Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
The North Nyasa Alkaline Province
Longitude: 34.62, Latitude: -14.72

Three outcrops of nepheline gneiss occur about 12 km northwest of Ncheu (Walshaw, 1965; Bloomfield, 1965b). The outcrops extend over about 8 km but are on the same strike and probably part of the same horizon. The most northerly occurrence is some 35 m wide, has a strong planar foliation and is banded, this being represented by mafic and felsic layers and accentuated by concordant pegmatites. Six horizons, three of which contain nepheline, grade into each other. The most southerly outcrop is much coarser than the others one area of gneiss containing nepheline crystals up to 5 cm across. In the nepheline gneisses thin bands of biotite and calcic amphibole are separated by layers composed of oligoclase and nepheline. Feldspar is the predominant mineral with nepheline comprising 2-12%. Accessories include cancrinite, magnetite, green spinel and titanite. A single rock analysis is given by Bloomfield (1965b).

BLOOMFIELD, K. 1965b. A reconnaissance survey of alkaline rocks in the northern and central regions. Records, Geological Survey of Malawi, 5: 17-63.WALSHAW, R.D. 1965. The geology of the Ncheu-Balaka area. Bulletin, Geological Survey of Malawi, 19: 1-96.
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