The Mount Cameroon 1959 compound lava flow field: morphology, petrography and geochemistry     

Manga S. Njome  Cheo E. Suh  R. Stephen J. Sparks  Samuel N. Ayonghe  J. Godfrey Fitton 《Swiss Journal of Geoscience》2008,101(1):85-98

  相似文献   

Vertical accuracy evaluation of freely available latest high-resolution (30 m) global digital elevation models over Cameroon (Central Africa) with GPS/leveling ground control points.     

Loudi Yap  Ludovic Houetchak Kandé  Robert Nouayou  Nasser Abdou Ngouh  Marie Brigitte Makuate 《International Journal of Digital Earth》2019,12(5):500-524

Digital Elevation Models (DEMs) contain topographic relief data that are vital for many geoscience applications. This study relies on the vertical accuracy of publicly available latest high-resolution (30?m) global DEMs over Cameroon. These models are (1) the ALOS World 3D-30?m (AW3D30), (2) the Shuttle Radar Topography Mission 1 Arc-Second C-Band Global DEM (SRTM 1) and (3) the Advanced Spaceborne Thermal Emission and Reflection Global DEM Version 2 (ASTER GDEM 2). After matching their coordinate systems and datums, the horizontal positional accuracy evaluation was carried out and it shows that geolocation errors significantly influence the vertical accuracy of global DEMs. After this, the three models are compared among them, in order to access random and systematic effects in the elevation data each of them contains. Further, heights from 555 GPS/leveling points distributed all over Cameroon are compared to each DEM, for their vertical accuracy determination. Traditional and robust statistical measures, normality test, outlier detection and removal were used to describe the vertical quality of the DEMs. The test of the normality rejected the hypothesis of normal distribution for all tested global DEMs. Overall vertical accuracies obtained for the three models after georeferencing and gross error removal in terms of Root Mean Square (RMS) and Normalized Median Absolute Deviation (NMAD) are: AW3D30 (13.06?m and 7.75?m), SRTM 1 (13.25?m and 7.41?m) and ASTER GDEM 2 (18.87?m and 13.30?m). Other accuracy measures (MED, 68.3% quantile, 95% quantile) supply some evidence of the good quality of AW3D30 over Cameroon. Further, the effect of land cover and slope on DEM vertical accuracy was also analyzed. All models have proved to be worse in the areas dominated by forests and shrubs areas. SRTM 1 and AW3D30 are more resilient to the effects of the scattering objects respectively in forests and cultivated areas. The dependency of DEMs accuracy on the terrain roughness is evident. In all slope intervals, AW3D30 is performing better than SRTM 1 and ASTER GDEM 2 over Cameroon. AW3D30 is more representative of the external topography over Cameroon in comparison with two others datasets and SRTM 1 can be a serious alternative to AW3D30 for a range of DEM applications in Cameroon.  相似文献   

Variation in mantle lithology and composition beneath the Ngao Bilta volcano,Adamawa Massif,Cameroon volcanic line,West-central Africa     

Robert Temdjim  Merlin Patrick Njombie Wagsong  Arnold Julien Nzakou Tsepeng  Stephen Foley 《地学前缘(英文版)》2020,(2):665-677

Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line.The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite.They commonly contain olivine,orthopyroxene,clinopyroxene and spinel although the dunite is spinel-free.Amphibole is an essential constituent in the lherzolites.Mineral chemistry differs between the three types of peridotite:olivines have usual mantle-like Mg#of around 90 in lherzolites,but follow a trend of decreasing Mg#(to 82)and NiO(to 0.06 wt.%)that is continuous in the dunites.Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#,indicating a reaction that removes Opx and introduces Cpx,olivine,amphibole and spinel.This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite,which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole.Thermobarometric estimates place the xenoliths at pressures of 11–15 kbar(35–50 km)and temperatures of 863–957
C,along a dynamic rift geotherm and shallower than the region where carbonate melts may occur.The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.  相似文献   

Evolution of Mount Cameroon volcanism: Geochemistry,mineral chemistry and radiogenic isotopes (Pb,Sr, Nd)     

