Ensembles structuraux et principales phases de déformations panafricaines dans la zone mobile du Nord Cameroun, région de Poli

Deux cycles de migmatisation et deux phases majeures de déformation donnent à la zone mobile ses principales caractéristiques structurales. La première migmatisation (environ 650 m.a. ? ), affectant les séries de Poli, est liée à la mise en place des granites orientés de Bakonwa, concordants et diffus. Suit une phase de serrage qui contribue à verticaliser les structures régionales de gneiss, puis des cisaillements. Intervient ensuite une seconde migmatisation, avec la mise en place des granites de Wangai, discordants. Ces granites préludent à la mise en place des intrusions granitiques circonscrites post-orogéniques

Microchemical characterization of placer gold grains from the Meyos-Essabikoula area, Ntem complex, southern Cameroon

Gold occurs as a native metal, usually containing silver, and in some cases mercury, copper, and palladium. It may also occur as inclusions within sulfur-rich minerals, such as pyrite and arsenopyrite. The style and variety of gold mineralization is influenced by the geological setting, chemistry of the ore fluids, and the nature of their interactions with rocks. Gold grains liberated from bedrock into surficial sediments during weathering and erosion are chemically stable and may be characterized according to their mineralogy: i.e the alloy composition and suite of mineral inclusions revealed within polished sections, characteristics faithful to gold from the hypogene source. This approach has been applied to placer gold grains from the Meyos-Essabikoula area, Cameroon, where the source of gold is not yet confirmed due to poor outcrop exposure. A total of 221 alluvial gold grains from 10 sites, tributaries of Sing and Bivele River over the Ntem Complex have been studied using Electron Probe Micro-Analysis (EMPA) to determine the concentration of minor alloying metals, (notably Au, Ag, Cu, and Hg) and Scanning Electron Microscopy (SEM) in order to evaluate the assemblage of mineral inclusions within the gold. Most of the grains are sub-rounded with pitted surfaces and inclusions of pyrrhotite, acanthite, and chalcopyrite were observed. The grains are AuAg alloys ranging from 54.4 to 99.8 wt% Au, 0.1–48.4 wt% Ag, 0.1–0.8 wt% Hg and 0–0.3 wt% Cu. The presence of Fe oxide (magnetite) inclusions containing Cr and V (to around 5 wt %) has not been reported elsewhere and suggests a strong interaction between hot reducing ore fluids and local mafic lithologies

Spatio-temporal evolution of the West African monsoon during the last deglaciation

On the basis of a multi-proxy data set from the Gulf of Guinea (eastern equatorial Atlantic) we reconstruct the spatio-temporal evolution of the West African monsoon (WAM) and present evidence for a decoupling between latitudinal shifts of the rain belt and WAM intensification. The onset of deglacial monsoon invigoration at ∼16,600 years before present lagged northward migration of a weak rainfall zone by ∼2800 years. Conversely, during the Younger Dryas (YD) time interval, WAM precipitation was severely reduced but we find no evidence for a large-scale retreat of the rainfall front. This observation is not in agreement with the hypothesis of a large-scale shift of the intertropical convergence zone south of the tropical WAM region during the YD. Our results can be better reconciled with the newly emerging concept of a strong influence of Tropical Easterly and African Easterly Jets on modern WAM

Petrography and geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: Implications for their sources and geological setting.

The Nagoundéré Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as the Adamawa-Yade batholith. The granites were emplaced into a ca. 2.1 Ga remobilised basement composed of metasedimentary and meta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The granitoids comprise three groups: the hornblende–biotite granitoids (HBGs), the biotite ± muscovite granitoids (BMGs), and the biotite granitoids (BGs). New Th–U–Pb monazite data on the BMGs and BGs confirm their late Neoproterozoic emplacement age (ca. 615 ± 27 Ma for the BMGs and ca. 575 Ma for the BGs) during the time interval of the regional tectono-metamorphic event in North Cameroon. The BMGs also show the presence of ca. 926 Ma inheritances, suggesting an early Neoproterozoic component in their protolith. The HBGs are characterized by high Ba–Sr, and low K2O/Na2O ratios. They show fairly fractionated REE patterns (LaN/YbN 6–22) with no Eu anomalies. The BMGs are characterized by higher K2O/Na2O and Rb/Sr ratios. They are more REE-fractionated (LaN/YbN = 17–168) with strong negative Eu anomalies (Eu/Euasterisk operator = 0.2–0.5). The BGs are characterized by high SiO2 with K2O/Na2O > 1. They show moderated fractionated REE patterns (LaN/YbN = 11–37) with strong Eu negative anomalies (Eu/Euasterisk operator = 0.2–0.8) and flat HREE features (GdN/YbN = 1.5–2.2). In Primitive Mantle-normalized multi-element diagrams, the patterns of all rocks show enrichment in LILE relative to HFSE and display negative Nb–Ta and Ti anomalies. All the granitoids belong to high-K calc-alkaline suites and have an I-type signature. Major and trace element data of the HBGs are consistent with differentiation of a mafic magma from an enriched subcontinental lithospheric mantle, with possible crustal assimilation. In contrast, the high Th content, the LREE-enrichment, and the presence of inherited monazite suggest that the BGs and BMGs were derived from melting of the middle continental crust. Structural and petrochemical data indicate that these granitoids were emplaced in both syn- to post-collision tectonic settings