Alkaline Rocks and Carbonatites of the World

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Bambouto

stripes

Occurrence number: 
030-00-023
Country: 
Cameroon
Location: 
Longitude: 10.08, Latitude: 5.67

This large volcanic complex, which extends in a northeast-southwesterly direction for over 50 km, has not been mapped in detail but it is clearly depicted on the geology map accompanying Gèze(1943) and, rather more simply, by Tchoua (1973). To the north it merges with the Oku volcanic centre (No. 22). Although dating of lavas gives ages of 14-23 Ma Bambouto still retains some of its original morphology and structure the highest peaks, up to 2679 m, being located around the rim of a 10 km diameter caldera; Tchoua (1972a) was able to distinguish two episodes of caldera formation. Gèze (1943 and 1953) recognised a lower basaltic series and an upper series of trachytes, trachyphonolites and phonolites and his map indicates that the latter series occupies much of the higher ground. Tchoua (1973) identified extensive areas of ignimbrite which outcrop in a 40 km diameter zone around the calderas and may be related to caldera formation. The basalts (Jérémine, 1943) are highly porphyritic rocks with olivine and clinopyroxene phenocrysts, plagioclase, which may be partly replaced by analcime, magnetite and a little calcite. They are ne normative (Jérémine, 1943; Fitton, 1987). The trachytes sometimes contain modal quartz, commonly have alkali feldspar phenocrysts and may include kaersutitic amphibole, aenigmatite and sodic pyroxene. These rocks grade through trachyphonolites into phonolites which appear to be concentrated in the summit areas. They contain phenocrysts of feldspar and a green, presumably sodic, pyroxene in a matrix of feldspar, nepheline, aenigmatite, sometimes aegirine and probable analcime. There are seven rock analyses in Deruelle et al. (1991) and Tchoua (1969) gives five analyses of ignimbrites. Rb-Sr, Sm-Nd, U, Pb and O isotopic data for three lavas and two xenoliths 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). Analyses of 15 basaltic rocks, including major and trace elements and Nd and Sr isotopic data, and some mineralogical data, are given by Marzoli et al. (2000).

Age: 
K-Ar dating indicates that the basaltic activity took place between 14.9 and 5.8 Ma and that the more evolved rocks were extruded between 22.7 and 15.9 Ma (Gouhier et al., 1974; Fitton and Dunlop, 1985). Marzoli et al. (1997) obtained dates of 17.8-15.1 Ma by 40Ar/39Ar on feldspar.
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.GÈZE, B. 1943. Géographie physique et géologie du Cameroun occidental. Mémoires du Muséum National d'Histoire Naturelle. Paris, N.S. 17: 1-271.GÈZE, B. 1953. Les volcans du Cameroun occidental. Bulletin Volcanologique, 13: 63-92.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.JÉRÉMINE , E. 1943. Contribution a l'ètude pétrographique du Cameroun Occidental. Mémoires du Muséum National d'Histoire Naturelle. Paris, NS 17: 273-320.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. TCHOUA, F. 1969. Découverte d'ignimbrites dans la région de Dschang (Cameroun). Annales de la Faculté des Sciences, Université Fédérale du Cameroun. Yaoundé, 2: 77-94.TCHOUA, F. 1972a. Sur la formation des caldéras des Monts Bambouto (Cameroun). Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences. Paris, 274D: 799-801. TCHOUA, F. 1973. Sur l'existence d'une phase initiale ignimbritique dans le volcanisme des Monts Bambouto (Cameroun). Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences. Paris, 276D: 2863-6.
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