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Located on the central part of the continental section of Cameroon Volcanic Line, Oku Mountains are one of the largest massifs of this volcano-tectonic structure. These mountains contain ultramafic mantellic xenoliths of peridotites and pyroxenites accidently reared by the Ibal-Oku recent basalts . Petrographically, the peridotites are composed of Fe-rich lherzolites and wehrlites (mg# between 82.7 and 89.9) whereas pyroxenites which constitute the majority of the xenoliths consist of websterites and olivine-websterites (mg# range 80.1–83.6). Some of the pyroxenites Cpx embody apatite and calcite and numerous fluid inclusions . From a geochemical point of view, the study of these xenoliths shows that the upper mantle of the Oku region in general and that of Ibal-Oku in particular has undergone at least two major events: the variations observed among the major elements and some compatible elements can be described in terms of partial melt, whereas the observed heterogeneities in trace elements and rare earths are related to carbonatitic metasomatism . Peridotites are Fe-rich, likely as a result of basaltic melt percolation . Texturally, these xenoliths have secondary protogranular and porphyroclastic textures and bear marks of deformation from the upper mantle. This mantle underwent a lithospheric uplift marked by the presence of clinopyroxenes exsolution lamellae in the orthopyroxenes and screws towards it.

Lack of published data on the petrography and quality of drinkable water sources in Santa Sub Division renders the water quality in the locality doubtful. This research work was therefore aimed at evaluating the quality of spring water and its effects on human health since they depend on springs as their major source of drinking water. This work examines some petrographic, physicochemical and bacteriological properties and anthropogenic activities on water sources. Rock and water samples were collected from Mbei, Mbu, Njong and Santa Akum localities and analyzed using standard methods. Petrographic studies revealed three rock types: basalts, trachytes, and rhyolites. Organoleptic and physicochemical parameters fell within WHO limits indicating no contamination except for iron. Water facies were HCO 3 –CO 3 –Cl, Ca–Na–K, SO 4–Ca–Mg and HCO 3 –Ca–Mg while the major ions were HCO3 - , K + , Ca 2+ and Mg 2+ with Total dissolved solids having the highest concentration (147.6 for both seasons), suggesting an influence of rock silicate weathering and ion exchange processes. However, pathogens were present in all the springs thus prevalence of waterborne diseases in Santa. This poor water quality was likely due to poor hygiene and anthropogenic activities. This water may lead to health implication to the community. Keywords : Santa Sub-division, spring quality, bacteria, contamination, petrography, anthropogenic.

This study presents geochemical data for mafic lavas from Mt. Oku to investigate the mantle source for Mt. Oku lavas and regional variations in isotopic ratios along the Cameroon Volcanic Line (CVL). The investigated mafic lavas present trace element and isotope signatures akin to ocean island basalts (OIB) and cover the entire range of the CVL. Major and trace element compositions of the studied lavas indicates that they have undergone fractionation of olivine + clinopyroxene + Fe-Ti oxide ± Cr–spinel with insignificant crustal contamination. The lavas were generated by The highest value of radiogenic 206Pb/204Pb (20.7) so far measured along the CVL is reported in this study. The isotopic features of the lavas suggest that the HIMU signatures are dominantly inherited from the asthenosphere (primary source) and melting of enriched components hosted in the subcontinental lithospheric mantle (SCLM; secondary source). Lavas from Mt. Oku present more comprehensive ranges of Sr–Nd–Pb isotopic composition than those from the continent-ocean-boundary (COB). The isotopic composition of Mt. Oku lavas with 206Pb/204Pb > 19.5 differs from those of the COB but are similar to FOZO. The isotopic composition of Mt. Oku lavas (87Sr/86Sr = 0.7030–0.7036, 143Nd/144Nd = 0.5127–0.5129, 206Pb/204Pb = 17.9–20.7) suggest that the lavas are the result of contributions from the asthenosphere and the SCLM. Using a combination of Sr–Nd–Pb isotope display on a 3D plot and Sm/Yb, we identify four essential components denoted; A, B, C and D involved in the petrogenesis of Mt. Oku lavas and the approximate melting depth. While A and B are from the asthenosphere with HIMU mantle flavour, C and D owe their origin from the SCLM with EM1 and DMM characteristics, respectively. Although depleted in Sr-Nd isotopes, D is radiogenic in 206Pb/204Pb, indicating the influence of metasomatism. Therefore, the petrogenesis of Mt. Oku lavas involves the mixing of at least three mantle end-members: HIMU–DMM–EM1. The “fanning” of samples from”A" towards the other components (B, C and D) indicates a HIMU–like end–member dominance in Mt. Oku magmatism. Considering the geophysical studies on mantle convection and plumes, we have developed a schematic model that explains the petrogenesis of the CVL lavas. The continuous spectrum from relatively depleted to HIMU characteristics of CVL lavas is associated with the progressive change in source components (i) SCLM with ± pockets of enriched metasomes to (ii) SCLM + asthenosphere sources and finally to (iii) typically asthenospheric source material ± pockets of enriched metasomes. Lavas derived from stage (i) exhibit geochemical variability involving DMM–like and EM1–like signatures. The magma generated at Stage (iii) is dominated by the HIMU mantle component and accounts for over 85% of CVL lavas.

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.

