The Cameroon Line (CL) appears as a SW-NE straight line characterized by an intense volcanic activity of basaltic composition with alkaline plutonic complexes, including the Nlonako ring complex (NRC) emplaced in the southern part of the CL. Many hypotheses have been proposed to explain the origin and petrogenetic evolution of these complexes but very few of these works focus or attempt to propose their structural emplacement model, though structural data recorded by them can permit such studies. Using petrographic, structural data and space images, we propose an emplacement model of the NRC and other ring complexes along the CL. The NRC is a ring complex of 10 km diameter slightly elongated in the NNE-SSW direction mainly composed of plutonites among which syenites, gabbros, diorites and biotite-amphibole granites, and a few volcanites made up of rhyolites and basalts occurring as veins or boulders in syenites. This complex was emplaced as sill intrusive body within Pan-African k-feldspars megacrysts granites and gneiss host rocks under fractures control. These fractures developed as result of stress release consecutive to the readjustment of the crust during and after the late Cretaceous general extension, therefore facilitating the upwelling of the mantle plume and the generation of magma that vertically uplift. The progressive magmatic pressure decreases after the NRC emplacement in addition to conjugated fractures networks developed at superficial crust level finally lead to the cauldron subsidence of the NRC summit. This subsidence was facilitated by the downward sliding of rocks along the WNW-ESE fault, finally leading to the formation of a caldera at the summit of the NRC. The NRC and other anorogenic complexes aligned along the CL are located in a tension gash form by the Cretaceous sinistral activation of the N70E Adamawa fault. This left lateral wrench movements developed a tension gash, overprinted on pre-existing transcurrent mega-faults with as result, the development of "en-echelon" fractures. Stress release during the late Cretaceous general extensional phase probably accelerated the uplift of magmas and emplacement of ring complexes along the "en-echelon" regional fractures in an extensional setting during the Tertiary (60 - 30 Ma). This therefore explained the alignment of anorogenic complexes along the N30E CL corridor that are highly correlated to lineament networks and Pan-African megafaults
The biotite granite of Dschang composing of quartz, feldspar and biotite, is a haplogranite I-type calc-alkaline granitoid. It is weakly peraluminous with a ferroan affinity and exhibits fractionated feature. This granite was generated at the minimum temperature and pressure of crystallization of 650°C/4 Kb. Its emplacement is syn-central Cameroon shear zone like most of post-collisional granitoids of the Pan-African fold belt in central Africa, and it displays N-S to NNE-SSW magmatic foliation dipping 30° to 60° towards W or WNW. The evolution of the major elements shows a generalized decrease correlation trends look firstly like fractional crystallization. The depleted Sr content related to decreasing Al 2 O 3 and CaO concentrations, indicate the role played by plagioclase during differentiation. The trace elements normalized to the primitive mantle, show negative anomalies out of Nb, Ta, Sr and Ti, and positive anomalies out of Ba, Rb, Th, comparable with those of the continental crust. The isotopic data Sr-Nd are rather homogeneous suggesting a single source for the biotite granite of Dschang. Ɛ Nd600Ma (-16 to -18) and T DM (1.9 to 2.1 Ga) confirm the formation of the magma of the biotite granite of Dschang starting from an old Paleoproterozoic crust by partial melting around 600 Ma. This melting which seems to have spread in the area and at several levels of the continental lithosphere, hence magmas were mixing throughout their evolution, considering the diversity of the granites and their sources, is certainly caused by great movements along CCSZ, at the post-collisional period of Pan-African orogenesis in central Africa.
The granitoids of Guéra Massif are composed of biotite-granite, amphibole-biotite granite and gabbro-diorite and commonly contain micro granular mafic enclaves which vary from monzogabbro to syenite composition. They are metaluminous, high-K calc-alkaline to shoshonitic series. Gabbro-diorite rocks are magnesian while amphibole-biotite granites are magnesian to ferroan, and biotite granites are ferroan. They are enriched in LREEs relative to HREE and display negative anomalies in Nb, Ta and Ti. Fields relationships, petrology and geochemistry indicate that mixing and mingling processes could be more relevant for the genesis of granitoids associated to fractional crystallization. Thus, the presence of mafic enclaves of gabbro-diorite composition in the granites, the resumption of alkaline feldspar xenocrystals in the gabbro-diorites, as well as the linear correlation between the granites and the gabbro-diorites and the intermediate position of the mafic enclaves between the two formations, enable us to propose magmatic mixing as the major process that presided over the evolution of the Guéra granitoids. The delamination of the continental lithosphere during the post-collisional phase of the Pan-African orogeny would have caused the partial melting of the subduction-modofied mantle and lower continental crust and thus produced the magmas of the Guéra granitoids.
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