Alkaline Rocks and Carbonatites of the World

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Numbi

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Occurrence number: 
038-00-014
Country: 
Comoros
Location: 
Longitude: 28.9, Latitude: -1.77

An elliptical intrusion of 8x3.5 km Numbi is emplaced in Precambrian Burundian schists and quartzites. The principal rock type is a nepheline syenite of microcline, which is generally aligned, albite, abundant nepheline, cancrinite, sodalite, green biotite, melanite, titanite, opaque phases, fluorite and apatite; chevkinite and lavenite are accessories. A variant, which is usually found near the margins of the complex, contains aegirine and aegirine-augite, but is free of biotite; this rock type is particularly rich in chevkinite and fluorite. Chemical analyses are available. A small area of ankaratrite with leucite is described by Denaeyer (1958) from 2 km southeast of the Numbi occurrence. This is undoubtedly younger and related to the Virunga volcanic field (No. 6).

Age: 
A four point whole-rock regression line on samples of granite and syenite from Numbi, Kirumba and Upper Rivubu (Burundi) yielded 648(17 Ma (Cahen et al., 1979 and 1984), and a Rb-Sr whole rock regression on ten syenites gave 830(51 Ma (Kampunzu et al., 1998a).
References: 
CAHEN, L., LEDENT, D. and VILLENEUVE, M. 1979. Existence d'une chaine plissee Proterozoique Superieur au Kivu Oriental (Zaire). Donnees geochronologiques relatives au supergroupe de l'Itombwe. Bulletin de la Societé Belge de Géologie, 88: 71-83.CAHEN, L., SNELLING, N.J., DELHAL, J. and VAIL, J.R. 1984. The geochronology and evolution of Africa. Clarendon Press, Oxford. 512 pp.DENAEYER, M.-E. 1958. Les syénites feldspathoïdiques du Kivu et leur rôle dans la genèse des laves des Virunga. Bulletin de la Société Belge de Géologie de Paléontologie et d'Hydrologie (Bruxelles), 67: 459-87.KAMPUNZU, A.B., KRAMERS, J.D. and MAKUTU, M.N. 1998a. Rb-Sr whole rock ages of the Lueshe, Kirumba and Numbi igneous complexes (Kivu, Democratic Republic of Congo) and the break-up of the Rodinia supercontinent. Journal of African Earth Sciences, 26: 29-36.
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith