Petrology of Spinel-Lherzolite Xenoliths from Mazéléand Others Northen Xenoliths Localities of Cameroon Volcanic Line: Exchange Reactions and Equilibrium State

The alkaline volcanism of the Cameroon Volcanic Line in its northern domain has raised many fresh enclaves of peridotites. The samples selected come from five (05) different localities (Liri, in the plateau of Kapsiki, Mazélé in the NE of Ngaoundéré, Tello and Ganguiré in the SE of Ngaoundéré and Likok, locality located in the west of Ngaoundé). The peridotite enclaves of the above localities show restricted mineralogical variation. Most are four-phase spinel-lherzolites, indicating that this is the main lithology that forms the lithospheric mantle below the shallow zone. No traces of garnet or primary plagioclase were detected, which strongly limits the depth range from which the rock fragments were sampled. The textures and the wide equilibrium temperatures (884˚C - 1115˚C) indicate also entrainment of lherzolite xenoliths from shallow depths within the lithosphere and the presence of mantle diapirism. The exchange reactions and equilibrium state established in this work make it possible to characterize the chemical composition of the upper mantle of each region and test the equilibrium state of the phases between them. Variations of major oxides and incompatible elemental concentrations in clinopyroxene indicate a primary control by partial melting. The absence of typical “metasomatic” minerals, low equilibration temperatures and enriched LREE patterns indicate that the upper mantle below septentrional crust of Cameroun underwent an event of cryptic metasomatic enrichment prior to partial melting. The distinctive chemical features, LREE enrichment, strong U, Ce and Pr, depletion relative to Ba, Nb, La, Pb, and T, fractionation of Zr and Hf and therefore ligh high Zr/Hf ratio, low La/Yb, Nb/La and Ti/Eu are all results of interaction of refractory peridotite residues with carbonatite melts.

A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

The origin and petrogenesis of the Cameroon Volcanic Line （CVL）, composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, het- erogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between Si02 58-64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability feld. Melting models suggest small degree （〈3%） partial melting of mantle bearing （6-10%） garnet for Mr. Etinde, the Ngaoundere Plateau and the Biu Plateau, and 〈5% of garnet for the oceanic sector of the CVL, Mr. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope sys- tematics suggest that mixing in various proportions of Depleted MORB Mantle （DMM） with enriched mantle 1 and 2 （EM1 and EM2） could account for the complex isotopic characteristics of the CVL lavas. Low Mg number （Mg# - 100 x MgO/（MgO ＋FeO）） and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in NbJTa and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle het- erogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

Mineralogy and Magmatic Processes of Cenozoic Intraplate Alkaline Volcanic Rocks of Bafang and Its Environs (Cameroon Volcanic Line, West Africa)

Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An67.97-15.84Ab69.19-30.43Or20.59-1.51) and anorthoclases (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Plagioclases are the most abundant amount these feldspars. Anorthoclases appear only in mugearite (BAF 3 and BAF 37) the most differentiated of the studied alkaline-basalts. In High Magnesian basalt, (HMg-B) plagioclases are labradorites (An67.97-59.30Ab38.74-30.43Or2.75-1.60) and sanidine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), whereas in Low Magnesian basalt (LMg-B) there are labrador (An67.4.75-51.96Ab44.98-33.72Or3.06-1.51), andesine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), oligoclase (An26.65-15.84Ab69.19-63.57Or20.59-8.55) and anarthoclase (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Olivines are magnesian (Fo86.7-50.1) and ferriferous (Fo48.8-37.8). In HMg-B, olivine are only magnesian. These olivines are chrysolites and hyalositerite. In LMg-B, olivines are magnesian and ferriferous with the predominance of ferriferous. They are chrysolites, hyalositerite and hortonolite. Pyroxenes are Ca, Mg and Fe clinopyroxenes. There are diopsides (Wo51.94-45.02En44.41-33.16Fs16.42-10.70) and augites (Wo44.88-43.64En41.03-37.04Fs18.25-14.43). Oxides are magnetites represented by ulvospinel (Usp90-75Sp2-7Mt5-23). Fractionation of ferromagnesian minerals (opaque oxide, olivine and pyroxene) is the main differentiation process. Two stages of fractional crystallization can be distinguished: the first stage comes with basanites and the second with hawaiites to mugearites. Chemical compositions of phenocrystals in studied basaltics lavas record signatures of magma recharge by pulsatory intrusions of new magma into the existing magma reservoir before the eruptions.

