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).
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