Évolution volcanologique du mont Manengouba (Ligne du Cameroun) ; nouvelles données pétrographiques, géochimiques et géochronologiques<br /> Volcanological evolution of the mount Manengouba (Cameroon line), new petrographical, geochemical, and geochronological data

Le mont Manengouba est un complexe volcanique polygénique de la Ligne du Cameroun édifié au Quaternaire, entre 1,55 et 0 Ma, en trois phases principales. La première phase, de 1,55 à 0,7 Ma, correspond à la construction du volcan Elengoum. La deuxième, entre 0,7 et 0,56 Ma, est marquée par l'effondrement de la partie sommitale de l'Elengoum. La troisième, de 0,56 à 0 Ma, voit l'édification du volcan Eboga, puis la formation de la caldeira, et comporte une activité fissurale adventive à partir de 0,48 Ma. Les produits émis définissent une série alcaline sodique, allant des basaltes à néphéline à des trachytes. Toutes ces laves ont évolué par cristallisation fractionnée dans une chambre magmatique périodiquement réalimentée. The mount Manengouba is a polygenic volcanic complex of the Cameroon line, which was built in the Quaternary, between 1.55 and 0 Ma, during three stages. The first stage, from 1.55 to 0.7 Ma, corresponds to the building of the Elengoum volcano. The second, between 0.7 and 0.56 Ma, points to the collapse of the Elengoum summit. The third, from 0.56 to 0 Ma, fits with the erection of the Eboga volcano and its caldera sinking, and, from 0.48 Ma, with the adventive fissural activity. The products define an alkaline sodic series, from nepheline-bearing basalts to trachytes. All the lavas evolved by fractional crystallization in a periodically replenished magmatic chamber

The Caldera of Mount Bambouto: Volcanological Characterization and Classification

Mount Bambouto culminates at 2744 m (Meletan Mountain) where an elliptical caldera of 16 × 8 km is found. Although that caldera has been a subject of numerous scientific works, complementary studies were needed to bring out additional data used to classify it through the Caldera DataBase of Geyer and Marti (2008). It emerges that Bambouto Caldera codes are 2 and 203 because it is respectively located in Africa and Central Africa according to the numbering system developed in the Catalog of Active Volcanoes of the World. The collapse type of the caldera is piecemeal; this relies on the fact that the caldera floor is uneven. Several rocks crop out in the caldera; accordingly, its code is B, I, T, P, and Ig viz. basalts, intermediate rocks, trachytes, phonolites, and ignimbrites. Bambouto depression is the ignimbrite caldera because it is associated with thick ignimbrite sheer, that ruled its collapse. The chemical analysis of rocks reveals that the magmatic series of Bambouto Caldera is of alkaline type. It has been built through the continental rifting of extensional type (RC-EXT). The collapse process has been followed by post-caldera protrusion of trachytic and phonolitic domes; then, its codes are Type-S and type-MS

Antidiarrhoeal activity of aqueous and methanolic extracts of Oxalis corniculata Klotzsch in rats

The antidiarrhoeal activity of the aqueous and methanolic extracts of Oxalis corniculata (Oxalidaceae) was evaluated on castor oil-induced diarrhoea in rats and on small muscle intestinal transit. At orally administrated doses of 160, 320 and 640 mg/kg of body weight, the two plant extracts significantly (

The geodiversity of Lefo and Santa-Mbu Calderas (Bamenda Mountains, Cameroon Volcanic Line): Factor for socioeconomic activities

Lefo and Santa-Mbu calderas are located on the Bamenda Mountains. They are two geological structures that accompanied the construction of the Bamenda Mountains. The geological processes that accompanied their collapse gave them a significant number of geological features. These elements, known as geodiversity components, constitute real assets for ecosystem services in these more or less populated volcanic environments. The present work aims to highlight the ecosystem services of these two calderas in relation to the socio-economic activities they govern. To this end, field and laboratory work was carried out. They revealed that the ecosystem services of the two calderas include: (1) regulation services because the calderas are covered in places by forest reserves, notably that of Bali Ngemba and Bafut Ngemba, and by a grassland with trees that play a crucial role in climate regulation. In addition, these areas are subject to natural hazards such as mass movements and volcanic eruptions; (2) supporting services as they provide a base for human activities, a habitat for wildlife species; (3) provisioning services because the calderas have fairly fertile soils that favour a fairly diversified agricultural activity, making the floor of the Lefo caldera a coffee growing area. These calderas are covered by a herbaceous grassland that favours pastoral activity in the region. In addition, the rocks of these calderas are used in construction, especially in the foundations and walls of houses; (4) cultural services as they provide a setting for traditional ritual practices and plants with therapeutic properties. In addition, it has several geomorphosites whose scientific and additional values favour the implementation of geotourism and offer geological features that are indispensable for the understanding and functioning of the Volcanic Line of Cameroon. These calderas are undeniable assets for the balance of the ecosystem

Mantle Xenoliths from Ibal-Oku (Oku massif, North-west Region, Cameroon): Imprints of Superimposed Carbonatitic and Silicic Metasomatisms

Mantle xenoliths have been discovered in Ibal-Oku basalts from Oku Massif, Cameroon Volcanic Line. These xenoliths analyzed in term of major elements by scanning electron microscope, atomic emission spectrometry, traces and rare earth elements by mass spectrometry are peridotites and pyroxenites. Peridotites comprise Fe-rich lherzolites, harzburgites and wehrlites. Pyroxenites comprise websterites, olivine-websterites, clinopyroxenites and olivine-clinopyroxenites. Mineralogically, olivine Fo% values and NiO content vary from 85 to 91 and 0.26 to 0.43 wt.%, respectively. Orthopyroxene is enstatite, Mg# values and Al content ranging from 0.83 to 0.92 and 0.12 to 0.27 atom per formula unit (apfu), respectively. Clinopyroxene is augite and diopside, Mg# values and Al content ranging from 0.83 to 0.93 and 0.23 to 0.37 apfu, respectively. Spinel is aluminous, Cr# and Mg# values ranging from 0.07 to 0.23 and 0.67 to 0.82, respectively. Micas are biotites (Fe#: 0.52-0.76). Feldspars, which are secondary are sanidine, andesine and labradorite. Geochemically, peridotite Mg# values vary from 82.7 to 89.9 and pyroxenites from 80.1 to 83.6. The major element variations and some compatible elements are described in terms of partial melting (14-15 vol.% in lherzolites and 17-18 vol.% in harzburgites), whereas the heterogeneities in trace elements are related to carbonatitic/silicic metasomatism

Evolution of volcanism in graben and horst structures along the Cenozoic Cameroon Line (Africa): implications for tectonic evolution and mantle source composition

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

Contribution of GIS to Hydromorphometric Characterization of the Nkoup Watershed (Nun Plain-Cameroon)

The Nkoup watershed (10°35’-10°47’E and 5°27’-5°42’N) is a volcanic zone situated in Nun Plain West Cameroon. The high fertility of the soils makes it a strategic agropastoral area where water resources are heavily exploited and used for several purposes. Due to human activities, soils and water resources are deteriorating, giving birth to water pollution and hydromorphological hazards. This work aims to determine the hydromorphometric parameters of the Nkoup watershed so that the data obtained help in the sustainable management of water resources and conservation of soil. To achieve this aim, various data were collected from DEM dataset derived from SRTM and processed in specialized software (QGIS and ArGIS). The simplified hydrological balance was calculated using the upstream approach. The Nkoup watershed has: Axial length Lax = 25.8 km, Axial Width Wax = 11.1 km, Perimeter P = 132.6 km, Area A = 173.7 km2 , Average Altitude Ha = 1726.3 m, Compactness Index Icomp = 2.8, Relief ratio Rr = 3.9 m/km, Circularity ratio Rc = 0.1, Elongation ratio R = 0.1, Drainage texture ratio Rt = 0.6, Drainage density Dd = 0.5 km/km2 . Stream Frequency Fs = 0.4, Channel Sinuosity Index CSI = 0.8, Stream gradient Sg = 0.6 and global slope Index Ig = 6.8 m/km. The specific height Difference Ds = 89.4 m shows moderate relief. The precipitation and evapotranspiration are unevenly distributed. With P = 187.7 mm/an, ETP = 953.4 mm/an, Q = 4.2 m3 /s, R = 762.5 mm/an, ETR = 832.3 mm/an and I = 282.9 mm/an. The Nkoup, 36.9 km long, has a sinuous aspect due to the low slope and the high CSI. The piezometric levels vary according to the seasons and the groundwater flow follows the N-S direction as surface flow

The discovery of late Quaternary basalts on Mount Bambouto: Implications for recent widespread volcanic activity in the southern Cameroon Line

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Geohazards (floods and landslides) in the Ndop Plain, Cameroon Volcanic Line

International audienceThe Ndop Plain, located along the Cameroon Volcanic Line (CVL), is a volcano-tectonic plain, formed by a series of tectonic movements, volcanic eruptions and sedimentation phases. Floods (annually) and landslides (occasionally) occur with devastating environmental effects. However, this plain attracts a lot of inhabitants owing to its fertile alluvial soils. With demographic explosion in the plain, the inhabitants (143,000 people) tend to farm and inhabit new zones which are prone to these geohazards. In this paper, we use field observations, laboratory analyses, satellite imagery and complementary methods using appropriate software to establish hazard (flood and landslide) maps of the Ndop Plain. Natural factors as well as anthropogenic factors are considered. The hazard maps revealed that 25% of the area is exposed to flood hazard (13% exposed to high flood hazard, 12% to moderate) and 5% of the area is exposed to landslide hazard (2% exposed to high landslide hazard, 3% to moderate). Some mitigation measures for floods (building of artificial levees, raising foundations of buildings and the meticulous regulation of the flood guards at Bamendjing Dam) and landslides (slope terracing, planting of trees, and building retaining walls) are proposed

Xenoliths of dunites, wehrlites and clinopyroxenites in the basanites from Batoke volcanic cone (Mount Cameroon, Central Africa): petrogenetic implications

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