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Teitchou M.I.,Institute Of Recherches Geologiques Et Minieres | Teitchou M.I.,University of Yaounde I | Gregoire M.,University Paul Sabatier | Temdjim R.,University of Yaounde I | And 3 more authors.
Special Paper of the Geological Society of America | Year: 2011

Basaltic lavas of Nyos volcano (Cameroon) mostly contain mantle peridotite xenoliths consisting of spinel-bearing lherzolites and harzburgites. Based on the trace-element patterns, especially rare earth element (REE) patterns, two groups of samples have been distinguished: group 1 samples are characterized by spoonshaped REE patterns, and group 2 samples show light (L) REE-enriched patterns. Mineralogical characteristics together with major-and trace-element compositions point to a low degree of partial melting (less than 5%) and metasomatic processes. The latter mechanism explains in particular the LREE content of bulk rocks and clinopyroxenes and the occurrence of hydrous minerals in some samples. All the metasomatic features observed in both groups of samples are related to more or less alkaline-and carbonated-mafic silicate melts. These melts are related to the magmatic activity of the Cameroon volcanic line, leading in particular to the eruption of the host lava xenoliths. © 2011 The Geological Society of America. All rights reserved.

Kankeu B.,Institute Of Recherches Geologiques Et Minieres | Greiling R.O.,Karlsruhe Institute of Technology | Nzenti J.P.,University of Yaounde I | Bassahak J.,Institute Of Recherches Geologiques Et Minieres | Hell J.V.,Institute Of Recherches Geologiques Et Minieres
Neues Jahrbuch fur Geologie und Palaontologie - Abhandlungen | Year: 2012

Field and microstructural work supported by anisotropy of magnetic susceptibility (AMS, magnetic fabric) studies were applied to identify the sequence and character of Pan-African structures in gneissic rocks and late-tectonic granitoids in a part of the Central Africa Shear Zone (CASZ) system (Meiganga area, central Cameroon). The area is bounded by two steeply dipping, NE-SW trending Pan-African shear zones, the Mbéré- Djérem (MDSZ, in the NW) and the Meiganga (MSZ, in the SE) shear zones, respectively, enclosing a central area, composed mostly of high-grade gneisses and orthogneisses, including reworked Palaeoproterozoic gneisses in the southwestern part (domain I), and mainly late-tectonic granitoid intrusions of variable size in the northeastern part (domain II). In the crystalline basement rocks of domain I, the early planar fabric (S1) was folded during D2 (folding and foliation) and D3 (shearing, wrenching, folding) deformational events. Oblate AMS (strain) ellipsoids characterize the dominant orthogneisses. Magnetic foliations curve from NE-SW to E-W with variable dips. Magnetic lineations trend mostly NE-SW with shallow to medium plunges. This fabric defines both small-scale and km-scale folds with WSW plunging axes sub-parallel with the marginal shear zones. Around the MSZ, AMS ellipsoids are typically prolate, whereas both prolate and oblate shapes are developed around the MDSZ. Magnetic foliations trend ENE-WSW with mostly steep dips. Magnetic lineations trend also ENE-WSW with shallow plunges. This fabric, with dominantly steep foliations and subhorizontal lineations implies that the MSZ and the MDSZ are Pan-African strike-slip shear zones with subordinate components of compression. The spatial separation of coeval kinematic domains of prolate and oblate shape dominated fabrics is interpreted as the product of partitioning between coeval simple shear-dominated strain in the MDSZ and MSZ, and pure shear-dominated strain in domain I. In granites of domain II, variably oriented magnetic foliations and subhorizontal lineations are interpreted as indicating emplacement during strike-slip deformation at the end of Pan-African orogeny. At a regional scale, the present results imply that central and eastern Cameroon with the CASZ, which represent the internal domains of the Pan-African orogen in central Africa, are dominated by high angle faults and partitioned compression and strikeslip tectonics. These structures are also comparable with those of NE Brazil, along strike prior to Atlantic opening. © 2012 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.

Mbassa B.J.,Institute Of Recherches Geologiques Et Minieres | Mbassa B.J.,University of Yaounde I | Njonfang E.,University of Yaounde I | Benoit M.,University Paul Sabatier | And 6 more authors.
Mineralogy and Petrology | Year: 2012

The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62 wt.% SiO 2. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0. 7031 < ( 87Sr/ 86Sr) initial < 0. 7043; 1. 03 < ε Ndi < 5. 17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28. 2 Ma, almost similar to 27. 40 ± 0. 6 Ma K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL. © 2012 Springer-Verlag Wien.

Mbassa B.J.,Institute Of Recherches Geologiques Et Minieres | Kamgang P.,University of Yaounde I | Gregoire M.,University Paul Sabatier | Njonfang E.,University of Yaounde I | And 6 more authors.
Comptes Rendus - Geoscience | Year: 2016

The Mbengwi plutonics consist of intermediate to felsic granitoids forming a continuous magmatic series from monzonite to granite and mafic intrusions. Their mineralogical composition consists of quartz, plagioclases, K-feldspars, biotite, muscovite, and amphibole. The accessory phase includes opaque minerals + titanite ± apatite ± zircon, while secondary minerals are pyrite, phengite, chlorite, epidote, and rarely calcite. These plutonics are assigned high-K calc-alkaline to shoshonitic series, metaluminous to weakly peraluminous and mostly belong to an I-type suite (A/CNK = 0.63-1.2). They are typically post-collisional, with a subduction signature probably being inherited from their protoliths emplaced during the subduction phase. The Sr and Nd isotopic data evidence that these plutonics result from melting of the lower continental crust with variable contribution of the oceanic crust. Their geochemical features are similar to those of western Cameroon granitoids related to the Pan-African D1 event in Cameroon. © 2015 Académie des sciences.

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