Alkaline Rocks and Carbonatites of the World

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Oku

stripes

Occurrence number: 
029-00-022
Country: 
Cameroon
Location: 
Longitude: 10.55, Latitude: 6.33

The Oku massif is a large multiple volcanic field with a diameter of nearly 100 km. There are four major stratovolcanoes namely Mount Oku, Mount Babanki, Nyos and Nkambe (Derruelle et al., 1991). Mount Oku reaches 3011 m and is built of basaltic and hawaiitic lavas which range from slightly ne to slightly hy normative (Fitton, 1987). These were succeeded by trachytes and these in turn by large volumes of trachytic and rhyolitic ignimbrites which have thicknesses up to 1000 m. There followed further trachytic lavas, tuffs and breccias. The final activity involved production of pyroclastic cones and explosion craters, one of which is now occupied by Lake Oku, from which emanated flows in the north of basalt rich in peridotite and pyroxenite xenoliths and in the south rhyolitic and phonolitic lavas. The trachytic lavas contain phenocrysts of sanidine and fewer of hedenbergite, fayalite, arfvedsonite, richterite and aenigmatite. Analyses of a rhyolite and a rhyolitic welded tuff are given by Fitton (1987) and Rb-Sr, Sm-Nd, U, Pb and O isotopic data on a rhyolite and a transitional basalt are in Halliday et al. (1988) and Hf, Sr, Nd and Pb isotope data on two further lavas are given by Ballentine et al. (1997). The mineralogy and rock chemistry of three spinel lherzolite xenoliths from Lake Enep and the vicinity of lake Enep are described in detail by Lee et al. (1996). Analyses of 7 basaltic rocks, including major and trace elements and Nd and Sr isotopic data, and some mineralogical data, are given by Marzoli et al. (2000).Mount Babanki lies 15 km southwest of Oku, comprises basaltic and trachytic lavas and has a summit area occupied by explosion craters. Nkambe is located northeast of Oku and consists of basaltic and trachytic lavas, but Nyos is built essentially of basaltic lavas and pyroclastics including surge deposits. An aerosol of water and CO2 was released from Lake Nyos in August 1986 creating a heavy toxic cloud which swept down valleys to the north killing more than 1700 people. There are many references on this event, but see the papers published after an International Conference on the occurrence (Guern and Sigvaldason, 1989).

Age: 
K-Ar on trachyte gave 22.7(1 (Gouhier et al., 1974) and further analyses on two rhyolites and a hawaiite ranged from 17 to 23 Ma (Fitton and Dunlop, 1985). Marzoli et al. (1997) give a date of 24.9 Ma by 40Ar/39Ar on feldspar. Well formed cinder cones are indicative of relatively recent activity.
References: 
BALLENTINE, C.J., LEE, D.-C. and HALLIDAY, A.N. 1997. Hafnium isotopic studies of the Cameroon line and new HIMU paradoxes. Chemical Geology, 139: 111-24.DERUELLE, B., MOREAU, C., NKOUMBOU, C., KAMBOU, R., LISSOM, J., NJONFANG, E., GHOGOMU, R.T. and NONO, A. 1991. The Cameroon Line: a review. In A.B. Kampunzu and R.T. Lubala (eds), Magmatism in extensional structural settings. 274-327. Springer-Verlag, Berlin.FITTON, J.G. 1987. The Cameroon line, West Africa: a comparison between oceanic and continental alkaline volcanism. In J.G. Fitton and B.G.J. Upton (eds), Alkaline igneous rocks. 273-91. Geological Society of London Special Publication 30.FITTON, J.G. and DUNLOP, H.M. 1985. The Cameroon line, West Africa, and its bearing on the origin of oceanic and continental alkali basalt. Earth and Planetary Science Letters, 72: 23-38.GUERN, F.LE, and SIGVALDASON, G.E. (eds) 1989. The Lake Nyos event and natural CO2 degassing, I. Journal of Volcanology and Geothermal Research (Special Issue), 2-3: 97-275.HALLIDAY, A.N., DICKIN, A.P., FALLICK, A.E. and FITTON, J.G. 1988. Mantle dynamics: a Nd, Sr, Pb and O isotopic study of the Cameroon Line volcanic chain. Journal of Petrology, 29: 181-211.LEE, D.-C., HALLIDAY, A.N., DAVIES, G.R., ESSENE, E.J., FITTON, J.G. and TEMDJIM, R. 1996. Melt enrichment of shallow depleted mantle: a detailed petrological, trace element and isotopic study of mantle-derived xenoliths and megacrysts from the Cameroon Line. Journal of Petrology, 37: 415-41.MARZOLI, A., RENNE, P. and PICCIRILLO, E.M. 1997. 40Ar/39Ar data on volcanics of the continental Cameroon Line (Africa): time related migration of trachytic volcanism. Plinius, 18: 141-2.MARZOLI, A., PICCIRILLO, E.M., RENNE, P.R., BELLIENI, G., IACUMIN, M., NYOBE, J.B. and TONGWA, A.T. 2000. The Cameroon volcanic line revisited: petrogenesis of continental basaltic magmas from lithospheric and asthenospheric mantle sources. Journal of Petrology, 41: 87-109.
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