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The geology of the Lewisian rocks north of Loch Torridon, Ross-Shire, Scotland

Cresswell, David

The geology of the Lewisian rocks north of Loch Torridon, Ross-Shire, Scotland Thumbnail


David Cresswell


The Lewisian rocks of Loch Torridon consist of banded acid gneisses enclosing a number of basic and ultrabasic bodies and cut by a swarm of NW-SE trending basic dykes and by a later set of pegmatite dykes and a granite sheet.
No definite conclusion has been reached as to whether the gneiss complex was derived from igneous or sedimentary parent rocks or a combination of both, although previously described metasedimentary gneisses are considered to be possibly magmatic derivatives of early basites (in the case of the "epidotites") and deformed quartz veins (in the case of the "quartzites").
The dyke swarm is believed to have been derived from two distinct tholeiitic parent magmas; the earlier type-TD basite suite owes its origin to a normal magmatic differentiation process, whereas the later type-TB basite and ultrabasite suite may have formed by an abnormal magmatic differentiation process involving possibly the crystallization of hydrous minerals (hornblende).
The rocks of the complex have been affected by at least nine deformation episodes of which five (D1-D5) occurred prior to the dyke intrusion and the remainder (D6-D9) after the dyke intrusion. In most of the deformation episodes, folds, lineation and axial-planar foliation have been produced.
The earliest structure, S1 foliation, has been inferred from the form of interference structures formed in the DJ deformation. The S2 foliation, although never found undeformed, is believed to have had an original NW-SE steeply dipping attitude. The s3 foliation, which is predominant in the north of the area has been affected by a flattening phase (D3A) and large-scale folding (D3B) about an axis plunging moderately SW. D4 has produced small-scale folds and related NW-SE striking S4 axial-planar foliation. The attitude of S4 and style and
scale of F4 folds show variations through the area suggesting that in the north F4 was formed under brittle conditions, whereas in the south the conditions favoured plastic flow.
S4 shows large-scale variations in attitude which are thought to be a result of D4 rather than of later large-scale folding. It is believed that the maximum compressive stress during D4 was along a N-S shallow dipping axis. D5 has produced both major and minor F5 folds with SE-plunging axes, affecting sub-horizontal belts of S4. L5 is co-axial to L4 within individual belts.
The post-dyke structures revealed from a study of the dykes involved four separate episodes, D6-D9. The S6 foliation is parallel or sub-parallel to dyke margins but only occasionally can be positively identified since S7 is also parallel to dyke margins and is thus generally co-planar to S6. D7 comprises F7 folds, S7 axial-planar foliation and L7 lineation in the dykes which increase in intensity towards the south. S7 is co-planar to S4 and S6 and L7 is co-axial to L4 in the associated gneisses. The D8 structures are restricted to narrow NW-SE striking belts and consist of Fa minor folds and L8 lineation. The last penetrative structure is the S9 mylonite banding affecting certain NW-SE striking belts and is co-planar to S4 and S7.
The earliest metamorphic episode is M4 which is syntectonic to D4. Further metamorphic episodes occur syntectonically with D6 (M6), D7 (M7) and a retrogressive episode occurs between D8 and D9. M4, M6 and M7
are of almandine amphibolite facies, whereas the late retrogressive metamorphism is of greenschist facies. The distinction between M4, M6 and M7 is made mainly on textural grounds, although the formation of garnet in M6 distinguishes it from M4 and M7. The earliest
recognizable migmatitic episode pre-dates D3. Following D3 there occurred a phase of migmatization which was in part magmatic, producing lit-par,-lit trondhjemitic pegmatites, and in part metasomatic, producing feldspar augen in the acid gneisses. A third migmatitic phase accompanying M.4 has produced syntectonic granitization in the acid gneisses and early basites in the south, producing rocks of dioritic composition from the early basites. In the north, this migmatization is post-tectonic, producing thin pegmatite veins parallel to F4 axial planes, Finally a potassic metasomatism and late retrogressive metamorphism has produced microcline-muscovite-quartz-bearing assemblages of all rock types.


Cresswell, D. (1969). The geology of the Lewisian rocks north of Loch Torridon, Ross-Shire, Scotland


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