Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Chilwa Province
Longitude: 35.28, Latitude: -15.03

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). The nepheline syenite is relatively uniform but there are pegmatitic patches and minor included rafts of fenitized gneiss and angular xenoliths and schlieren of what appear to be metamorphosed basic volcanic rocks. There are a few phonolite dykes on the main hill but Bloomfield (1965a) considered that the larger of the satellite hills comprises a number of parallel dykes of microfoyaite. The smaller satellite consists predominantly of fenitized gneiss with a small agglomerate vent and an area of eudialyte-aegirine-albite rock. The Junguni nepheline syenites are coarse-grained and the most silica undersaturated of the Chilwa province nepheline syenites (Woolley and Jones, 1987), apart from the small volume nepheline syenites associated with the Chilwa Island and Tundulu carbonatite centres. They consist of sub-prismatic perthite, prismatic or interstitial nepheline, alkali pyroxene, biotite, minor eckermannite to arfvedsonite and sodalite, which may be abundant i.e. up to 90% by volume. Among a wide range of accessories are pyrophanite, eudialyte, rosenbuschite, mangan-titan-lavenite, wohlerite, kupletskite, strontian fluorite, pyrochlore, britholite and astrophyllite. The pyroxenes, amphiboles and micas are described in detail by Woolley and Platt (1988) who also give analyses of many of the rarer accessory phases. Two rock analyses are given by Bloomfield (1965a) and six by Woolley and Jones (1987), including two rocks with 17.2 and 20.5% Na2O and 3.5 and 4.4% Cl reflecting abundant sodalite.

Three titanite fission-track ages ranged from 128.8(20.1 to 137.4(24.9 Ma (Eby et al., 1995).
BLOOMFIELD, K. 1965a. The geology of the Zomba area. Bulletin, Geological Survey of Malawi, 16: 1-193.EBY, G.N., RODEN-TICE, M., KRUEGER, H.L., EWING, W., FAXON, E.H. and WOOLLEY, A.R. 1995. Geochronology and cooling history of the northern part of the Chilwa Alkaline Province, Malawi. Journal of African Earth Sciences, 20: 275-88.WOOLLEY, A.R. and JONES, G.C. 1987. The petrochemistry of the northern part of the Chilwa alkaline province, Malawi. In J.G. Fitton and B.G.J. Upton (eds), Alkaline igneous rocks. 335-55. Geological Society of London, Special Publication 30.WOOLLEY, A.R. and PLATT, R.G. 1988. The peralkaline nepheline syenites of the Junguni intrusion, Chilwa province, Malawi. Mineralogical Magazine, 52: 425-33.
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