《地学前缘(英文版)》2020,11(6):2157-2168

Mount Cameroon volcano has erupted several times in the 20th Century with documented eruptions in 1909, 1922, 1954, 1959, 1982, 1999 and 2000. Evidence of historic volcanism is represented by several older lava flows and lahar deposits around the flanks of the volcano. This study aims to assess the evolution of Mount Cameroon volcanism through its eruptive history via interpretation of mineralogical, whole rock geochemical and Pb, Sr, Nd isotope data generated from historic and recent lava flows. In this study, samples were collected from the 1959, 1982, 1999 and 2000 eruptions and from several historic eruption sites with unknown eruption dates.Evaluation of major and trace element data demonstrates that Mount Cameroon is geotectonically associated with within-plate Ocean Island Basalt Settings. More than 90% of the studied historic lavas (n ​= ​29) classify as tephrites and basanites whereas the modern lavas (n ​= ​38) are predominantly trachybasalts, demonstrating evolution from primitive to evolved lavas over time typically in response to fractional crystallization. Petrographically, the lavas are porphyritic with main mineral phases being olivine, clinopyroxene, plagioclase feldspars and Fe–Ti–Cr oxides. The 1982 lavas are predominantly aphyric and dominated by lath-shaped flow-aligned plagioclase in the groundmass. Olivine chemistry shows variable forsterite compositions from Fo_60–89. Clinopyroxenes vary from diopside through augite to titanaugite with chemical composition ranges from Wo₄₅En₃₂Fs₇ to Wo₅₁En₄₇Fs₁₇. Plagioclase feldspars vary from labradorite (An_56–70) to bytownite (An_80–87). For the Fe–Ti–Cr oxides, calculated ulvöspinel component shows a wide variation from ulv_38–87. CIPW-normative classification on the Di-Ol-Hy-Qz-Ne system shows that all Mount Cameroon lavas are nepheline-normative (Ne ranges from 4.20 wt.% to 11.45 ​wt.%).Radiogenic isotope data demonstrate that Mount Cameroon lavas are HIMU (or high μ ​= ​²³⁸U/²⁰⁴Pb), characterized by ²⁰⁶Pb/²⁰⁴Pb ​= ​20.19–20.46, ²⁰⁷Pb/²⁰⁴Pb ​= ​15.63–15.69, ²⁰⁸Pb/²⁰⁴Pb ​= ​40.01–40.30, ⁸⁷Sr/⁸⁶Sr ​= ​0.70322–0.70339 (ε_Sr ​= ​−21.37 to −18.96) and ¹⁴³Nd/¹⁴⁴Nd ​= ​0.51276–0.51285 (ε_Nd ​= ​+2.29 to +4.05). The historic lavas show stronger HIMU signature relative to the modern lavas, suggesting evolution towards less HIMU signatures over time. This study has revealed that Mount Cameroon volcanism has evolved from primitive magmas characterized by stronger HIMU signatures with high ^206/204Pb and ^208/204Pb isotopes, low SiO₂ and high Mg, Ni, Cr content towards lower HIMU signatures with relatively higher SiO₂, lower Mg, Cr and Ni compositions. The geochemical and isotopic changes, which account for the evolution of magmatism on Mount Cameroon occur over long periods of time because all the modern lavas erupted within the last 100 years are isotopically homogeneous, with very limited variation in SiO₂ compositions.  相似文献   

Tree population dynamics of three altitudinal vegetation communities on Mount Cameroon (1989�C2004)     

Philip F.FORBOSEH  Terry C.H.SUNDERLAND  James A.COMISKEY  Michael BALINGA 《山地科学学报》2011,8(4):495-504

Changes in permanent sample plots in the lowland,submontane and montane forests on Mount Cameroon(4,095 m above sea level),an active volcano,are described for 15 years from 1989 to 2004.Throughout the study period,the stocking level of trees with a diameter at breast height(DBH) ≥ 10 cm in the three forests were lower than in pan-tropical stands suggesting a significant impact of volcanic and human-related activities on the vegetation communities on the mountain.Annual mortality rates in the submontane and montane forests were consistent with those reported for comparable altitudinal ranges in the Blue Mountains of Jamaica.The annual mortality rate was higher in the lowland forest than other lowland sites included.Divergence between recruitment and mortality rates was large suggesting that the three vegetation communities have not reached their climax.The seven-year difference in half-life of large trees(with a DBH ≥ 50 cm) in the submontane and montane forests suggests an altitudinal effect on turnover of larger trees that in turn contributes to the frequent small stature of high altitude forests.There was little evidence of an altitudinal effect on species turnover and growth rate.This finding supports generalizations about the zero effect of growth on the stature of high altitude trees.Understanding forest dynamics is crucially important in the management of tropical montane environmentsand in this instance particularly so given the recent creation of the Mount Cameroon National Park.  相似文献   

Petrology and geochemistry of the banded iron-formations from Ntem complex greenstones belt,Elom area,Southern Cameroon: Implications for the origin and depositional environment     

