Caractérisation structurale et magnétique du pluton granitique de Misajé (Nord-Ouest Cameroun)

The objective of this study was to map the Misajé pluton based on the magnetic susceptibility, petrographic, geochemical and structural (classic) data in order to integrate it in the tectonic evolution of the Central African Fold Belt. It appears at the end of this work that, the Misajé granitic pluton, elongated NNE-SSW is located in the westhern domain of the Pan-African Belt in Cameroon. It was emplaced around 569-560 Ma in the ante-Pan-African basement rocks, composed of biotite hornblende migmatite and amphibolites reactivated in the Pan-African. It (Misajé granitic pluton) consists of biotite hornblende granite, biotite granite, granodiorite and leucocratic granite, differenciated by fractional crystallization.The study area has recorded three deformation phases. The D1 phase, characterized by an E-W foliation in the basement rocks, has been progressively transposed by dominant D2 phase, observed in the basement rocks, biotite hornblende granite and biotite granite. This D2 phase, orientated NE-SW to N-S is dated between 569 ± 12 Ma in the biotite hornblende granite and 560 ± 9 Ma in the biotite granite. The D3, dated at 532 ± 35 Ma is observed in the biotite hornblende orthogneiss. It is characterized by dextral strike-slip with a non-coaxial deformation, which permit (1) the development of the N-S structures and dextral shear movement, (2) the emplacement of leucocratic granite. The microstructural study shows that, in the Misajé pluton, the deformation started during the magmatic state and followed up till the mylonitic state.The rocks of the Misajé granitic pluton are dominated by paramagnetic behavior (54 % of stations). The presence of ferromagnetic minerals reveals that the carriers of the susceptibility are mainly dominated by magnetite that can be euhedral or anhedral. The T Jelinek factor shows that the dominant strain ellipsoids are plano-linear, nevertheless, 80 % of stations have P values less than 1.2, suggesting that, the emplacement of the Misajé pluton is dominated by magmatic flowing. This magmatic flow induced a flattening on the edge of the massif. The domiance of the dynamic flow in the emplacement of the Misajé pluton, the presence of flat minerals edging the pluton, the presence of preferred orientation of feldspar megacrysts, the presence of a discrete NE- SW shear zone in biotite granite and the magnetic structures (foliation and lineation) suggest a syn-kinematic dome emplacement in a NNE-SSW. These structures are parallel to the major regional structures described in Mayo Nolti and Godé Gormaya Shear Zone (MNSZ and GGSZ) and folds developed during the extensional collapse of western Gondwana, as described in eastern Nigeria and NE Brazil.L’objectif de cette étude était d’établir à partir des données de susceptibilité magnétique, de pétrographie, de géochimie et de structurale (classique), une cartographie des formations du pluton de Misajé afin de l’intégrer dans l’évolution de la Chaîne Panafricaine d’Afrique Centrale (CPAC). A l’issu de ce travail, nous retiendrons que le pluton de Misajé, allongé NNE-SSW est situé dans le domaine Ouest de la CPAC au Cameroun. Ce pluton s’est mis en place par injections successives autour de 569-560 Ma dans un socle anté panafricain constitué de migmatite à biotite et amphibole et d’amphibolites, réactivé au Pan-Africain. Quatre ensembles pétrographiques mis en place par cristallisation fractionnée le composent : la granodiorite, le granite à biotite et amphibole, le granite à biotite et le granite leucocrate. La région de Misajé est affectée par trois phases de déformations. La phase D1, caractérisée par une foliation E-W, observée dans l’encaissant a été progressivement transposée par la phase D2 simultanément observée dans l’encaissant, dans le granite à biotite et amphibole et dans le granite à biotite. Cette phase D2, orientée NE-SW à N-S est datée entre 569 ± 12 Ma dans le granite à biotite et amphibole et 560 ± 9 Ma Ma dans le granite à biotite. La phase D3, datée à 532 ± 35 Ma est décrite dans les orthogneiss à biotite et amphibole comme étant la phase de mylonitisation du granite à biotite et amphibole qui se transforme en orthogneiss à biotite et amphibole. Elle est caractérisée par un mécanisme de déformation ductile au cours duquel une déformation non-coaxiale a permis (1) le développement des structures N-S et une déformation par cisaillement dextre et (2) la mise en place du granite leucocrate. L’étude microstructurale permet d’envisager pour le pluton de Misajé une déformation continue (du stade magmatique au stade mylonitique) au cours de sa mise en place. Les roches du pluton granitique de Misajé sont à dominance paramagnétique (54 % de stations). La contribution ferromagnétique existante révèle que les minéraux porteurs de la susceptibilité sont dominés par la magnétite qui peut être automorphe ou xénomorphe. Le facteur T de Jelinek montre que les ellipsoïdes de déformation dominants sont plano-linéaires, cependant, 80 % de stations ont des valeurs de P < 1,2 ; indiquant une mise en place dominée par l’écoulement dynamique. Cet écoulement magmatique a induit un aplatissement en bordure du massif. La domination de l’écoulement magmatique dans la mise en place du pluton de Misajé, la présence des minéraux aplatis en bordure du pluton, la présence des mégacristaux de feldspath orientés, la présence d’un couloir de cisaillement discret NE-SW idenfié par la méthode d’ASM dans le granite à biotite et les structures (foliation et linéation) magnétiques, permettent d’envisager pour le massif de Misajé une mise en place syn-cinématique en coupoles ou dômes dans un continuum cinématique NNE-SSW, parallèlement aux grandes structures régionales (Cisaillement de Mayo Nolti et le Cisaillement de Godé-Gormaya) et aux chaînes plissées développées lors de l’assemblage du Gondwana Ouest, telle que décrite à l’Est du Nigéria et au NE du Brésil

