss sampled in the southeast corner of the quarry indicates a late Archaean age of c.2850 Ma for the igneous protolith (Friend et al., 2008). The strong fabric within the orthogneisses is at least partly Caledonian in age, as it carries a mineral and extension lineation that is parallel to the regional L2 lineation within the Moine rocks. A 40Ar/39Ar age of c.421 Ma obtained from hornblende within the gneisses here is interpreted to date cooling through a closure temperature of c.500°C either during or following D2 (Dallmeyer et al., 2001).

References[edit]

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