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.

Spinel-Bearing Lherzolite Xenoliths from Hossere Garba(Likok,Adamawa-Cameroon):Mineral Compositions and Geothermobarometric Implications

Spinel-bearing lherzolite xenoliths from the Hossere Garba (1272 m.a.s.l) volcano on the Adamawa Plateau, is located in Likok village, at about 35 km to WSW of Ngaoundere. These xenoliths (~11 cm size) have been sampled into the host basaltic lava flows from the NE flank of the Hossere Garba volcano. These xenoliths characterized by porphyroclastic texture consisted of olivine (~55 vol.%), orthopyroxene (~19 vol.%) and clinopyroxene (~21 vol.%) crystals. Spinel crystals (~5 vol.%) are red brown and interstitial between the crystals of olivine and pyroxenes. CaO contents are low (<0.08 wt%) in olivine and similar to those estimated (CaO: 0.05 - 0.1 wt%) for the mantle origin. The values of AlVI/AlIV ratio range between 1.1 and 1.3 for the Cr-diopside crystals from Hossere Garba xenoliths. The constant value of the volumes V(Cell) and V(M1) for clinopyroxene compositions, indicates the similar pressures. Hossere Garba represents a residual sequence issued from partial melting of a mantle source. Similar compositions have been recorded in minerals of ultramafic xenoliths from other ultramafic xenoliths domains of the Cameroon Line and the Adamawa Plateau.

Study of Multi-Origin Hazards and Assessment of Associated Risks in the Lefo Caldera (Bamenda Volcano, Cameroon Line)

The Bamenda Volcano (BV) (2621 m) is a stratovolcano situated in the Cameroon Line (CL). BV includes Mount Lefo (2534 m) which is situated on its southern slopes and contains one elliptical caldera (3 × 4 km). This caldera is propitious to farming and breeding activity. Despite these profitable assets, Lefo caldera (LC) is an amphitheater of the occurrence of multi-origin hazards that have direct or indirect impacts on the biodiversity and human patrimony. The most present hazards are those of meteorological origin. Numerous combined factors (steepest slopes, heavy rainfalls, weathered state of volcanic products…) rule these hazards. These factors gave rise to the occurrence of landslides, gullies erosion and rock falls which occur precisely on caldera northern and eastern rims. Hazards of anthropogenic origin are based on the destruction of the vegetation cover by the population for dealing, firewood and building issues. Moreover, during the breeding activity, the cattle covers the caldera throughout the day;this unevens the topography and destroys the meadow. Hazards of volcanological origin are not yet occurring in LC;but the recent Lake Monoun (1984) and Nyos (1986) CO2 eruptions, mount Cameroon eruptions (1999 and 2000) and mount Oku Lake event (2011) which are close to the BV and, the age of 0 Myr of basalt, constitutes a subject of controversy toward the reactivation of the Cameroonian hotspot faults. The assessment of risks in LC was based on the average income of breeding activity and house investment. The economy of LC is valued at about US$527472.527. The level of such risk will be important in case of loss of human beings and the destruction of geomorphosites’ values. In order to reduce the level of looses in this zone, hazard and risks maps are provided in this paper as well as some recommendations.

A Typical Sublithospheric Mantle beneath the Tello Region, SE-Ngaoundéré (Cameroon Line)

Peridotite xenoliths,raised to the surface by alkaline basalts or kimberlites,provide us direct information on the processes and composition of the upper mantle.They are the major source of information on the state of stress,pressure and temperature in the deep mantle.They are thus a source of petrological and geochemical information that is generally not available on the Earth’s surface.Fresh spinel-lherzolite xenoliths exhibit a protogranular components of the Tello volcano.The Tello is the continental sector of the Cameroon Line,located in the South East of the town of Ngaoundéréat 75 km approximately between(N7°13’,N7°14’)and(E13°40’and E13°60’).Minerals’composition of the xenoliths is~64%olivine,~24%orthopyroxene,~11%clinopyroxene and~1%spinel.Significant variation in(Cr/Cr+Al)of the system shows the reciprocal nature of the spinel solution.The Tello spinel lherzolites show internal chemical homogeneity and represent a normal upper mantle.Their mineral chemistries suggest equilibrium condition of 830°-925°and 1.4 GPa-2.3 GPa.These data suggest that there is good correlation bracket between increasing activity of Al2O3 and decreasing of practionning of TiO2 into spinel.The AlIV and AlVI contents vary by 0.05-0.2 and 0.03-0.2 respectively.The majority of samples caracterise the lithospheric mantle.