Alkaline Rocks and Carbonatites of the World

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Chikangawa (South Vipya)


Occurrence number: 
The North Nyasa Alkaline Province
Longitude: 33.8, Latitude: -11.87

This 6.5x4.5 km intrusion is emplaced in gneisses and consists of nepheline syenites that vary widely in grain size, degree of foliation and presence or absence of mafic banding (Bloomfield, 1965b; Gaskell, 1973). Most rocks are leucocratic with obvious patches of biotite, which is the only major mafic mineral. The rock consists essentially of plates of microcline with aggregates of smaller grains of albite/oligoclase, up to 40% nepheline, which forms oval and lobate grains with rounded inclusions of feldspar, biotite, cancrinite, natrolite and accessories including calcite, magnetite, pyrite, pyrrhotite, zircon, crystals of which are abundant in some soils (Bloomfield, 1965b), apatite and pyrochlore. More mafic layers are mineralogically similar but with a greater abundance of biotite and more clearly defined foliation. Pegmatite veins occur in places and are distinguished by cores of lemon-yellow cancrinite. An analysis of a single nepheline syenite is in Bloomfield (1965b).

A K-Ar determination on biotite gave 410(16 Ma (Snelling, 1965a) and a Rb-Sr whole rock determination 650(40 Ma (Bloomfield 1968).
BLOOMFIELD, K. 1965b. A reconnaissance survey of alkaline rocks in the northern and central regions. Records, Geological Survey of Malawi, 5: 17-63.BLOOMFIELD, K. 1968. The pre-Karoo geology of Malawi. Memoir, Geological Survey of Malawi, 5: 1-166.GASKELL, J.L. 1973. The geology of the Mzimba area. Bulletin, Geological Survey of Malawi, 37: 1-30.SNELLING, N.J. 1965a. Age determination unit. Overseas Geological Surveys Annual Report (for 1964), 28-38.
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