Alkaline Rocks and Carbonatites of the World

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Mlanje (Mulanje)

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Occurrence number: 
103-00-040
Country: 
Malawi
Region: 
Chilwa Province
Location: 
Longitude: 35.63, Latitude: -15.95

The Mlanje Massif is a spectacular mountainous area rising to some 3000 m from the Palombe Plain (750 m) and extending over 640 km2. The margins of the massif, particularly on the western side, rise as near vertical cliffs (see, for instance, Garson and Walshaw, 1969, Plate VI). The physiography of the area has been described and illustrated by Dixey (1927) and a popular account of an expedition to the mountain is that of van der Post (1957). Mlanje comprises a cluster of coalescing intrusions of syenite, quartz syenite and granite with a small area of extrusive rhyolitic rocks, the origin of which is far from clear. Syenitization or fenitization of the enveloping Precambrian gneisses extends for up to one kilometre, but some contacts are sharp with no alteration. About 60% of the complex was mapped by Walshaw (Garson and Walshaw, 1969) as quartz syenite grading into some small areas of syenite, with the rest predominantly granite. However, the Chambe area on the northwestern margin of the complex appears to be a distinct intrusion comprising several rings of syenite although, as discussed at length by Walshaw (Garson and Walshaw, 1969), there do not appear to be any intrusive contacts between them. All the plutonic rocks of the complex are dominated by perthite with exsolved albite and oligoclase. The quartz syenites appear to grade into the granites but the Chambe rocks are distinguished by the absence or presence of only minor quartz. Pyroxene is present in most rocks, invariably partly replaced by amphibole, and Platt and Woolley (1986) have demonstrated that it defines two distinct trends of sodic salite-aegirine-augite-aegirine in the Chambe rocks and sodic ferrosalite-sodic hedenbergite-aegirine hedenbergite-aegirine in the rest of the complex. Amphibole is ubiquitous and varies from green and brown katophorite and ferrorichterite to arfvedsonitic and riebeckitic varieties. Biotite is widespread but accessory fayalite, aenigmatite, astrophyllite, chevkinite, unidentified RE minerals and yttrofluorite are confined to granites of the main complex and titanomagnetite to Chambe rocks. At several places around the complex, but most notably on the Tuchila Shelf, an elevated plateau area at nearly 2000 m, xenoliths up to 100 m across of hornfelsed lavas and minor agglomerates occur that are cut by veins of pegmatite, quartz syenite and granite. The least altered parts of the lavas reveal a trachytic texture and all the rocks consist of alkali feldspar, quartz, biotite, hornblende, riebeckite, aenigmatite and in some fayalite. Minerals of metamorphic origin include andalusite, corundum and sillimanite. The biggest area of volcanic rocks forms a section with a vertical thickness of 24 m more than half of which is agglomerates in which graded bedding is discernable. Garson and Walshaw (1969) suggest that the Tuchila volcanics originated in one of the centres northwest of Mlanje, e.g. Namangali (No. 103-00-039), but it also seems possible that they represent an early extrusive phase from the Mlanje centre itself. Numerous analyses of the Mlanje plutonic rocks, and three of the volcanic rocks, are given in Garson and Walshaw (1969) and analyses of pyroxene, amphibole, mica, aenigmatite, astrophyllite and fayalite in Platt and Woolley (1986); descriptions and analyses of monazite, fluocerite and bastnasite from a granite sample are given by Platt et al. (1987).

Economic: 
There are several areas of bauxitisation on Mlanje but the only potentially economic ones are on the Lichenya Plateau in the southwest of the complex. Reserves of 50-60 million tons averaging 43% Al2O3, 14% Fe2O3 and 17% free quartz have been proved (Garson and Walshaw, 1969).
Age: 
K-Ar determinations on biotite from syenite and quartz syenite gave 116(6 and 128(6 Ma (Garson and Walshaw, 1969).
References: 
DIXEY, F. 1927. The Mlanje Mountains of Nyasaland. Geographic Review, 17: 611-26. GARSON, M.S. and WALSHAW, R.D. 1969. The geology of the Mlanje area. Bulletin, Geological Survey of Malawi, 21: 1-157.PLATT, R.G. and WOOLLEY, A.R. 1986. The mafic mineralogy of the peralkaline syenites and granites of the Mulanje complex, Malawi. Mineralogical Magazine, 50: 85-99.PLATT, R.G., WALL, F., WILLIAMS, C.T. and WOOLLEY, A.R. 1987. Zirconolite, chevkinite and other rare earth minerals from nepheline syenites and peralkaline granites and syenites of the Chilwa Alkaline Province, Malawi. Mineralogical Magazine, 51: 253-63.VAN DER POST, L. 1957. Venture to the interior. Penguin Books, Harmondsworth. 239 pp.
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