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A study of the granites and associated lewisian rocks of Loch Laxford, Sutherland

A study of the granites and associated lewisian rocks of Loch Laxford, Sutherland Thumbnail


The area studied, approximately 12 sq. km. in extent, lies at the landward end of Loch Laxford. Field mapping on a scale of about 1: 2,500 was followed by detailed petrographical, geochemical, and spectrochemical analyses.
The rock groups can be divided into 1. four groups of gneisses, namely the Claise Fearna., Badnabay, Weaver' s Bay and Laxford Bay gneisses; 2. the Early Igneous Rocks, comprising metaperidotites, hornblendites, garnet-pyriclasites, amphibolites, plagioclasites, and metadiorites; 3. the Trondhjemitic Injection complex; and 4. the Granitic Injection complex.
It is concluded that two metamorphic episodes have affected the rocks thus: an Early granulite facies (- ?Securian) metamorphism and a Later amphibolite facies (Laxfordian) metamorphism.
The gneisses are metasedimentary in origin, the parent rook being greywaoke. The metagreywackes have suffered both metamorphisms and are now represented by trondhjemitic, quartz-dioritic, granodioritic, and granitic gneisses. They were intruded by those Early Igneous rooks that appear to form part of differentiating igneous series, that is, the metaperidotites, hornblendites, the meta­ gabbro-anorthosite rocks, and the metadiorites. This igneous series has also suffered two metamorphisms. Certain amphibolites appear to be equivalent to the Scourie dyke swarm and are considered to be meta-tholeiites intruded after the granulite facies metamorphism but prior to the amphibolite facies metamorphism.
The Trondhjemitic Injection complex developed during the early part of the Laxfordian metamorphism and was accompanied by an extensive sodium metasomatism. Some trondhjemites are folded whereas others cut the Laxfordian foliation. The Granitic Injection complex developed during the Laxfordian metamorphism and may have partly overlapped the Trondhjemitic Injection complex. The Injection complex proper (The Cnoc nan Cro
group) has produced a series of microgranites, granites and pegmatites intruded concordantly into the pre-existing rocks. Extensive potassium metasomatism, associated with the complex, has produced the badnabay group - a series of microgranites, microsyenites, and other granitic rocks - and microgranite-trondhjemite bodies. The Laxford Quartzofeldspathites are granitic bodies developed locally by anatexis.
The Laxfordian phase of folding has produced NW-SE folds plunging SE and an associated axial plane foliation that dips to the SW. A late phase of E-W folding is also seen. NE-SW and WNW-ESE jointing, occasionally filled by pegmatitic material, represents the final phase of the Laxfordian metamorphism.

Publication Date Apr 1, 1966


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