Alkaline Rocks and Carbonatites of the World

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Kirumba

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Occurrence number: 
038-00-004
Country: 
Comoros
Location: 
Longitude: 29.3, Latitude: -1.08

The approximately 30 km2 Kirumba complex is limited on three sides by Precambrian schists and quartzites, but the eastern margin is against lavas of the extensive Virunga volcanic field (No. 6). Four major groups of intrusive rocks are described by Denaeyer (1958), the first of which consists of mica-bearing syenites that vary from silica oversaturated to marginally undersaturated types but they do not appear to be peralkaline. The second group comprises feldspathoidal syenites including nepheline-sodalite-cancrinite, sodalite-cancrinite, sodalite and cancrinite syenites. There are a range of textural variants including aplitic, porphyritic and pegmatitic types. These rocks include microcline perthite and albite, occasional zeolite, aegirine, biotite, melanite, titanite, apatite, zircon, fluorite and opaque phases; accessories include pyrochlore, chevkinite and possible cerite and lavenite. A cancrinite-aegirine syenite, which contains accessory thorite, is referred to by Denaeyer (1959) as lujavrite. A rock composed of microcline, albite, aegirine and melanite and forming narrow pegmatitic dykes is named ordosite while another pegmatitic rock is a melteigite consisting essentially of nepheline and aegirine. Denaeyer (1959) gives chemical analyses of all the major rock types. He later discovered a small area of carbonatite at the northeastern margin of the complex within syenites, which are albitized. The carbonatite is a rauhaugite composed principally of ankerite of which rock analyses will be found in Denaeyer (1966) and carbon and oxygen isotopic data in Denaeyer (1970).

Age: 
Biotite from nepheline syenite and syenite gave K-Ar ages of 555(17 and 665(33 Ma (Cahen and Snelling, 1966). An eight point Rb-Sr whole rock isochron gave 803(22 Ma (Kampunzu et al., 1998a).
References: 
CAHEN, L. and SNELLING, N.J. 1966. The geochronology of Equatorial Africa. North-Holland, Amsterdam. 195 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.DENAEYER, M.E. 1966. Sur la présence d'une carbonatite ankéritique (rauhaugite) en bordure du complexe alcalin de Kirumba (Kivu). Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences, Paris, 263D: 9-12.DENAEYER, M.E. 1970. Rapports isotopiques (O et (C et conditions d'affleurement des carbonatites de l'Afrique centrale. Comptes Rendus Hebdomadaire des Séances de l'Académie des Sciences, Paris, 270D: 2155-8.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