Chilwa Island

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. Chilwa Island, situated in the southwestern quarter of Lake Chilwa, is about 4 km in diameter and almost wholly composed of carbonatite surrounded by fenitized Precambrian granulites and syenites, the outer margin of the fenitized zone being hidden beneath the waters of the lake. The central area of the island is built of carbonatites that form a flat-topped, horse-shoe-shaped hill (see Garson and Smith, 1958, Frontispiece), reaching some 400 m above lake level. The contact zone with the physiographically lower surrounding fenites forms steep slopes thickly covered with vegetation. Carbonatite also occurs intruding fenites on Marongwe Hill, which is part of an isolated area of outcrop forming the northwestern part of the island, and as rare sheets amongst the fenites. There are a number of small plugs, sheets and dykes of a range of igneous silicate rocks that cut both the carbonatites and fenites, but these are areally much subordinate to the carbonatites. There are three distinct carbonatites in the central complex (Garson and Smith, 1958): an outer sovite, which forms the high plateau, an intermediate ‘ankeritic sovite’, and a central ‘sideritic carbonatite’. The contacts of the sovite with the fenite are sharp but generally complex with numerous fragments and blocks of the fenite in the carbonatite and much carbonatite veining of the fenite. The sovite was probably intruded as a series of steeply-inclined dykes sometimes incorporating screens of fenite which were largely replaced. The inner contact of the sovite with the ankeritic sovite is sharp but intrusive relationships seem to indicate that the ankeritic sovite is later. Contacts with the innermost sideritic carbonatite are rarely seen and their relationship is far from clear but Garson (Garson and Smith, 1958) indicates that the ankeritic sovite grades inwards from creamy coloured rocks to dark brown carbonatites nearer the central sideritic carbonatite. On the basis of mineralogy five types of sovite were distinguished and described in detail by Garson and Smith (1958) namely (a) aegirine and aegirine-biotite sovite with accessory niobian rutile, apatite, pyrochlore and magnetite; (b) calcite rocks with only minor amounts of other phases; (c) sovite with minor siderite or its pseudomorphs and accessory pyrochlore, quartz, synchysite and barite - these rocks comprise the bulk of the sovites; (d) apatite-rich sovite in which the apatite is concentrated in thin, flat lenticles and (e) sovite with a little ankerite that forms veinlets throughout but is particularly abundant towards the margins of the ankeritic sovite. The ankeritic sovite, as well as the main body, also forms numerous thin, steeply-inclined dykes and sheets that cut and replace the sovites. It contains accessory pyrochlore, microcline, magnetite, florencite, synchysite and apatite. The sideritic carbonatite consists of carbonate which has been almost totally replaced by iron and manganese oxides and these rocks are traversed by manganiferous bands up to 0.5 m across. There are drusy patches of calcite, florencite and bastnasite. An analysis of this carbonatite indicates 29% CaO, 12% MgO, 11% Fe2O3 and 6.7% MnO2. The origin of these rocks is obscure and it would seem probable that they are secondary. Bands of pyrochlore-rich carbonatite, up to 0.5 m thick, occur within the sovites and are described in considerable detail by Garson and Smith (1958). They sometimes apparently form dykes but also occur as ‘dragged out’ and partly digested zones within the sovite. They comprise calcite, ankerite, apatite, amphibole, phlogopite, crystals of pyrochlore up to 3 mm in diameter, columbite, fluorite, spinel, feldspar and pyrite. Nepheline syenite forms several plugs and sheets cutting carbonatite and fenite northwest of the main carbonatite intrusion. In general it consists of orthoclase, nepheline, variably altered to cancrinite, aegirine-augite zoned to aegirine and biotite. One occurrence that intrudes sovite has generated an aurole of melanite-rich carbonatite. In the vicinity of the nepheline syenites is a dyke, up to 3 m thick, of wollastonite-melanite ijolite. Dykes and sheets in the carbonatite and the fenites, the latter often concentric to the main intrusion and inward dipping, include nephelinite, phonolite, trachyte, alnoite and camptonite. The nephelinites are commonly extensively replaced by carbonate and contain augite, biotite, olivine, nepheline and dark green spinel. Garson and Smith (1958) suggest that the trachytes are rheomorphosed fenites. There is on the summit plateau a plug about 100 m in diameter and several dykes of camptonite, as well as an irregular sheet 300 m long of alnoite, the latter consisting of xenocrysts of mica, magnetite, pyroxene and hornblende in a matrix of carbonate, melilite pseudomorphed by carbonate, mica, pyroxene and magnetite. The last phase of activity is represented by veins containing fluorite, quartz and barite. The fenites comprise a broad, but widely variable, inner zone of what Garson (Garson and Smith, 1958) called feldspathic breccia, which is always the immediate country rock to the carbonatites. Around this are syenitic and quartz fenites, the outer limits of which are not found on the island. The feldspathic breccias consist essentially of orthoclase with minor quartz and iron oxides. The syenitic and quartz fenites display all variations from primary mineralogys of oligoclase to andesine, orthoclase, hornblende, ortho- and clino-pyroxene, biotite and quartz to fenites characterised by a net veining and consisting of perthite, aegirine, minor sodic amphibole and biotite. The fenites have been described in detail by Woolley (1969), who presents a dozen whole rock analyses including a feldspathic breccia with >14% K2O. Cutting the fenites at several localities, but notably at Sonko Point, on the eastern side of the island, are vents choked with agglomerate. That at Sonko Point is about 400x150 m and consists of angular to rounded fragments up to 0.5 m across of basement rocks in all stages of fenitization, feldspathic breccia and sovite in a matrix of comminuted fragments of the same material with in places much calcite. Analyses of carbonatites, including some trace element data, will be found in Garson and Smith (1958) and of a range of igneous silicate rocks in Woolley and Jones (1987). Simonetti and Bell (1994) made a detailed geochemical study of the complex and present major and trace element data for all rock types, including fenites. They also determined initial Nd, Pb and Sr isotopic data for the same rocks and C and O isotopes for the carbonatites, these data being discussed and modelled in detail.