The Loch Borralan alkaline igneous complex

• Main Author: Woolley, A. R.
• Published: Royal Holloway, University of London 1965
• Subjects: 552
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599566

The Complex is emplaced partly among Cambrian limestones, and partly among thrust Lewisian and Cambrian rocks of the Benmore Nappe. It has the form of a laccolith but consists of two distinct igneous phases of which the later occupies the higher part of the mass. The earlier rocks have been emplaced mainly before, am the later rocks after the principal movements on the Benmore Thrust. The earlier rocks are of four main types : spotted rocks and rocks derived from them, borolanites, potash-feldspar-mica-ore rocks and potash-feldspar-muscovite rocks; rocks of Ledmore type, shonkinites, ma.lignites and ledmorites; ultramafio rocks, cromaltite, hornblendite ani pyroxenite; and finally miscellaneous rocks including nepheline syenites, fenitised quartzites and a. phlogopite-rich rock. The spotted rocks of the 'Eastern Tract' are derived from a feldspar porphyry by metamorphic and metasomatio processes involving mineralogioa.l, textural and chemical changes. The spotted borolanites are not pseudo-leucite rocks. The rocks of Ledmore type are derived from a shonkinite-pulaskite magma, but some members have been changed, by nephelinisation and melani tisation, to ledmorites am borolanites of Ledmore type. It is suggested that the ultramific rooks were formed by metamorphic segregation representing chemical culminations of iron, lime, magnesia and titania. In contrast to the earlier rocks, the later rocks, quartz syenites perthosites am nordmarkites, differentiated from a magua by processes of progressive crystallisation.The earlier and later rocks are shown to belong to ohemically distinct groups, and a similar grouping can be recognised among the rocks of the nearby Loch Ailsh Complex, as well as among the hypabyssal rocks that are distributed more widely in the Assynt district. Finally, it is postulated that the earlier rocks were formed by metasanatism, and by anatexis and mobilisation, these processes taking place in advance of am through the agency of a syenitic or granitic magma which was eventually' emplaced within the metasomatised assemblage.