Structural characterization of the Misaj, granitic pluton (NW Cameroon): constraints from magnetic and field observations

International audienceThe Misaj, granitic pluton, emplaced between 569 and 560 Ma in an amphibolitic and gneissic host rock, comprises four petrographic units namely biotite-hornblende granite (BHG), granodiorite (Gd), biotite granite (BG), and leucocratic granite (LG). Four major tectonic events have been described in the studied area: a D-1-early tectonic event, responsible of the E-W flat foliation which has been progressively transposed by a D-2 tectonic event. A D-2 event has developed heterogeneous simple shear in a dextral transpressive context with moderate to strong dipping NE-SW striking foliation; a D-3 tectonic event has lead to a sinistral N-S ductile shear characterized by N- to ENE-striking foliation and E-W strike-slip shear corridors and a D-4 tectonic event that developed N-S dextral ductile strike-slip deformation. The magnetic study of the pluton, based on the AMS parameters, reveals the coexistence of both paramagnetic (dominated by iron-bearing silicates; 54 % of sites) and ferromagnetic (due to the occurrence of PSD and MD grains of magnetite or other ferromagnetic minerals; 46 % of sites) behaviors. Magnetic foliation shows best poles at 55/82 for the whole pluton, 95/32 in BHG, and 273/83 in BG, and the magnetic lineation trends are mostly NNE-SSW with best lines at 210/8, 198/19, and 36/3, respectively. The trend of the magnetic lineation in BG indicates an S-shape trajectory, suggesting a sinistral sense of shear motion along discrete E-W corridors situated at the northern and southern ends. Kinematic indicators in BG point to a sinistral sense of shear, suggesting its emplacement during the D-3 event. The close relationship between K (1) and K (3) points to a syn-kinematic emplacement and crystallization of the Misaj, granitic pluton during the Pan-African event, and the tectonic evolution of the study area is considered to be coeval with the tectonic evolution of the trans-Saharan Pan-African belt of eastern Nigeria

Major elements, trace elements and Sr-Nd-Pb isotopes form lavas of lakes Nyos, Wum, Elum and Oku sampled in the Oku Volcanic Group of the Cameroon Volcanic Line

Lake Nyos is located at the summit of a stratovolcano in the Oku Volcanic Group (OVG) along the Cameroon Volcanic Line. The sudden release of magmatic CO2 trapped at the bottom of Lake Nyos in August 1986 caused historical casualties of 1750 people and over 3000 cattle. New geochemical data of volcanic rocks from the Nyos volcano and the first available data for volcanic rocks from other maar-bearing volcanoes (Lakes Elum, Wum and Oku) in the OVG are presented and compared. Lavas from the Nyos, Elum and Wum volcanoes show similarities in major and trace elements and Sr?Nd?Pb isotopes, suggestive of a similar mantle source. However, this source is slightly different from that of the Oku volcano. The samples from Lake Oku have lower alkali, higher TiO2 and more depletion and enrichment in most incompatible trace elements than those from the Nyos, Elum and Wum volcanoes. These differences and those observed in the Sr?Nd?Pb results are consistent with a heterogeneous source for lavas in the OVG. Trace element compositions suggested the presence of garnet in the source (< 6% garnet) and modelled melting results indicate < 2% partial melting of the source material. Isotope data plot within the focal zone, extending towards enriched mantle 1 (EM1; e.g. Lakes Oku and Nyos samples). This indicates the involvement of at least three mantle components: depleted mid-ocean ridge basalt mantle, high-µ and EM1 components in the magmatism of the lavas studied. The contributions of these components in different proportions, originating from asthenospheric and subcontinental lithospheric mantle sources, can account for the observed variations in geochemical characteristics. The geochemical characteristics of the studied lavas indicate that the magma source need not necessarily have an abnormal CO2 concentration to pose a potential threat. Degassing of an ordinary magma chamber and the migration of gas to the bottom of the lakes through cracks and faults can lead to the accumulation of CO2 in lake bottoms. This is controlled by tectonic parameters (fractures and faults) that enhance degassing from the magma chamber to the lake bottom and physical parameters of the lake (e.g. size, depth, temperature and solubility) that control CO2 stability