Alkaline Rocks and Carbonatites of the World

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Kasungu

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Occurrence number: 
103-00-010
Country: 
Malawi
Region: 
The North Nyasa Alkaline Province
Location: 
Longitude: 33.45, Latitude: -13.05

Kasungu is a striking, steep-sided mountain that rises about 400 m above the Kasungu Plain. It is built of nepheline syenite that forms an intrusion approximately circular in plan and about 1.8 km in diameter (Bloomfield, 1965b). The marginal contacts are obscured by soil cover but there is a long, narrow roof pendant of biotite-hornblende gneiss near the summit. The nepheline syenite is described as a melasyenite by Bloomfield (1965b) and has a marked foliation in many places. Vertical, up to 30 cm thick, mafic dykes, commonly with sheared margins, cut and enclose angular fragments of the nepheline syenite, but are in turn traversed by stringers of nepheline syenite. There are two, apparently interlayered, varieties of nepheline syenite (Eby et al., 1998). One consists of turbid perthite, nepheline, mafic patches of aegirine-augite, amphibole and biotite, occasional scapolite and accessory titanite, apatite, zircon, magnetite and/or ilmenite. The second contains two discrete feldspars, clear microcline and sodic plagioclase, both with simple margins and commonly with 120° triple junctions, nepheline, evenly distributed aegirine-augite, amphibole and biotite, with accessory titanite but no opaque phase. Like Chipala (No. 103-00-009) the pyroxene comprises populations of Al-poor and Al-rich aegirine-augite, considered by Woolley et al. (1996) to be respectively of igneous and metamorphic origin. They also note the presence of scapolite of an unusually sodic composition. Analyses of three nepheline syenites and a mafic dyke, including a wide range of trace elements, and of two biotites, are given by Eby et al. (1998), and one nepheline syenite by Bloomfield (1965b).

References: 
BLOOMFIELD, K. 1965b. A reconnaissance survey of alkaline rocks in the northern and central regions. Records, Geological Survey of Malawi, 5: 17-63.EBY, G.N., WOOLLEY, A.R., DIN, V. and PLATT, G. 1998. Geochemistry and petrogenesis of nepheline syenites: Kasungu-Chipala, Ilomba, and Ulindi nepheline syenite intrusions, North Nyasa Alkaline Province, Malawi. Journal of Petrology, 39: 1405-24.PETERS, E.R. 1969. The geology of the Kasungu area. Bulletin, Geological Survey of Malawi, 25: 1-55. WOOLLEY, A.R., PLATT, R.G. and EBY, G.N. 1996. Relatively aluminous alkali pyroxene in nepheline syenites from Malawi: mineralogical response to metamorphism in alkaline rocks. The Canadian Mineralogist, 34: 423-34.
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