The Zindeng phonolitic flow forms a NW-SE elongated body located on the SE flank of Mount Bambouto (West-Cameroon), along the Cameroon Volcanic Line. It covers basalt emplaced on top of the Dschang biotite megacrystic granite. The magnitude of the magnetic susceptibility (Km) in the phonolitic lava flow varies between 196 μSI and 15769 μSI, indicating a dominantly ferromagnetic sensus lato behavior (Km > 1000 μSI; 89% of the stations) due to the presence of pseudo-single to multi-domain magnetite grains as deduced from hysteresis data. The low values of the degree of magnetic anisotropy, the dominantly oblate shape of the AMS ellipsoids and the concordance between lava plane and magnetic foliation point to flow-related magnetic fabrics. The pattern of the magnetic foliations and lineations indicate that the SE part of the phonolite was emplaced as a dome, whereas the NW part corresponds to a flow domain sensus stricto. It is suggested that the viscous phonolitic magma rose from a feeding conduit below the southeastern part of the lava flow and propagated laterally towards the NW.

Lake Nyos is located at the summit of a stratovolcano in the Oku Volcanic Group (OVG) along the Cameroon Volcanic Line. The sudden release of magmatic CO 2 trapped at the bottom of Lake Nyos in August 1986 caused historical casualties of 1750 people and over 3000 cattle. New geochemical data of volcanic rocks from the Nyos volcano and the first available data for volcanic rocks from other maar-bearing volcanoes (Lakes Elum, Wum and Oku) in the OVG are presented and compared. Lavas from the Nyos, Elum and Wum volcanoes show similarities in major and trace elements and Sr–Nd–Pb isotopes, suggestive of a similar mantle source. However, this source is slightly different from that of the Oku volcano. The samples from Lake Oku have lower alkali, higher TiO 2 and more depletion and enrichment in most incompatible trace elements than those from the Nyos, Elum and Wum volcanoes. These differences and those observed in the Sr–Nd–Pb results are consistent with a heterogeneous source for lavas in the OVG. Trace element compositions suggested the presence of garnet in the source ( 2 concentration to pose a potential threat. Degassing of an ordinary magma chamber and the migration of gas to the bottom of the lakes through cracks and faults can lead to the accumulation of CO 2 in lake bottoms. This is controlled by tectonic parameters (fractures and faults) that enhance degassing from the magma chamber to the lake bottom and physical parameters of the lake (e.g. size, depth, temperature and solubility) that control CO 2 stability.

Mantle xenoliths hosted by Oligocene alkaline basalts of the Kapsiki Plateau, northern end of the Cameroon Volcanic Line consist of group I spinel and plagioclase peridotites, mainly protogranular and accessorily porphyroclastic. The sub-continental lithospheric mantle here is heterogeneous and encloses both depleted and fertile components. Minerals exhibit wide range major element compositions compared to Nyos and Kumba grabens equivalent rocks. Spinel occurs as homogeneous brown crystals or as composite (brown-core–dark-rim) crystals when in contact with diopside or swatted in melt pools. Clinopyroxene crystals are either spinel exsolution-bearing or exsolution-free, the latter being often skeletal or frameworked and riddled with intracrystalline melt pools. Intraxenolith melt pockets and veinlets are always associated to plagioclase-bearing samples. Feldspars depict two distinctive compositions (An37–66Ab57–32Or6–2 and An3–7Ab52–62Or31–48) partly attributed to host xenolith type and to the involvement in the spinel and/or diopside melting reaction of an infiltrating alkali and carbonate-rich liquid. Petrographic and geochemical data discriminate melt pockets from their host basalts, excluding thus infiltration of basaltic melt as prospective origin. Thermo-barometric estimates reveal that prior to their entrainment the Kapsiki mantle xenoliths experienced two P–T equilibrium stages resulting in subsolidus re-equilibration from spinel- to plagioclase-facies conditions. Furthermore mineral textural relations show that the occurrence of plagioclase and melts inclusions is linked to spinel and/or diopside breakdown, likely subsequent to decompression and/or metasomatic induced melting events predating Oligo-Miocene volcanism.

The Barombi Koto volcanic field (BKVF) is located northeast of Mount Cameroon and constitutes a portion of the Kumba graben, one of the monogenetic volcanic fields of the Cameroon volcanic line (CVL). Tortonian fissural eruptions yielded picritic flows reworked by subsequent explosive eruptions, which generated two maars and ten cinder cones. The lavas comprise picrobasalts, basanites, alkali basalts, and hawaiites. Some clinopyroxene crystals exhibit a Cr-reverse zoning (0.15 to 0.58 wt%) that we attribute to a subsequent magma Cr-enrichment. Lavas are alkaline and range from primitive to moderately evolved. They are similar to volcanics of the other monogenetic volcanic fields of the CVL (e.g., Tombel, Nyos), but contain less incompatible elements than polygenetic volcanics. The absence of correlations on bivariate plots of most of the incompatible elements, the wide variability of Zr/Nb and Ba/La ratios (4.41–6.04 and 6.87–11.14, respectively) and the values of Dy/Yb, La/Yb, Nb/Y and Zr/Y ratios suggest that the different volcanic centres correspond to independent plumbing systems, resulting from low degrees of melting (2.12–6.85 wt%) of a heterogeneous asthenospheric mantle source characterized by a HIMU prevailing component.