The Cnoc nan Cuilean area of the Ben Loyal igneous complex

Abstract

The Ben Loyal igneous complex forms a group of mountains, situated about six miles to the south of Tongue in northern Sutherland. The three plutonic masses of the complex are (a) The Ben Loyal range, (b) Cnoc nan Cuilean, and (c) Beinn Stumanadh. Though all three are composed essentially of syenites, each presents distinct petrographic features, and the Cnoc nan Cuilean mass, unlike the other two, shows a development of basic contact and basic marginal igneous rocks. The present paper is concerned primarily with the petrology of the Cnoc nan Cuilean mass, but the Ben Loyal intrusion is also considered in connexion with the form and structure of the complex. The country rocks are dominantly highly siliceous granulites of the Moine Series, which, beyond the area affected by the intrusions, dip at 20°- 30° to the south-east. As the igneous areas are approached, the strike tends to become parallel to the intrusion margins and the dips become steeper. It is suggested that the Ben Loyal and Cnoc nan Cuilean intrusions are irregular cones in shape, with their apices pointing downwards and with marginal sheet-like apophyses. The igneous rocks of the Cnoc nan Cuilean area are (a) the main syenite (pulaskite), composed, essentially of anorthoclase and albite (largely intergrown as perthite) and aegirineaugite (b) variable marginal syenites, consisting of more melanocratic syenites, "basic patches" (composed of aegirineaugite) and evident xenoliths. These are traversed by complex sets of pegmatite and aplite veins and small dykes. In the vicinity of the igneous mass the Moine granulites have been transformed metasomatically into rocks ranging from highly albitic granulites to aegirine-augite-schisis and hornblende-schists. Chemical and mineralogical evidence shows that the alkalies (principally soda) and alumina were "fixed" first and also possessed the greatest mobility. Later, such additional constituents as lime, magnesia and iron oxides formed, aegirine-augite and hornblende: these minerals attain their maximum development in the immediate vicinity of the contact. The source of the metasomatizing agencies is difficult to ascertain, but it seems very improbable that they were supplied by a syenitic magma. It is therefore suggested that "primary" alkalumina emanations displaced cafemic constituents from deep-seated "rocks of Lewisian type" thereby providing the materials necessary for the development of the marginal metasomatic rocks. The rocks of Lewisian type, which consist of hornblende-gneisses and schists, occur interbanded with the Moine granulites and, although they are not exposed at the surface nearer than the north-western slopes of Ben Loyal, there is structural evidence for their existence below the Cnoc nan Cuilean mass. The igneous contact itself is often characterized by transitions from basic schists to basic marginal syenites. A significant feature of these phenomena is the assumption of igneous characters by the more felspathic foliae of the schists and their injection into the basic bands. The contact transitions are considered to be rheomorphic phenomena, representing the final stage in the process of metasomatism, whilst the variable marginal syenites are interpreted as due to the hybridization of modified basic-metasomatic rocks (as represented by the "basic patches") by felspathic syenite magma("migma") of rheomorphic origin.