Alkaline Rocks and Carbonatites of the World

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Ngara Hill

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Occurrence number: 
103-00-011
Country: 
Malawi
Region: 
The North Nyasa Alkaline Province
Location: 
Longitude: 33.58, Latitude: -13.3

An east-west-trending arcuate area of aegirine gneisses and aegirine-bearing quartzo-feldspathic granulites and gneisses extends for about 6 km and is up to 1.5 km wide. It forms a series of parallel ridges on which stands the sharp peak of Ngara Hill (Bloomfield, 1965b). At Ngara Hill the gneisses are banded on scales from 2-3 cm to several 10s of metres. Various scales of banding are illustrated by Bellingham and Bromley (1973) and involve combinations of aegirine, aegirine-biotite and mafic aegirine gneiss with all the contacts sharp. Typical aegirine gneisses contain 10-20% aegirine, which in some rocks is partly replaced by a riebeckitic amphibole, about 30% microcline, a little andesine, up to 35% quartz and accesory magnetite, titanite, zircon, apatite, allanite and biotite, with the last a major phase in gneisses found near the summit of the hill; pyrochlore has also been noted. A rock analysis is given by Bloomfield (1965b).

References: 
BELLINGHAM, K.S. and BROMLEY, J. 1973. The geology of the Ntchisi-Middle Bua area. Records, Geological Survey of Malawi, 27: 1-38.BLOOMFIELD, K. 1965b. A reconnaissance survey of alkaline rocks in the northern and central regions. Records, Geological Survey of Malawi, 5: 17-63.
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