Alkaline Rocks and Carbonatites of the World

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Nyamuragira (Nyamlagira, Namlagira) `

stripes

Occurrence number: 
038-00-007
Country: 
Comoros
Location: 
Longitude: 29.2, Latitude: -1.4

This, the most active volcano of the Virunga field, is a symmetrical shield volcano having an elevation of 3056 m (about 1600 m above Lake Kivu) and measuring 20 km across the base. A summit crater is 2 km in diameter with walls up to 100 m (Denaeyer et al., 1965, Fig. 3). Lavas overlie Precambrian basement rocks as well as Pleistocene and Recent deposits and extend up to 60 km from the central vent. In the south and east Nyamuragira products abut those from the Nyiragongo, Mikeno and Visoke volcanoes. The greatest activity appears to have been in the late Pleistocene but between 1900 and 1982 low viscosity lavas were erupted 17 times, mainly from rifts on the flanks. The production rate during the period 1900-82 was about 12.2x106 m3 per year, suggesting that the volcano was formed within the last 40,000 years (Ueki, 1983; Aoki et al., 1985). The lavas define a series from olivine basanite through tephrite to phonolitic tephrite and tephritic phonolite, of which phonolitic tephrite is by far the most abundant (Aoki et al., 1985). Phonolitic tephrite varies from aphyric to porphyritic (0-12% phenocrysts) with phenocrysts of salite, and more rarely chromian diopside, olivine, occasionally with cores of chromian spinel, and sometimes plagioclase and titanomagnetite. Olivine basanite and tephrite are porphyritic to highly porphyritic rocks (up to 40% phenocrysts) but with the same phenocryst assemblage; with decreasing olivine and salite phenocrysts they grade into phonolitic tephrite. Tephritic phonolite differs in its more felsic nature. The groundmass of the whole series is similar and ranges from glassy to holocrystalline granular; it comprises plagioclase, salite, olivine, titanomagnetite, leucite and apatite with accessories including titanian biotite, anorthoclase-sanidine, nepheline, hornblende, ilmenite, sodalite and pyrrhotite. Analyses of olivine, clinopyroxene, feldspar, leucite, nepheline, sodalite, hornblende, titanian biotite, iron oxides and pyrrhotite will be found in Aoki et al. (1985), who also give whole rock analyses, including a range of trace elements. A strontium isotope study, based on the same specimens, is that of Aoki and Kurasawa (1984) and Aoki et al. (1981) report data for fluorine. Further petrography and chemistry of lavas and ejected blocks are reported by Holmes and Harwood (1937) who classify many of the rocks as 'kivite'; strontium isotope data on some of this material is given by Bell and Powell (1969). Collected rock analyses are in Denaeyer and Schellinck (1965). A number of papers, many on geophysical topics, are included in a volume edited by Hamaguchi (1983), while a useful account of early work on the volcano, including a bibliography of activity between 1894 and 1956, will be found in Richard and Padang (1957). Accompanying Thonnard and Denaeyer (1965) is a coloured geological map on a scale of 1:50,000.

References: 
AOKI, K. and KURASAWA, H. 1984. Sr isotope study of the tephrite series from Nyamuragira volcano, Zaire. Geochemical Journal, 18: 95-100.AOKI, K., ISHIKAWA, K. and KANISAWA, S. 1981. Fluorine geochemistry of basaltic rocks from continental and oceanic regions and petrogenetic application. Contributions to Mineralogy and Petrology, 76: 53-9.AOKI, K., YOSHIDA, T., YUSA, K. and NAKAMURA, Y. 1985. Petrology and geochemistry of the Nyamuragira volcano, Zaire. Journal of Volcanology and Geothermal Research, 25: 1-28.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.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.HAMAGUCHI, H. (ed) 1983. Volcanoes Nyiragongo and Nyamuragira: geophysical aspects. Faculty of Science, Tohoku University, Sendai, Japan. 130 pp. 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.RICHARD, J.J. and PADANG, M.N. 1957. Active volcanoes of the world. Part 4 Africa and the Red Sea. International Volcanological Association, Napoli, Italy. 118 pp.THONNARD, R.L.G. and DENAEYER, M.-E. 1965. Carte volcanologique des Virunga (Afrique centrale). Feuille No. 1. 1:50,000 and Introduction generale et Notice Explicative de la Feuille No 1. Centre National de Volcanologique (Belgique), BruxellesUEKI, S. 1983. Recent volcanism of Nyamuragira and Nyiragong. In H. Hamaguchi (ed) Volcanoes Nyiragongo and Nyamuragira: geophysical aspects. 7-18. Faculty of Science, Tohoku University, Sendai, Japan.
Location: 
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