《Chemie der Erde / Geochemistry》2015,75(3):375-387

Banded iron-formations (BIFs) form an important part of the Archaean to Proterozoic greenstone belts in the Southern Cameroon. In this study, major, trace and REE chemistry of the banded iron-formation are utilized to explore the source of metals and to constraint the origin and depositional environment of these BIFs. The studied BIF belongs to the oxide facies iron formations composed mainly of iron oxide (mainly magnetite) mesobands alternating with quartz mesobands. The mineralogy of the BIF sample consists of magnetite and quartz with lesser amount of secondary martite, goethite and trace of gibbsite and smectite. The major element chemistry of these iron-formations is remarkably simple with the main constituents being SiO₂ and Fe₂O₃ which constitute 95.6–99.5% of the bulk rock. Low Al₂O₃, TiO₂, and HFSE concentrations show that they are relatively detritus-free chemical sediments. The Pearson’s correlation matrix of major element reveals that there is a strong positive correlation (r = 0.99) of Al with Ti and no to weak negative correlation of Ti with Mn, Ca and weak positive correlation of Si with Ca, suggesting the null to very minor contribution of detrital material to chemical sediment. The trace elements with minor enrichments are transition metals such as Zn, Cr, Sr, V and Pb. This is an indicator of direct volcanogenic hydrothermal input in chemical precipitates. The studied BIF have a low ΣREE content, ranging between 0.41 and 3.22 ppm with an average of 0.87 ppm, similar to that of pure chemical sediments. The shale-normalized patterns show depletion in light REE, slightly enrichment in heavy REE and exhibit weak positive europium anomalies. These geochemical characteristics indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The absence of a large positive Eu anomaly in the studied BIF indicates an important role of low-temperature hydrothermal solutions. The chondrite-normalized REE patterns are characterized by LREE-enriched (Mean La_CN/Yb_CN = 8.01) and HREE depletion (Mean Tb_CN/Yb_CN = 1.61) patterns and show positive Ce anomalies. With the exception of one sample (LBR133), all of the BIF samples analyzed during this study have positive Ce anomalies on both chondrite- and PASS-normalized plots. This may indicate that the BIFs within the Elom area were formed within a redox stratified ocean. The positive Ce anomalies in the studied samples likely suggest that the basin in which Fe formations were deposited was reducing with respect to Ce, probably in the suboxic or anoxic seawaters.  相似文献   

U–Pb Zircon Age Constraining the Source and Provenance of Gem‐Bearing Late Cenozoic Detrital Deposits,Mamfe Basin,SW Cameroon     

Nguo Sylvestre KANOUO  Khin ZAW  Rose Fouateu YONGUE  Frederick Lin SUTHERLAND  Sebastien MEFFRE  Emmanuel NJONFANG  Changqian MA  Milan Stafford TCHOUATCHA 《Resource Geology》2012,62(3):316-324

Zircons and other heavy minerals (corundum, rutile, ilmenite, magnetite, sillimanite) are identified in the Nsanaragati gem corundum placer deposit, in the western part of the Mamfe sedimentary basin, SW Cameroon. These alluvial minerals have different morphological characteristics and zircons, in particular, vary mostly in colour and shape. They are reddish, brownish, yellowish, pink or colourless. These minerals form rounded and sub‐rounded alluvial grains, prismatic, pyramidal or dipyramidal crystals. Reddish zircons retain their original crystallographic shape. Trace element and U–Pb isotopic geochemical analyses of these reddish zircons, using the LA‐ICP‐MS method give significant Hf (4576–6334 ppm), Th (46–1565 ppm) and U (66–687 ppm) contents, with Th/U ratio ranging from 0.6 to 3.0. The ²⁰⁶Pb/²³⁸U corrected mean age gave 12.39 ± 0.55 Ma, which characterizes an Upper Cenozoic (Serravallian) magmatic event. The zircons are probably sourced from a magmatic field in the South eastern boundary of the Cross River Formation. The Cameroon Volcanic Line of basaltic and alkaline lavas and intrusions which lie east of the Mamfe Basin mostly range in age from 37 Ma to <1 Ma. The zircons may also relate to the Mount Bambouto plateau lavas which lie northeast of the Mamfe sedimentary basin and have an eruptive age range of 21–14 Ma. The oldest Nsanaragati reddish zircon ages overlap within error with the end stages of the Bambouto eruptions. This eruptive or a related episode provides a potential source for megacrystic reddish zircons within the Nsanagarati placer deposit.  相似文献   

Denudation rates on cratonic landscapes: comparison between suspended and dissolved fluxes,and 10Be analysis in the Nyong and Sanaga River basins,south Cameroon          下载免费PDF全文

Vincent Regard  Sébastien Carretier  Jean‐Loup Boeglin  Jules‐Rémy Ndam Ngoupayou  Jean‐Guy Dzana  Jean‐Pierre Bedimo Bedimo  Jean Riotte  Jean‐Jacques Braun 《地球表面变化过程与地形》2016,41(12):1671-1683

