Tchabal Gangdaba Massif in the Cameroon volcanic line : a bimodal association

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Geochemistry of the Bayonplutonic Complex – Western Cameroon

The BayonNeoproterozoic plutonic complex located in Western Cameroon intrudes gneisses of Paleo to Neo Proterozoic age. The complex is composed of gabbro, monzogabbro and monzonites frequently crosscut by trachytic and granitic veins. The primary mineral assemblages of the gabbro and monzogabbro is plagioclase (An30 - An69), clinopyroxene (En40 – 42 Fs 12 – 18 Wo 45 - 47) hypersthene (En62 – 65 Fs34 – 37 Wo1- 4) and orthoclase (Or78-Or91) whilebiotite, magnetite, Ilmenite and apatite constitute accessory minerals. Monzonite is formed of plagioclase (An22 to An39), orthoclase (Or80-Or87),clinopyroxene (En38-39 Fs14-20 Wo45-46), biotite and quartz. Amphiboles occur as secondary minerals. Ilmenite and apatite are accessory minerals in monzonite.The rocks are mafic to intermediate in composition (41 – 61 wt % SiO2) and transalkaline with high-K and have shoshonitic features. Bayon plutonic rocks have highabundance of Ba, Sr ,V and Zr but possesses low concentrations of Rb, Sc, Y and Th.Gabbroic rocks show moderately fractioned REE patterns (LaN/LuN= 14 – 27) with none or negligible Eu anomalies. The monzonite shows also moderate fractionatedpatterns (LaN/LuN= 20 – 27) with fairly positive Eu anomalies. All the studied rocks show flat HREE features. The primitive mantle- normalized element patterns are almost homogeneous with negative anomalie in Ta, Nb, Th, P and Ti. Sm/Nd-wr-Cpx-Pl ages of the complex are 580 ± 13 Ma; 553 ± 32 Ma for the monzogabbro and 547 ± 26 Ma for the monzonite. The Nd/Sr isotopic compositions show that the Bayon plutonic rocks were generated by partial melting of subcontinental lithospheric mantle. The depleted mantle Nd model age TDM of 1.6 – 1.7 Ga indicates that the studied rocks originated by partial melting of Mesoproterozoic mantle.KEYWORDS: West Cameroon, Panafrican, Pluton, Geochemistry, partial metin

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