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Rocks of the Moine Series occurring in the Cluanie area are divided into three main groups, namely : Upper Psammitic, Pelitic, and Lower Psammitic. This general succession is similar to that in the Morar area and maintains throughout the southern portion of the Northern Highlands.
Most of the country in the vicinity and to the east of the Cluanie intrusion is composed of rocks of the Upper Psammitic Group which almost invariably show gentle dips. To the west of the intrusion the Moine rocks are generally highly inclined. Nappe structures are present.
Prior to or during early Caledonian folding the Moine Series was intruded by basic sheets and dykes now represented by amphibolites and hornblende-schists. Regional injection by trondhjemitic material, and, slightly later, localized intrusion of granodioritic to adamellitic orthogneiss, occurred contemporaneously with the folding.
The Cluanie igneous intrusion is one of the "Newer Granites". It cuts the regional injection and is clearly later than it. The intrusion consists of only one major rock type, namely a porphyritic hornblende-biotite-granodiorite which is cut by E.-W. trending dykes of microdiorite, quartz-microdiorite and trondhjemite-porphyrite and aplite. Round much of the margin of the intrusion the strike and dip of the Moine rocks conforms to the attitude of the igneous contact. This is a characteristic feature of other Caledonian "granites" and indicates that strong magma pressure accompanied their intrusion.
The granodiorite and other minor intrusions have been metasomatically altered by hydro thermal solutions rich in alkalis. During the alteration, which may be described as albitization, quartz was replaced by felspar and the plagioclase became progressively more sodic. Cleavelandite was commonly formed at a late stage in the albitization. Solutions causing the albitization in the main followed lines of intense jointing trending between north-north-east and north-north-west, though in some cases no such relationship between jointing and albitization occurs, the solutions having apparently percolated upwards through solid granodiorite or trondhjemite.
The injection complex and the Cluanie granodiorite are considered to have been formed by the rise of granitic material and magma consequent upon melting of the acid crust at depth during the Caledonian orogeny.
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