South Cameroon is located in a tropical and tectonically quiescent region, with landscapes characterized by thick highly weathered regolith, indicative of the long‐term predominance of chemical weathering over erosion. Currently this region undergoes huge changes due to accelerated mutations related to a growing population and economical developments with associated needs and increasing pressures on land and natural resources. We analysed two of the main south Cameroon rivers: the Nyong River and Sanaga River. The Sanaga catchment undergoes a contrasted tropical climate from sub‐humid mountainous and humid climate and is impacted by deforestation, agriculture, damming, mining and urbanization, especially in the Mbam sub‐basin, draining the highly populated volcanic highlands. By contrast, the Nyong catchment, only under humid tropical climate, is preserved from anthropogenic disturbance with low population except in the region of Yaoundé (Méfou sub‐basin). Moreover the Nyong basin is dam‐free and less impacted by agriculture and logging. We explore both denudation temporal variability and the ratio between chemical and physical denudation through two catchment‐averaged erosion and denudation datasets. The first one consists of an 11‐year long gauging dataset, while the second one comes from cosmogenic radionuclides [CRNs, here beryllium‐10 (¹⁰Be)] from sand sampled in the river mainstreams (timescale of tens to hundreds of thousands of years). Modern fluxes estimated from gauging data range from 5 to 100 m/Ma (10 to 200 t/km²/yr); our calculations indicate that the usual relative contribution of chemical versus physical denudation is 60% and 40%, respectively, of the total denudation. Beryllium‐10 denudation rates and sediment fluxes range from 4.8 to 40.3 m/Ma or 13 to 109 t/km²/yr, respectively, after correction for quartz enrichment. These fluxes are slightly less than the modern fluxes observed in Cameroon and other stable tropical areas. The highest ¹⁰Be‐derived fluxes and the highest physical versus chemical denudation ratios are attributed to anthropogenic impact. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Soil and plant composition in the Noun river catchment basin,Western Cameroon: a contribution to the development of a biogeochemical baseline   总被引：1，自引：0，他引：1  

Clémentine Njofang  Jörg Matschullat  Akoa Amougou  Jean Pierre Tchouankoué  Hermann Heilmeier 《Environmental Geology》2009,56(7):1427-1436

Soils and selected edible plants of the Noun river catchment basin of western Cameroon were sampled to investigate the distribution of trace elements, based on the preliminary idea of unusual anomalies. Analytical techniques for trace elements included ICP-AES, GF-AAS, and ICP-MS. Further soil analyses comprised the mineralogy and contents of the biogenic elements carbon, nitrogen and sulphur (CNS). The trace element concentrations in the soils reflect those of the lithogeochemical background of the pluto-volcanic rocks of the region. This is consistent with the results from the mineralogical analyses and physicochemical parameters such as pH, taken in the field, which also do not suggest any geochemical anomaly. Most trace elements analyzed in the plants showed concentrations that reflect those of the soils (Al, Fe, Ti, and Rb). However, some trace elements were enriched in the plants as compared to the soils, such as Zn, Cu, Cd, Mo (excluding yam), Ni (peanut), Ba (peanut), Sr (peanut, bean), and B. Trace elements such as As, Cr, V, and Se were not bioavailable for all the analyzed plants. Besides, trace elements such as Cu, Zn, Mo, Fe, Al, Ni, B, Ti, Rb, Cs, and Ba were in the range of phytotoxicity and reached or exceeded human food tolerance level (Cu). The plants with seeds showed a higher absorption of trace elements compared to plants with tubercles.  相似文献   

Preliminary evidence of environmental changes at Lake Bambili (Cameroon, West Africa) since 24,000 BP     

J. Curt Stager 《Journal of Paleolimnology》1999,22(3):319-330

Preliminary analyses of diatoms, phytoliths, and siliceous protozoan plate records in a 16 m sediment core from Lake Bambili (Cameroon, West Africa; 2264 m AMSL) provide evidence of pronounced climatic changes in the West Cameroon Highlands since ~24,000 ¹⁴ C yrs BP. Percentages of planktonic diatoms rose with increased precipitation:evaporation ratios around 24,000 BP, ~15,000-9500 BP, and ~2400-2000 BP. Since 15,000 BP, Bambili appears to have experienced climatic changes of comparable timing and magnitude, but with signs in opposition to those registered in the West African lowlands. Much of this pattern may be attributable to variability in montane stratiform cloud formation, which in turn is related to paleo-wind regimes and upwelling dynamics in the Gulf of Guinea.  相似文献