Alkaline Rocks and Carbonatites of the World

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Kahusi

stripes

Occurrence number: 
038-00-015
Country: 
Congo (Kinshasa)
Location: 
Longitude: 28.68, Latitude: -2.2

Kahusi comprises both intrusive and extrusive suites which occupy an area of about 9x9 km. There is, however, a considerable disparity in the ages of the suites (see below) which inevitably makes their petrogenetic relationship problematical. The intrusive part is an annular structure of syenites and granites which are overlain in the south and east by an arc of volcanic hills, of which Kahusi itself is the highest. Only parts of both suites are peralkaline. The plutonic units are as follows (Biayi-Kalala, 1983; Kampunzu et al., 1985; Lubala et al., 1985): annite-lepidomelane syenite, quartz syenite with ferrohastingsite and annite, biotite microgranite and granites, including varieties with amphibole + biotite, arfvedsonite, arfvedsonite + aenigmatite and arfvedsonite + aegirine. Details of the mineralogy are tabulated by Kampunzu et al. (1985). The volcanic rocks include rhyolitic flows and tuffs with some trachytes. A number of individual flows and units have been mapped by Boutakoff (1956) and Kampunzu et al. (1985) consider their distribution to be typical of a cauldron, the curved line of volcanoes possibly standing above a ring fault. The rhyolites are quartz- and feldspar-phyric with sodic amphibole in some varieties. The trachytes contain feldspar, and sometimes quartz, phenocrysts, with biotite in most varieties (Biayi-Kalala, 1981). All the rhyolites are highly potassic and the trachytes particularly so (K2O 12.68%; Na2O 0.55%) by which they differ markedly from the intrusive suite in which the alkalis are approximately equal. Analyses for major elements of both suites are given by Kampunzu et al. (1985), for the intrusive rocks by Biayi-Kalala (1983) and for the trachytes by Biayi-Kalala (1981).

Age: 
K-Ar dates on quartz porphyry and riebeckite granite of 134(7 Ma and 55(3 Ma were obtained by Bellon and Pouclet (1980), but determinations, also by K-Ar, on four samples by Vellutini et al. (1981) ranged from 452(11 to 493(12 Ma.
References: 
BELLON, H. and POUCLET, A. 1980. Datations K-Ar de quelques laves du Rift-ouest de l'Afrique Centrale; implications sur l'évolution magmatique et structurale. Geologische Rundschau, 69: 49-62.BIAYI-KALAYA, W. 1981. Les trachytes dans les laves acides du Kahuzi (Kivu, Zaïre): données pétrographiques et chimiques. Rapport Annuel Pour l'Annee 1980 du Départment de Géologie et de Minéralogie du Musée Royal de l'Afrique Centrale, Tervuren, Belgique, 179-94.BIAYI-KALALA, W. 1983. Les syénites et les granites du complexe du Kahuzi (Kivu, Zaïre): données pétrographiques et chimiques. Rapport Annuel du Départment de Géologie et de Minéralogie du Musée Royal de l'Afrique Centrale, Tervuren, Belgique, (1981-1982): 47-59. BOUTAKOFF, N. 1956. Les massifs volcaniques du Kahusi et du Biega (Kivu, Congo Belge). Mémoires de l'Institut Géologique de l'Université de Louvain, 9 (5): 1-41. KAMPUNZU, A.B., LUBALA, R.T., MAKUTU, M.N., CARON, J.-P., ROCCI, G. and VELLUTINI, P.J. 1985. Les complexes alcalins de la région interlacustre à l'est du Zaïre et au Burundi: un example de massifs anorogeniques de relaxation. Journal of African Earth Sciences, 3: 151-67.LUBALA, R.T., KAMPUNZU, A.B. and MAKUTU, M.N. 1985. Un inventaire des complexes anorogéniques du Burundi, du Rwanda et du Zaïre. Journal of African Earth Sciences, 3: 169-74.
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