The Glenelg-Ratagain igneous complex

Summary The Glenelg&Ratagain complex is situated approximately six miles east of Glenelg, Inverness-shire, and is the most north-westerly of the Newer Granites of Scotland. Like other intrusions of that age, it is composite in character, though differing from most in the presence of biotite-pyroxenite and nordmarkitic syenite. To the northeast the igneous rocks intrude paragneisses similar to those of Lewisian age farther west; elsewhere, the country rock is acidic gneiss of Moine type with sporadic politic bands. Four intrusions have been recognized. The first, of granitic composition, is only sparsely represented, forming small discontinuous outcrops around the western side of the complex. The second, a diorite, was closely followed by one of syenitic material and, at a somewhat later date, by a fourth of adamellitic composition. Around the margins of the complex the diorite carries quartz and, on the western side, passes, apparently by gradation, into rocks of granodioritic composition. It seems likely that assimilation of the earlier “ granite ” has led to marginal acidification of the diorite. Considerable variation is seen in the diorite, the following varieties having been noted as well as the normal rock : fine-grained pyroxene-mica-diorite, brown horn-blende-mica-diorite and a coarse hornblende-diorite. Masses of biotite-pyroxenite and “ appinite ” occurring in the diorite area are probably xenolithic. Late magmatic liquors, possibly augmented by the assimilation of the earlier acidic rock, have attacked the crystallized material, with the replacement of andesine by more sodic plagioclase, the formation of potash felspar and the conversion of pyroxene to a green hornblende, thus giving rise to a variety of so-called “ modified diorites ”. The succeeding intrusion of syenitic magma, following rapidly upon the emplacement of diorite, probably before its final consolidation, has, by extensive hybridization, given rise to a wide range of monzonitic and similar rocks. The adamellite of the fourth intrusion is richer in alkalis than most comparable Scottish granites and is believed to have originated by hybridism at depth between the syenites and a more normal calc-alkaline granodiorite. The nordmarkitic syenite is comparable both chemically and mineralogically with similar rocks from Ben Loyal and Loch Ailsh and was probably derived from the same source at depth. The emplacement of such a magma between two intrusions of distinctly calc-alkaline affinities indicates the co-existence of alkaline and calc-alkaline magma beneath the Highland region during Caledonian times.