Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Chilwa Province
Longitude: 35.75, Latitude: -15.6

The low hills of the Nkalonje area are, according to Garson (1963 and 1965a), the remnants of a ring-complex centred on Nkalonje Hill, but the area between the hills is obscured by recent lake deposits and soils. Nkalonje forms a steep-sided, conical hill rising some 150 m above the plain and is a breccia and agglomerate-filled vent which cuts fenitized Precambrian granulites and gneisses. There are satellite vents of agglomerate forming craggy features on Nyama Hill, which is a ridge continuous with, and to the east of, Nkalonje. On the south side of Nkalonje is a 500 m long intrusion of nepheline syenite with a smaller intrusion on Nyama Hill. Arcuate dykes of carbonatite, melanephelinite, nepheline syenite, phonolite and trachyte occur on Nkalonje and the hills of Matopon and Palone, and Garson (1963) found that the dykes are arcuate to a centre immediately south of Nkalonje Hill. The vent rocks of Nkalonje comprise: (a) Breccias consisting of fragments of Precambrian basement rocks, some fenitized, in a comminuted matrix. (b) Feldspathic breccias and agglomerates, which are rocks similar to the breccias of (a) but have been fenitized to orthoclase-rich rocks; blocks of trachyte are also abundant. Some of these rocks resemble recrystallised tuffisites and they contain vesicular patches of colourless fluorite. Near the summit of Nkalonje the agglomerate contains fragments of melanite melanephelinite. ( c) Agglomerates with a pseudotrachytic matrix produced by recrystallisation of the comminuted feldspathic matrix of the breccias. In places these rocks have been rheomorphosed and invade less altered breccias. (d) Trachytic inclusions are abundant as fragments in the breccias and agglomerates and trachyte dykes up to 3 m thick are very fine-grained and consist of feldspar laths in a feldspathic matrix with aegirine-augite, which is mostly altered. The nepheline syenite of Nkalonje is a medium- to coarse-grained rock of alkali feldspar, nepheline, pyroxene zoned to aegirine-augite, arfvedsonite and aenigmatite. The nepheline syenite on Nyama Hill is similar but the nepheline is partly altered to cancrinite and sodalite and the pyroxene much replaced by orange biotite. Two dykes of sovite up to 3 m thick occur on Nyama Hill and consist of calcite, aegirine, biotite, feldspathic patches rich in apatite, pyrochlore and accessory barite, synchysite and fluorite. Sovite sheets also occur on Matopon Hill and a 1 metre thick dyke of ‘siderite sovite’ is present on Nyama Hill and a similar rock forms the matrix to some agglomerates elsewhere on this hill. These carbonatites contain strontianite, ankerite, bastnasite, florencite and fluorite. An analysis of nepheline syenite and three of fenite, two of which have >13% K2O, are in Garson (1963 and 1965a).

GARSON, M.S. 1963. The geology of the Nkalonje area, Mlanje District. Records, Geological Survey of Nyasaland, 2: 91-110.GARSON, M.S. 1965a. Carbonatites in southern Malawi. Bulletin, Geological Survey of Malawi. 15: 1-128.
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