Within the Precambrian Dahomeyan basement gneisses of southeastern Ghana peralkaline and feldspathoidal gneisses form a layer between 7 and 20 m thick which can be traced almost continuously for 76 km (Kesse, 1985) in the vicinity of the towns of Somanya, Kpong and Pore (Holm, 1974). The alkaline gneisses are in contact with mafic and granitic gneisses, amphibolites and schists but with epidote amphibolites and mafic gneisses predominant near the contact. The alkaline gneisses are conformable with the adjacent rocks and contacts are sharp. They are foliated rocks with leucocratic nepheline syenitic gneiss the most abundant type, but mafic nepheline gneiss and feldspar-poor nepheline gneiss occur locally. Silica-saturated leucocratic gneisses are interlayered with the undersaturated rocks throughout their outcrop but Holm (1974) has identified silica-oversaturated peralkaline varieties at only three localities. The predominant leucocratic nepheline gneiss is variably fine- to coarse-grained, weakly to strongly foliated with 15-30% nepheline, 37-56% albite (An0-4), 12-28% microcline and 1-15% mafic minerals including biotite, aegirine-augite and amphibole. The last mineral is commonly zoned and varies from hastingsite, commonly as cores, to a sodic amphibole somewhat similar to mboziite (Holm, 1971). Accessory minerals include titanite, apatite, zircon, allanite, calcite, fluorite, cancrinite and opaques. With increase in amphibole and the appearance of garnet these rocks grade into mafic gneisses which form streaks and layers that may be several metres thick. Modally the mafic gneisses comprise 3-25% nepheline, 35-55% plagioclase (An11-17), 5-20% microcline, 15-30% hastingsitic amphibole and 5-10% andradite-rich grossular garnet with minor local aegirine-augite but only rare biotite. Titanite, apatite, zircon and calcite are the usual accessories with idocrase sometimes present. The feldspar-poor nepheline gneisses distinguished by Holm (1974) are rare, occurring as lenses a few centimetres thick. They are characterised by less than 5% feldspar and 33-59% nepheline, 10-52% amphibole, 5-13% sodian augite, 5-10% garnet and up to 5% cancrinite; large grains of titanite are present. It is estimated that 30-50% of the gneiss is composed of a syenitic variety, containing neither nepheline nor quartz, which forms layers from a few centimetres to a few metres in thickness within the nepheline-bearing gneisses. These syenitic rocks consist essentially of albite (50-75%) and microcline (2-22%) with muscovite (0-20%) and biotite (1-10%). The relatively rare silica-oversaturated gneiss consists of 10-15% quartz, 45-65% albite (An0-4), 20-30% microcline, 5-10% aegirine-augite and amphibole, which may be hastingsitic or richteritic. A rock spatially associated with the alkaline gneisses and extending for 25 km within a shear zone, is a calcite-biotite rock containing rounded blocks, up to about 50 cm across, of alkaline gneiss and other local lithologies (Kesse, 1985). Holm (1974) suggests that this rock was produced by metasomatic introduction of calcite into a mylonite zone, but Mani (1978, unpublished report quoted by Kesse, 1985) considered this rock to be carbonatite. Holm (1974) gives chemical and modal data on a range of alkaline gneisses and concludes that they were originally extrusive alkaline volcanic rocks. Analyses of amphiboles, pyroxenes and garnets are given by Holm (1971).
ALLEN, J.B. and CHARSLEY, T.J. 1968. Nepheline syenite and phonolite. Institute of Geological Sciences, Mineral Resources Division. Her Majesty's Stationery Office, London. 169 pp.HOLM, R.F. 1971. Some garnets, pyroxenes, and amphiboles from nepheline gneisses in Ghana. American Mineralogist, 56: 2111-22.HOLM, R.F. 1974. Petrology of alkalic gneiss in the Dahomeyan of Ghana. Bulletin of the Geological Society of America, 85: 1441-8.KESSE, G.O. 1985. The mineral and rock resources of Ghana. Balkema, Rotterdam and Boston. 610 pp.