Rock magnetic investigation of possible sources of the Bangui magnetic anomaly

International audienceThe Bangui magnetic anomaly (BMA) is the largest lithospheric magnetic field anomaly on Earth at low latitudes. Previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements, as well as from surface magnetic susceptibility measurements on rocks from the Panafrican Mobile Belt Zone (PMBZ). Here we combine magnetic field data modelling and rock magnetic property measurements (susceptibility and natural remanent magnetization, NRM) on many samples from this PMBZ and the surrounding formations. It reveals that NRM is a significant component of the total magnetization (Mt) of the BMA source, which reaches 4.3 A/m with maximum thicknesses of 38 and 54 km beneath the western and eastern parts of the BMA. Only the isolated and relatively thin banded iron formations and some migmatites show such Mt values. Thus we suggest that the thick BMA source may be composed either by overlapped slices of such metamorphic rocks, or by an iron-rich mafic source, or by a combination of these two geological structures

Kinematic evolution of the Mbakop Pan–African granitoids (western Cameroon domain): An integrated AMS and EBSD approach

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Time relationship between emplacement, fabric development and regional deformation of the Manchi granitic pluton (western - Cameroon domain)-an integrated AMS, CPO and microstructural investigation

International audienceIn this study, we integrate anisotropy of magnetic susceptibility (AMS) and microstructural data including quartz crystallographic preferred orientation (CPO) from SEM-EBSD from the Manchi Pan-African granitic pluton with the objective of deciphering the time relationship between fabric development, emplacement and regional tectonics. The Manchi pluton, located in the western Cameroon domain of the Pan-African Fold Belt of Central Africa in Cameroon, is made of hornblende-biotite granite (HBtG), biotite granite (BtG) and protomylonites of HBtG. The pluton intruded the high grade country rocks made of banded gneiss and amphibolites. AMS of the pluton is controlled dominantly by paramagnetic phases such as, Fe-bearing silicates (biotite and hornblende). This is inferred from the low Km values (-6 SI units) as well as from hysteresis data. The AMS ellipsoid shape is mostly oblate, implying that flattening component of the strain was important during granite emplacement. Microstructural observations reveal presence of magmatic to sub-magmatic and solid-state deformation textures developed at moderate- to low-temperatures. This is consistent with the presence of rhomb and basal slip systems established from quartz c-axis data through EBSD analysis. The pluton recorded top-toward SW to top-toward south sense of shear as established by the kinematic markers. This indicates a component of simple shear during tectonic evolution of the pluton. We infer that the magnetic fabrics recorded in the Manchi granitic pluton is a manifestation of the D2 regional deformation. The dominance of flattening strain and the concentric pattern of magnetic data are indicative of ballooning being an important mechanism under transpressive regime

Transpressional granite-emplacement model: Structural and magnetic study of the Pan-African Bandja granitic pluton (West Cameroon)

International audienceThe Pan-African NE-SW elongated Bandja granitic pluton, located at the western part of the Pan-African belt in Cameroon, is a K-feldspar megacryst granite. It is emplaced in banded gneiss and its NW border underwent mylonitization. The magmatic foliation shows NE-SW and NNE-SSW strike directions with moderate to strong dip respectively in its northern and central parts. This mostly, ferromagnetic granite displays magnetic fabrics carried by magnetite and characterized by (i) magnetic foliation with best poles at 295/34, 283/33 and 35/59 respectively in its northern, central and southern parts and (ii) a subhorizontal magnetic lineation with best line at 37/8, 191/9 and 267/22 respectively in the northern, central and southern parts. Magnetic lineation shows an `S' shape trend that allows to (1) consider the complete emplacement and deformation of the pluton during the Pan-African D-2 and D-3 events which occurred in the Pan-African belt in Cameroon and (2) reorganize Pan-African ages from Nguiessi Tchakam et al. (1997) compared with those of the other granitic plutons in the belt as: 686 +/- 17 Ma (Rb/Sr) for D-1 age of metamorphism recorded in gneiss; and the period between 604-557 Ma for D-2-D-3 emplacement and deformation age of the granitic pluton in a dextral ENE-WSW shear movement

Transpressional granite-emplacement model: Structural and magnetic study of the Pan-African Bandja granitic pluton (West Cameroon)

International audienceThe Pan-African NE-SW elongated Bandja granitic pluton, located at the western part of the Pan-African belt in Cameroon, is a K-feldspar megacryst granite. It is emplaced in banded gneiss and its NW border underwent mylonitization. The magmatic foliation shows NE-SW and NNE-SSW strike directions with moderate to strong dip respectively in its northern and central parts. This mostly, ferromagnetic granite displays magnetic fabrics carried by magnetite and characterized by (i) magnetic foliation with best poles at 295/34, 283/33 and 35/59 respectively in its northern, central and southern parts and (ii) a subhorizontal magnetic lineation with best line at 37/8, 191/9 and 267/22 respectively in the northern, central and southern parts. Magnetic lineation shows an `S' shape trend that allows to (1) consider the complete emplacement and deformation of the pluton during the Pan-African D-2 and D-3 events which occurred in the Pan-African belt in Cameroon and (2) reorganize Pan-African ages from Nguiessi Tchakam et al. (1997) compared with those of the other granitic plutons in the belt as: 686 +/- 17 Ma (Rb/Sr) for D-1 age of metamorphism recorded in gneiss; and the period between 604-557 Ma for D-2-D-3 emplacement and deformation age of the granitic pluton in a dextral ENE-WSW shear movement

Magnetic fabrics of the Miocene ignimbrites from West-Cameroon: Implications for pyroclastic flow source and sedimentation

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