Vulkanologische, petrologische und zeitliche Entwicklung der Bambouto-Caldera (Kamerun)

The Bambouto mountain, third largest massif of the Cameroon line culminating at 2740 m at Meletan is characterized at the summit by an ellipse-like collapse caldera (13 * 8 km). The caldera had been formed by succesive collapses, giving it a step-like structure. The volcanic activity is controlled by 3 eruptive mechanisms: an effusive type responsible for the lava flows, an intrusive type that is characterized by the trachyte and the phonolite domes, and an "explosive" type that accompanied nearly every volcanic eruption in the caldera and covered nearly all the Bambouto-massif with tuffs. 40Ar/39Ar laser dating of feldspar crystals helps to distinguish 2 principal volcanic phases, one between ca. 16 and ca. 11 Ma and the other between ca. 9 and ca. 5 Ma. Considering only the caldera, the trachytes are the first eruptions while the phonolites are the last. The subdivision of the volcanic phases into 3 volcanic series after Geze (1943) ist not applicable for the Bambouto mountains. 9 rock types including alkali-basaltes, basanites, tephrites, hawaiites, mugearites, latites, phonolites, rhyolites and trachytes (lavas and ignimbrites) were examined in the studied sector and have a microlic porphyritic oder aphyric structure, sometime vitroclastic (ignimbrite) in thin sections. Olivine, plagioclase clinopyroxene and titanomagnetite are the principal mineral phases of the mafic volcanics while the alkali-feldspar, and in lesser amounts clinopyroxene, plagioclase and titanomagnetite characterize the felsites. The chemical composition of the different minerals and their crystallization temperatures evolve from the mafic to the felsic rocks. The geochemical study emphasizes a compositional gap between the mafic and the felsic rocks: this suggests bimodal volcanism. When considering the behavior of trace and rare earth elements, the stratigraphy and the age criteria, 4 volcanic series (mostly incomplete) were identified. The crystal fractionation is the principal mechanism of the differentiation of the caldera rocks. The Nb/Zr and La/Ce ratios are pratically constant in all rocks, whatever the series or volcanic phase. This suggests their derivation from the same magmatic source, located probably in the upper mantle