Alkaline Rocks and Carbonatites of the World

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Mikeno

stripes

Occurrence number: 
038-00-009
Country: 
Comoros
Location: 
Longitude: 29.42, Latitude: -1.47

At 4437 m Mikeno is the second highest volcano of the Virunga field; it is deeply eroded and probably extinct. There is apparently no summit crater (Combe and Simmons, 1933), but the volcano is deeply eroded. Guibert et al. (1975) were able to distinguish four zones, which are depicted in a simple cross-section. Zone (a) comprises a mass of alkaline rhyolites which includes breccias. Based on age determinations Guibert et al. (1975) include as a sub-zone of (a) a distinctive leucitite, which is referred to as ‘mikenite’. Holmes and Harwood (1937) present detailed descriptions and a chemical analysis of mikenite. It comprises leucite euhedra up to 7 mm in diameter, which may have rims altered to nepheline and alkali feldspar, and rather larger augite phenocrysts. The groundmass contains greenish pyroxene, a little alkali feldspar, though more abundant in some varieties, nepheline, leucite, possible sodalite, abundant opaque phases, occasional melilite, a little plagioclase in some rocks and accessory apatite and perovskite in a glassy base. These rocks pass into olivine leucitites which contain olivine as phenocrysts. The second zone (b) comprises tuffs, agglutinates and lavas of phonolite, tephrite and rare trachyte and accounts for 80-90% of the total edifice. Zone (c) consists of phonolitic and tephritic lavas that appear to have formed a lava lake. The fourth, and terminal zone (d), is located in the summit area and consists of leucite phonolite agglutinate with sills of trachyte. Some general field observations of some of the lava flows and boulder deposits up to 8 m thick, the boulders being of leucitite, are given by Combe and Simmons (1933). Collected analyses of Mikeno rocks are in Denaeyer et al. (1965). Strontium isotope data on three specimens of mikenite are given by Bell and Powell (1969).

Age: 
Guibert et al. (1975) give K-Ar age determinations on 10 rocks and separated minerals representing all four zones. They range from 0.2(0.1 to 4.6(0.1 Ma. However, they conclude that the period of major activity was from 2.5 to 0.8 Ma and the terminal activity from 0.5 Ma to Recent.
References: 
BELL, K. and POWELL, J.L. 1969. Strontium isotopic studies of alkalic rocks: the potassium-rich lavas of the Birunga and Toro-Ankole regions, east and central equatorial Africa. Journal of Petrology, 10: 536-72.COMBE, A.D. and SIMMONS, W.C. 1933. The geology of the volcanic area of Bufumbira, South-west Uganda. Geological Survey of Uganda, Memoir, 3 (Part 1): 1-150.DENAEYER, M-E., SCHELLINCK, F. and COPPEZ, A. 1965. Recueil d’analyses des laves du fossé tectonique de l’Afrique Centrale (Kivu, Rwanda, Toro-Ankole). Annales, Musée Royal de l’Afrique Centrale, Tervuren, Belgique. Serie In 8(, Sciences Géologiques, 49: 1-234.GUIBERT, P., DELALOYE, M. and HUNZIKER, J. 1975. Contribution à l’étude géologique du volcan Mikeno, Chaîne des Virunga (République du Zaïre). Comptes Rendu des Sciences de la Société de Physique et de l’Histoire Naturelle de Geneve, 10: 57-66. HOLMES, A. and HARWOOD, H.F. 1937. The volcanic area of Bufumbira. Part 2. The petrology of the volcanic field of Bufumbira, south-west Uganda, and of other parts of the Birunga field. Geological Survey of Uganda, Memoir, 3 (Part 2): 1-300.
Location: 
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