Geochemistry and Geochronology of the Nlonako Alkaline Complex, Central Africa

The Nlonako Anorogenic Complex (NAC) is located in the southern continental sector of the Cameroon Line (CL). It intrudes the Cretaceous sediments and the Pan-african granitogneissic basement and is characterized by an association of diversified plutonic and volcanic rocks. Geochemical studies of major, trace and rare earth elements reveal an alkaline series which corresponds to a gabbro-diorite-monzonite-syenite-granite type. This series comprises two suites: a metaluminous suite made of basic and intermediate rocks, fayalite alkali granite and syenites; and a peralkaline suite including rhyolite and other granites. Both suites are derived from a single, hot and dry magma of basaltic composition. The evolution of the magma is dominated by fractional crystallization of mantle material and was progressively affected by crustal contamination at the end of the differentiation process. The conventional whole-rock Rb/Sr method was performed on three syenites and five granites. The ages obtained range between 44.6 and 49.7 Ma, confirming the Tertiary time of emplacement of the NAC.Keywords: Nlonako Anorogenic Complex (NAC), alkaline series, magma, fractional crystallization, crustal contamination, tertiary age

Ti-bearing aenigmatite from Djinga Tadorgal (Adamawa plateau) and Sao Tomé (Cameroon Line) phonolites: geochemical implications and application of the QUILF thermobarometer for the crystallization conditions

The chemical and structural compositions of aenigmatite from Djinga Tadorgal and Sao Tomé phonolites were compared to those previously described in the felsic lavas and nephelinite from the Adamawa Plateau and Cameroon Line, in order to determine their crystallization temperatures and pressures. Chemical analysis results and the application of QUILF thermobarometer, indicated the equilibrium temperatures between 655 °C and 791 °C, oxygen fugacity (fO2) below the FMQ buffer, pressures near 0.1 GPa and silica activities less than 1.0 for Djinga Tadorgal and Sao Tomé phonolites. The Ti-bearing aenigmatite from Djinga Tadorgal and Sao Tomé phonolites are a late and accessory mineral, resulting from the reaction between Tirich magnetite and Na2O-rich magmatic liquid.Keywords: Peralkaline lavas, mineral chemistry, silica activity, thermodynamic condition

Géochimie des laves basaltiques récentes des zones Nord et Est de Ngaoundéré (Cameroun, Plateau de l’Adamaoua, Afrique centrale): pétrogenèse et nature de la source

Des coulées et projections basaltiques récentes (basaltes: I.D

Contribution à l\'étude de l\'influence des activités humaines sur les écoulements des cours d\'eau dans le Sud Cameroun: cas de la déforestation.

L\'influence des modifications de l\'état de surface par l\'homme, sur les écoulements et les ressources en eau est un fait réel dans la zone tropicale. Dans cette étude, nous allons nous intéresser à la partie Sud Cameroun, où l\'on observe une déforestation intense depuis plusieurs décennies. La méthodologie adoptée pour étudier les tendances de l\'influence de la déforestation sur le comportement hydrologique des bassins versants est la méthode de la modélisation stochastique, pouvant prendre en compte à la fois les composantes dynamiques et le caractère aléatoire de l\'évolution des processus hydrologiques. De ce modèle, il ressort que la lame d\'eau écoulée varie de –19 % à 6 % et le coefficient de variation entre – 7 % à 6 %. Une étude comparative avec la prévision de l\'impact de l\'augmentation démographique révèle que les variations sont plus remarquables avec une diminution de lame d\'eau écoulée jusqu\'à près de –45 % et une augmentation du coefficient de variation de –12 % à 26 %.It is known fact that man\'s influence on the surface of the earth has modified not only the drainage patterns, but also the water resources of the tropical region. In this survey, we were more interested, in the Southern part of Cameroon, where an intense deforestation is being going on for several decades. The methodology adopted to study the tendencies of influence of deforestation on drainage basin is the stochastic modelling method, which takes into account the dynamic components and the uncertain character of the hydrologic evolutionary processes. Of this model, it is revealed that the runoff varies from -19 % to 6 % and the coefficient of variation ranges from - 7 % to 6 %. A comparative survey with the forecasting of the impact of demographic growth reveals that variations are more remarkable with a reduction of the runoff up to about - 45 % and an increase in the coefficient of variation ranging from -12 % to 26 %. Keywords: Hydrologie, déforestation, ressources en eau, modèle stochastique, prévisions hydrologiques.Hydrology, deforestation, water resources, stochastic model, hydrological forecasting. Journal des Sciences Pour l\'Ingénieur. Vol. 8 2007: pp. 45-5

Petrology of the Late Cretaceous peralkaline rhyolites (pantellerite and comendite) from Lake Chad, Central Africa

International audienc

Do decades of tropical rainfall affect the chemical compositions of basaltic lava flows in Mount Cameroon ?

To evaluate the effects of tropical rainfall on the compositions of basaltic lavas, we studied 20th century lava flows from Mount Cameroon in Africa. Weathering conditions are extreme because the climate is particularly warm and humid, and vegetation grows extremely quickly on the flows. The high rainfalls and dense vegetation contribute to rapid and intense degradation of the volcanic rocks and should cause significant changes in chemical composition. Such effects need to be quantified to constrain how young a lava flow must be so that its trace element and isotopic composition remains representative of the original magma. Fresh inner parts and altered flowtops of four different lava flows were sampled and analysed for major and trace elements as well as O, U, Sr, Nd and Pb isotopic compositions. Four samples of the 1999 eruption were also analysed to constrain the composition of fresh basalts. Almost all major and trace elements display similar concentrations in inner and outer parts of the same flow. This is notably the case for elements such as K, Rb and Sr, which are highly mobile during weathering. The lack of variation suggests that the overall composition of the lava flows has not been significantly affected. However, some systematic chemical changes are observed: Loss-on-ignition (LOI) and d18O increase slightly from inner parts of flows to near surface samples; Na and, to a lesser extent, U display significant losses in the outer samples. We interpret the Na loss in terms of hydration leading to exchange between Na+ and H+ ions. This process, associated with oxidation of Fe2+ to Fe3+, accounts for the larger loss-on-ignition in the outer parts of flows. A change in U contents is only observed in the 1922 flow, which is covered by dense vegetation. This emphasizes the role that complexation by organic ligands plays in U mobility. While U is not completely immobile, all volcanic rocks are in secular equilibrium ((234U/238U)—1), indicating limited interaction between meteoric waters and basalts. Sr and Nd isotopic compositions remain constant and although 206Pb/204Pb ratios vary from inner to outer parts of the lavas, the changes are not systematic and cannot be attributed to weathering. The absence of significant chemical mobility and substantial isotopic exchange suggests very limited interaction between water and lavas at the centimeter scale, due, most probably, to their relatively young age. This work shows that over a period of about 100 years, no detectable geochemical changes are observed. These results are very promising for petrological and geochemical studies of ocean island basalts located in tropical areas since they provide direct evidence of very limited trace element mobility at the century time scale