Mantle sources and magma evolution beneath the Cameroon Volcanic Line: Geochemistry of mafic rocks from the Bamenda Mountains (NW Cameroon)

International audienceWe report the mineralogy, geochemistry and geochronology of the mafic rocks from the Bamenda Mountains, part of the Cameroon Volcanic Line (CVL), in order to discuss the origin and evolution of the magmas in this part of the CVL. Mafic rocks in the Bamenda Mountains are basanites, basalts, hawaiites and mugearites with an alkaline affinity. K-Ar ages have been obtained on 10 samples and range from 17.6 Myr to present. Trace element and isotopic compositions (Sr-Nd-Pb) show that some samples among the oldest are slightly contaminated by a crustal component with high La/Nb and 87Sr/86Sr ratios and low Pb isotopic ratios. The mafic rocks strongly resemble OIB in their trace element compositions. Some samples possess a positive Sr and Eu anomaly which cannot be explained by a process of plagioclase accumulation. These anomalies are also observed in some pyroxenites found as xenoliths in the Adamawa volcanic province further north. Furthermore, non-contaminated samples have high Pb isotopic ratios and point towards an HIMU component similar to the St. Helena mantle plume. We propose that the Bamenda mafic magmas with positive Sr and Eu anomalies were formed by hybridization of asthenospheric melts with melts formed by the partial melting of pyroxenites. Samples without these anomalies result from the hybridization of the same asthenopheric melts with melts coming from the metasomatized, amphibole-bearing, lithospheric mantle

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

The Pan-African Kekem gabbro-norite (West-Cameroon), U-Pb zircon age, geochemistry and Sr-Nd isotopes: Geodynamical implication for the evolution of the Central African fold belt

International audienceThe Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U-Pb zircon analyses provide an age of 576 ± 4 Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ). The REE patterns display enriched LREE (LaN/YbN = 14.2-23.5) while HREE present a nearly flat profile (DyN/YbN = 1.3-1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb-Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6-10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068-0.7082), low εNd (−5 to −9) and old Nd TDM model ages (1.6-1.8 Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust. The ca. 576 Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle

Evaluation of the mineralizing potential of the Mbengwi plutonics, Northwestern Cameroon, Central Africa

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