Alkaline Rocks and Carbonatites of the World

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Chipala And Chipala East (Kasungu-Chipala)

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Occurrence number: 
103-00-009
Country: 
Malawi
Region: 
The North Nyasa Alkaline Province
Location: 
Longitude: 33.47, Latitude: -12.98

The Chipala nepheline syenite intrusion forms a small hill 5 km north of Kasungu (No. 103-00-010) with the smaller Chipala East intrusion a kilometre to the east (Bloomfield, 1965b; Peters, 1969). To the west of Chipala Hill a series of essentially horizontal sheets of nepheline syenite lie within the foliation of the Precambrian gneisses. The exact intrusive relationships are not clear as no contacts are exposed. The feldspar tends to develop an orientation and the rocks have a colour index of about 25 that led Bloomfield (1965b) to classify them as melasyenite. They are coarse-grained rocks of microcline perthite, with mafic minerals forming a vague, or marked, banding and irregular clots. The microcline is generally cracked and strained and may be traversed by zones of fine-grained, apparently re-crystallised, feldspar. Nepheline comprises 10-20% of most rocks, but can be minor. The mafic clots are dominated by pyroxene, amphibole and biotite, the first tending to form the cores and also occurring as myriads of grains enclosed in feldspar (Woolley et al., 1996). Accessory minerals include titanite, apatite, zircon and magnetite. A detailed investigation of the pyroxenes (Woolley et al., 1996) has shown that they comprise distinct groups of Al-poor and Al-rich aegirine-augites, the former interpreted as being of igneous origin and the latter metamorphic. Two rock analyses, including data for numerous trace elements, are given and discussed by Eby et al. (1998).

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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