Alkaline Rocks and Carbonatites of the World

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Mauze And Songwe

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Occurrence number: 
103-00-035
Country: 
Malawi
Region: 
Chilwa Province
Location: 
Longitude: 35.8, Latitude: -15.7

Mauze is a substantial intrusion of nepheline syenite of some 10x6 km the most easterly two thirds of which is located in Mozambique (Mozambique No. 114-00-010). It forms a mountain rising 800 m from the raised shore lines to the southeast of Lake Chilwa. At the western end of the intrusion, within Malawi, the nepheline syenite is cut by the Songwe vent of breccia and carbonatite. The nepheline syenite, referred to as foyaite in the literature (Dixey et al., 1937; Garson, 1965a), consists of perthite, nepheline, partly altered to cancrinite, aegirine-augite zoned to aegirine and biotite, with a sodic amphibole and sodalite in some varieties; accessory astrophyllite may occur and melanite and fluorite have been found in veins. A sodalite-rich variety forms a small intrusion on Chenga Hill, immediately west of Mauze, and dykes of phonolite and trachyte cut basement rocks on Chenga and other hills west of Mauze with eudialyte having been identified in phonolite (Garson and Walshaw, 1969). On Chiweni Hill, which is north of Mauze and crossed by the Malawi-Mozambique border, there is a thin (20 cm) dyke of melilite nephelinite. A broad zone of fenitized basement rocks extends along the northern margin of Mauze and a narrow zone to the west, while all the basement rocks exposed on Chenga Hill are also fenitized. Songwe is a conical, steep-sided hill which comprises an approximately circular vent 900 m in diameter of feldspathic breccia and agglomerate cut by arcuate dykes up to 45 m thick of carbonatite. Agglomerate also occurs at the eastern foot of Chenga Hill where it consists of angular and sub-rounded blocks up to 2 m across of melanephelinite, dolerite and various types of fenitized gneiss and granulite in a feldspathized matrix of the same material. The rocks in the main Songwe vent are similar but are more feldspathized to rocks consisting of Carlsbad-twinned K-feldspar in a trachytic matrix, which in extensive areas is carbonate-rich. Melanephelinite fragments on Chenga Hill are carbonated but nepheline, green pyroxene, biotite and melanite are recognisable. The carbonatite sheets consist mainly of calcite with patches of iron oxides, xenocrysts of orthoclase and accessories including apatite, pyrochlore, synchysite and bastnasite. One dark carbonatite consists of patches of iron and manganese oxides replaced by calcite and fluorite. Loose boulders of apatite-fluorite rock occur northwest of the summit of Songwe. Analyses of a foyaite, three phonolites and a calcite carbonatite will be found in Garson (1965a).

References: 
DIXEY, F., SMITH, W.C. and BISSET, C.B. 1937. The Chilwa Series of southern Nyasaland. Bulletin, Geological Survey of Nyasaland. 5: 1-85 (revised 1955).GARSON, M.S. 1965a. Carbonatites in southern Malawi. Bulletin, Geological Survey of Malawi. 15: 1-128.GARSON, M.S. and WALSHAW, R.D. 1969. The geology of the Mlanje area. Bulletin, Geological Survey of Malawi, 21: 1-157.
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