Throughout the paragneisses of southern Malawi, from just north of Nsanje southwards to the border with Mozambique, biotite-nepheline gneisses form long, narrow bands which have been deformed in broad open folds (Bloomfield, 1958 and 1968).
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Malawi is, for a relatively small country, particularly well endowed with alkaline rocks and carbonatites, and it is further fortunate that most of the country has been mapped at relatively large scales and described in numerous publications of the Geological Survey. Apart from the general sheet bulletins of the Survey there are also numerous memoirs and other Survey publications that are devoted to the alkaline rocks and carbonatites; many of these are referred to in the following pages. The alkaline rocks comprise essentially two provinces: the North Nyasa Alkaline Province, many of the rocks of which have been affected by the Pan-African tectono-thermal event, and the Chilwa Alkaline Province of southern Malawi the intrusions of which range from about 98 to 137 Ma. Brief descriptions of these provinces are given in the appropriate places. A general account of the geology and mineral resources of the country is that of Carter and Bennett (1973).
A thin radioactive dyke of carbonatite agglomerate occurs 6 km east-southeast of Kapiri Hill (No. 103-00-022), southeast of the village of Kanono (Garson, 1965b, Map 1).
Nsengwa Hill is a steep-sided feature rising some 350 m above the surrounding country the core of which is a plug of microsyenite that has been intruded and altered by a dyke-like body of agglomerate, carbonatite agglomerate and quartz-rich carbonatite.
Junguni forms a horseshoe-shaped hill of nepheline syenite 2.5 km in diameter that is surrounded by recent unconsolidated sediments; there are two small satellite intrusions to the northeast (Bloomfield, 1965a).
Chinduzi is situated at the western end of the Mongolowe-Chaone-Chikala line of nepheline syenite-syenite intrusions (No. 103-00-027) and is continuous with Mongolowe through a low saddle in which runs the main Zomba-Liwonde road.
The overlapping complexes of Mongolowe, Chaone, Chikala and Chinduzi, the last described separately (No. 103-00-026), are the most extensive centres of nepheline syenite-syenite in the northern part of the Chilwa Province.
The Zomba-Malosa complex is the largest intrusion in the northern part of the Chilwa province. It is pear-shaped in plan and extends southwards for 24 km from the Mongolowe intrusion (No. 103-00-027), with which it is in contact; the maximum width is 13 km.
Mpyupyu is a small conical hill rising from recent sediments fringing the southwestern shores of Lake Chilwa. It consists of hornblende-biotite syenites, some of which contain a little quartz. There are rare pyroxene cores to the amphibole which is in turn mantled by biotite.
Python Island, in the northern part of Lake Chilwa, contains a vent filled with blocks of gneiss and small areas of breccia consisting of angular fragments of slightly fenitized granulite and syenitic gneiss in a sparse comminuted matrix containing siderite and goethite.
The Chilwa Island centre was the first carbonatite to be recognised in Africa (Dixey et al., 1935), Smith appreciating that specimens of limestone sent to him by Dixey were similar to the carbonatites of the Fen complex.