b'The origin of the Bangui magnetic anomalyFeatureFigure 2.(a) Simplified tectonic map of Africa (after Milesi etal. 2010), showing the location and extent of the Archean cratons, intracratonic basins and the surrounding Precambrian and Paleozoic fold belts, which were affected by rifting processes during Mesozoic and by Cenozoic volcanism. WCAMZ: West and Central African Mobile Zone, WCARS: West and Central African Rift System, CARS: Central African Rift System; (b) Simplified geological map showing the Central African Orogenic Belt (CAOB) in the West and Central African rift system (WCARS) and the Congo craton (modified from Abdelsalam etal. 2002). TBSZ: Tchollire-Banyo Shear Zone; SSZ: Sanaga Shear Zone; CCSZ: Central Cameroon Shear Zone; C.V.L.: Cameroon Volcanic Line; (c) Main rheological domains of sub-Saharan Africa centred on the Saharan Metacraton (modified from Liegeois etal. 2013). Smc: Saharan Metacraton.Atlantic. Currently continental break-up and rifting occursThe granulites are post-collisional remnants characteristic of the along the boundary between the Nubian and Somalian plates,Eburnean orogeny. They outcrop at the northern edge of the marked by the 5000 km long East African Rift System. The BMACongo Craton in Cameroon (Toteu etal. 2006). These structures overlies, along a major mid-axis ENE-WSW, the Neo-Proterozoicwere taken up during a pan-African thermo-tectonic event in formations of South-Cameroon and the Central Africanthe mobile zone (Penaye etal. 2004; Toteu etal. 2004). According Republic, individualised during the Pan-African Orogeny; as wellto Pin and Poidevin (1987) all Central African granulites are as the Paleo-Proterozoic and Archean structures of the Congocontemporaneous with the Congo Craton in Cameroon. Craton. The dominant outcrop lithologies in the Congo CratonFigure1 shows the spatial distribution of granulites beneath the are Tonalite-Trondhjemite-Granodiorite (TTG), charnockites,BMA. The high concentration of iron oxides in these granulites high-K granites and greenstone belts (Dostal etal. 1985; Nedelecindicates that they derived from a very mafic parental magma etal. 1986; Feybesse etal. 1998; Shang etal. 2004; Owona,(Clark 1999). According to Pin and Poidevin (1987) the crust in 2008). According to Dostal etal. (1985) and Poidevin (1991) theits lower part would have a more mafic composition than at the Archean domain of the Central African Republic is dominatedbase of the surface rocks. The emplacement of mafic basalts by an orthogenetic basement in which multi-kilometre longin the lower crust during pan-African orogeny is a possibility. greenstone belts, granites and epi-metamorphic series of iron- Granulites are stable at depths of nearly 50 km under conditions bearing quartzites appear. of high pressure and temperature (Boukeke 1994). Moreover, petro-physical studies on samples (Ouabego etal. 2013) have The greenstone belts are mafic to ultra-mafic meta-volcanitesshown that mafic sources rich in iron have a particular natural (amphibolites, gneisses and metamorphosed Banded Ironmagnetic remanence of about 4.3 A/m, which would make Formations with magnetite; Shang etal. 2010; Ndime etal.them potential sources.2019). However, the Banded Iron Formations (BIFs) form quite localised outcrops (about 200 km) compared to theThe BMA actually covers a geographical area marked by major wavelengths of about 1000 km of the Bangui Magneticstructural features other than the Congo Craton. Notably, the Anomaly. BIFs are formations that generally extend over aCentral African Orogenic Belt is characterised by important hundred meters and may be 1 to 2 km thick (Alexandrowregional shear zones including the Central Cameroon Shear etal. 1973). Although being characteristic of the PrecambrianZone (CCSZ) and the Sanaga Shear Zone (SSZ). Regional phases basement under the Bangui Magnetic Anomaly, these BIFsof deformation (Rollin 1995; Feybesse etal. 1998; Ngako etal. alone do not explain the magnetic behaviour of the crust.2003; Toteu etal. 2004) in the Central African Orogenic Belt According to Boukeke (1994), the sources are deep, stronglyhave followed one another during pan-African thermo-tectonic magnetic and closely related to the Congo Craton structures.events. These thermo-tectonic events have affected the Congo Two types of sources could explain surface observables:Craton in Cameroon and Central Africa, with the development either the presence of mafic granulites or a demagnetisationof important thrust sheets. In general, the thrust sheets effect. (Figure2) as well as the regional shear zones (CCSZ and SSZ) FEBRUARY 2022 PREVIEW 44'