Mineralogy and geochemistry of pozzolans from the Tombel Plain,Bamileke Plateau,and Noun Plain monogenetic volcanoes in the central part of the Cameroon Volcanic Line

Pozzolans from the Tombel Plain,Bamileke Plateau,and Noun Plain,3 monogenetic volcanic fields in the central part of the Cameroon Volcanic Line(the Tombel Plain,Bamileke Plateau,and Noun Plain),were explored in order to constrain their petrology and make some predictions on their pozzolanicity.The rocks in this study include alkaline and subalkaline basalts,trachybasalts,and basanites.Most of these rocks present an overall composition that overlaps with primitive mantle,suggesting rapid ascent of magmas,limited crustal contamination and crystal fractionation of olivine,clinopyroxene,and feldspar.The pozzolans present enrichment of LREE relative to HREE and high chondrite normalized ratios of La/Yb and Tb/Yb.ranging between 7 and 20 and>1.9 respectively,similar to those of Ocean Island Basalts.Like other nearby volcanoes,partial melting in a dominantly garnet-bearing mantle zone can be assumed.Quantitative mineralogy by X-ray diffraction revealed various mineral phases with dominantly plagioclase,augite,olivine,and Fe-Ti oxides.The samples contains important amorphous phase up to 23,51,and 69 wt%in the Tombel Plain,Noun Plain,and Bamileke Plateau,respectively.This elevated amount of amorphous phases together with the sum of SiO2,Al2O3 and total Fe2O3(SAI=68.50-83.50>70 wt%)according to ASTM C618 standard and the sum of CaO,FeO,and MgO(CIM=14.5-30.52 wt%and 23.58-31.08 wt%)suggest interesting pozzolanicity character for the studied pozzolans.

The Bafoussam volcanic series:origin and evolution of the volcanism along the Cameroon volcanic line

The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data about mafic and felsic volcanic rocks.These rocks belong to two different series:A transitional series made of basalts,basaltic andesite,and trachytes and an alkaline mafic series with basalts,hawaiites,and basanites.New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma.The alkaline volcanism is younger,with ages ranging from10 to 4.5 Ma.Magmatic evolution in both series is accomplished through a fractional crystallization process,with the removal of olivine and clinopyroxene,while plagioclase does not seem to be a major crystallizing phase.All the samples are enriched in incompatible trace elements,but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas.Alkaline lavas have lower initial^(87)Sr/^(86)Sr and higher^(176)Hf/^(177)Hf and Pb isotopic ratios than the transitional lavas.Low La/Nb and high^(87)Sr/^(86)Sriratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust.They cannot be used for discussing the mantle source of the volcanic rocks from this series.Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source,with higher partial melting degrees(3-5%)for the transitional magmas than for the alkaline magmas(<2.5%).Combining trace elements and isotopic ratios,we show that the Bafoussam lavas formed from two different mantle sources.Transitional magmas formed from a pyroxenite-bearing enriched mantle with low Pb isotopic composition.This mantle source is present in all the oldest lavas from the CVL.Alkaline magmas formed from an HIMU-like mantle source,different from the Mt Cameroon HIMU mantle source.The depleted asthenospheric mantle is not involved in the Bafoussam magmatism and the two mantle sources are probably located in the lithospheric mantle,in agreement with recent geophysical models presenting the CVL as a consequence of the partial melting of the lithospheric mantle in response to edge convection along the margin of the Congo craton.

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

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.

Exploration of Potential Ore Deposits along the Cameroon Volcanic Line from Gravity and Magnetic Studies

A gravity and magnetic study has been carried out along the continental part of the Cameroon Volcanic Line (CVL), with the aim to explore the possibility of ore’s presence into the basement of this region. Different processing techniques have been applied, including the isostatic residual, the analytic signal and the Euler deconvolution to compute the Bouguer and magnetic anomalies. Following Euler solutions (result of Euler deconvolution) and the analytic signal results, four profiles crossing main structures on the isostatic residual have been used to enhance the structure of intrusions in the studied area. Despite the lack of constraints in the studied region, the results show that the basement is intruded by bodies of different density (2.57 g/cm3 to 2.87 g/cm3), different size and shape, with depths between 1 and 10 kilometers. These bodies have been interpreted as old rocks then as potential reservoirs of rare ores. In addition, volcanic rocks modeled have constituted potential reserves of other ores like graphite, sulfur, copper, iron.

Magmatism of the Beka Volcanic Massifs (Cameroon Volcanic Line, West-Central Africa): New Petrographical and Mineralogical Data

The Beka volcanic massifs are located northeast of Ngaoundere region, within the Adamawa plateau. It consists mainly of basanites, trachytes and phonolites. The petrographic study shows that all the basanite lavas have porphyritic microlitic textures with a more pronounced magmatic fluidity than the felsic lavas displaying trachytic textures. The lavas are composed of phenocrysts, microlites and microphenocrysts of olivine, clinopyroxene, plagioclase and iron-titanium oxides for the basanites and of greenish clinopyroxene, alkali feldspar, and titanomagnetite for the felsic lavas. Chemical microprobe analysis indicates that the olivine crystals are magnesian (Fo73-78). Clinopyroxene crystals have a composition of diopside (Wo47-) in the basaltic lavas and diopside near the hedenbergite pole in the trachytes phonoliths and titanomagnetite (TiO2: 21.13% - 22.36% and FeO: 68% - 68%). Chemical analyses on whole rocks show that all the lavas belong to the same series and the felsic lavas come from the differentiation of basanite lavas by fractional crystallization of the minerals therein. The basanites originate from a low rate of partial melting of an OIB-type mantle. Contamination and mixing processes are suspected. Lavas of similar composition are found in other volcanic centres of the Adamawa plateau and the continental and oceanic sectors of the Cameroon Volcanic Line, in particular the Kapsiki plateau, Mounts Cameroon and Bamenda.