The petrochemistry of some Lewisian granitic rocks

Abstract

Summary. The chemical compositions of eighty granitic rocks from the Lewisian of the North-West Highlands of Scotland, particularly from the Gairloch district, are set out in relation to geological occurrence and their normative proportions of albite, orthoclase, quartz, and anorthite compared with experimental data relating to the systems NaA1Si30s-KA1Si30s-SiO2-H20 and KA1SiaOs-NaA1SiaOsCaA12Si~Os-SiO2. The field of composition for these Lewisian rocks moves from Ab Q-rich for autochthonous granites to Ab = Or = Q (approximately) for parautochthonous granites to Or-rich for intrusive granites. This trend is related to the varying roles of mineral solubility under stress, selective melting, and potassium metasomatism. RANITIC rocks comprise a considerable proportion of theLewisian complex of the North-West Highlands of Scotland. Much of the rock cropping out between Cape Wrath, Durness and the Loeh Laxford district, and from Gruinard Bay to Loch Torridon and Raasay, is quartzo-feldspathie gneiss with hornblende and biotite, while dykes and sills of granite and pegmatite occur in these regions, being particularly common in the Loch Laxford and Ben Stack district (Peach et al., 1907, pp. 37-40). The origin of the quartzo-feldspathic gneisses was ascribed by Peach et al. (1907, p. 35) to the 'plastic deformation' of an assemblage of igneous rocks, and Phemister (1948, pp. 10-11) refers to them as orthogneiss.Recent investigations of the Lewisian gneisses and associated rocks in the Loch Tollie district, north of Gairloeh, Ross-shire (Bhattacharjee, 1963a), in the Gairloch district (Park, 1964) and in the district south of Gairloch (Ghaly, 1965) have related geological occurrence and structural history to the nature of the rock types. This detailed work provides a basis for the comparison of the petrochemistry of naturally occurring granitic rocks with the results of phase equilibrium studies of systems with compositions resembling those of granitic rocks. Other investigations in the Lewisian that have provided information concerning geological occurrence and structural history (e.g. Nisbet, 1961 ; Bowes, Wright, and Park, 1964) further assist relevant comparison of petrochemical and experimental data.