International audienceTombel graben and Mounts Bambouto are two volcanic fields of the typical system of alternating graben and horst structure of the Cameroon Volcanic Line. Tombel graben is a young volcanic field, whereas Mounts Bambouto horst is an old stratovolcano with calderas. Volcanic products in both settings have a signature close to that of Ocean Island Basalt implying a major role of FOZO (focal zone) component and varied contribution of depleted mantle (DMM) and enriched mantle (EM) components. The Cameroon Volcanic Line is a hot line essentially resulting from passive rifting. Eocene to Recent intraplate basaltic volcanism in the study area was probably a result of mantle upwelling coupled with lithospheric extension. The olivine basaltic magma of horst volcanoes evolved in a large-scale, steady-state magmatic reservoir via crystal fractionation and limited contamination to highly differentiated alkaline lavas (trachyte and phonolite). Conversely, rapid ascent of lavas along multiple fault lines of graben structures produced less evolved lavas (hawaiite) within small reservoirs. This model, evaluated for the study area, involves mantle upwelling inside zones of weakness in the lithosphere after intra-continental extension. It can be applied to other parts of the Cameroon Volcanic Line as well, and is similar to that described in other intra-continental rift-related areas in Africa

International audienceThe lavas of the Mount Cameroon, a Plio-Quaternary stratovolcano and the most important volcano along the Cameroon Volcanic Line (CVL), constitute a weakly differentiated alkaline series: mainly comprising basanites as well as alkaline basalts, hawaiites and mugearites. Ultramafic xenoliths (1–5 × 0.5–4 cm) of dunites, wehrlites and clinopyroxenites have been discovered in the basanites of a strombolian cone, located near Batoke on the South flank of the massif at an elevation of 500 m. K-Ar whole rock dating of the basanitic host rock has yielded an age of 0.73 ± 0.08 Ma. This result falls within the range of the seven new K-Ar age determinations of mafic lavas, between 2.83 Ma and the Present. These are the first K-Ar data on this massif. The 87Sr/86Sr ratios of basic lavas are low (0.703198–0.703344), and 143Nd/144Nd ratios are intermediate (0.512851–0.512773). These ratios are typical of a mantle origin. The main characteristics of the xenoliths are: (a) total FeO contents are 15.1 to 19.1 wt.% in olivines (chrysolite, Mg# ranging from 79 to 84) of xenoliths, and 4.7 to 6.9 wt.% in diopsides of xenoliths, (b) diopsides of the clinopyroxenites have up to 7.2 wt.% Al2O3 and 2.3 wt.% TiO2, (c) spinels occur as interstitial grains between chrysolite and diopside grains, i.e. Cr2O3-rich magnetites (19 to 21 wt.% Cr2O3) in the dunites as well as (22 to 25 wt.% Cr2O3) in the wehrlites and titanomagnetites (14 to 15 wt.% TiO2) in the clinopyroxenites. Mineralogical analyses show an important re-equilibration between the chrysolite xenocrysts and the host basanitic magma. We observed a decrease in Mg and Ni towards the rim, and an enrichment in all others cations like Fe, Mn, Ca, Si. The changes of Fe2+ / Mg2+ are the most important. The xenoliths are interpreted as cumulates: clinopyroxenite xenoliths have probably crystallized and fractionated at an early stage from the mafic (host basanitic) magma, while dunite and wehrlite xenoliths seem to have crystallized from a previous more primitive batch of magma. These alkaline liquids could have been derived from partial melting of a garnet- rich lherzolite in the upper mantle beneath the Cameroon Volcanic Line. The AlIV/AlVI ratios remain high (1.2 to 4.9) in the clinopyroxenes of the xenoliths. This suggests crystallization under a lower pressure than that of equilibration of the clinopyroxenes (ratios 0.6 to 0.8) found in typical mantle xenoliths from the CVL

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International audienceThe volcanic story of Mount Manengouba is related to four chronological stages: (1) forming of the early Manengouba shield volcano between 1.55 and 0.94 Ma, (2) building of the Eboga strato-cone between 0.94 and 0.89 Ma, (3) caldera collapse and silicic extrusions of the Elengoum Complex between 0.89 and 0.70 Ma, and (4) intra-caldera and flank activity between 0.45 and 0.11 Ma. The volume of the volcano is calculated at 320 km3 ± 5%. The volcanic rocks are attributed to two magmatic outputs. The first and main magma generation produced the shield volcano, the strato-cone, and the syn- to post-caldera extrusions, displaying a complete series from basanites to trachytes (magmatic Group 1). The second magma generation is limited to the late and flank activity evolving from basanites to trachy-phonolite (magmatic Group 2). Both magmatic groups belong to the under-saturated alkaline sodic series. Petrological calculations locate the magmatic reservoir between 37 and 39 km in the upper mantle for the Group 1 lavas, and between 42 and 44 km for the Group 2 lavas. Trachytes were generated in a secondary crustal reservoir. Magmatic series evolve with medium to low pressure fractional crystallization of olivine, pyroxene, oxides, feldspar, and apatite. Significant crustal assimilation is evidenced in trachytes. The magma of Group 1 was generated with 3-6% of partial melting of a moderately enriched source containing 3-7% of garnet. Melting took place in the spinel to garnet transition zone located at 70-90 km and around 25 kb. The magma of Group 2 resulted from a slightly higher partial melting from a less garnet-rich source that indicates uprising of the melting column in the upper part of transition zone. Sr, Nd, and Pb isotope data of the Manengouba rocks and neighboring lavas are analyzed and compared with those of the mafic lavas of the CVL. Three source components are distinguished: a depleted component originated from the asthenospheric swell, a radiogenic component linked to the contaminated lithosphere of the Neoproterozoic mobile belt, and an enriched component or the lithosphere possibly related to pre-rifting magmatic processes