The West Central African belt: a model of 2.5–2.0 Ga accretion and two-phase orogenic evolution

Abstract

A mapping, tectonic (deformation, kinematics), petrologic ( P– T paths) and geochronological approach has enabled us to define the Paleoproterozoic West Central African Belt (WCAB), to reconstruct its organization and its evolution, and to determine its position within the Congo–São Francisco craton. The belt extends from southwestern Cameroon to the Congo and evolved between 2500 and 2000 Ma. The initial tectonic stage had a continental character. It corresponds to the break up of an Archean craton (no ocean opening, no addition of juvenile material) with the appearance of a zone of tectonic instability comprising blocks whose boundaries are partly marked by sedimentary basins (Ogooué, Nyong, Ikobé–Waka) and plutonic rocks emplaced between 2515 and 2435 Ma. A first crustal shortening event (E1) occurred between ca 2235 and 2300 Ma. This event led to thickening, at first homogeneous and then heterogeneous (nappe tectonism of unknown vergence). The sediments deposited during the previous stage were thus buried to depths of 20–35 km (7–12 kbar at peak metamorphism) and underwent a prograde–retrograde type metamorphic evolution characterized by an anticlockwise P– T path. The retrograde metamorphic evolution was due to crustal thinning arising from isostatic uplift and erosion of the thickened zones. The crustal thinning was also accompanied by a new break up of nearby Archean continental blocks. This break up was associated with the opening, before 2145 Ma, of the pre-oceanic Francevillian extensional basins [deposition of formation A (FA) of the Francevillian sediments]. A second crustal thickening (event E2) took place after 2145 Ma through thrust tectonism that accreted the Archean block, and the sediments buried during the E1 event, onto the edge of the Congo craton. E2 was accompanied by further prograde–retrograde metamorphism characterized by a clockwise P– T path in the inner zone of the belt and an anticlockwise P– T path in the outer zone. The tectonic activity of this second event began in the inner zone, where the rocks underwent high-grade metamorphism (9 kb and 810°C) that peaked ca 2120 Ma. At the same time the foreland was subjected to brittle tectonism with accelerated subsidence of the Francevillian basins and the deposition of marine sediments (flysch; formations FB and FC) from 2100 Ma onwards. The thickened inner zone was then transported onto the outer zones of the belt, which were thus buried and locally reached anatectic conditions at peak metamorphism (600°C and 8 kb). The lithotectonic pile was finally transported onto the autochthonous Archean Chaillu Block (=Congo craton), this transport being completed ca 2050–2040 Ma in a transitional ductile/brittle context with rapid erosion of the mountain belt giving rise to the deposition of terrigenous molasse (formations FD and FE) in the Francevillian basins. A third tectonic event (E3) saw the return, ca 2040 Ma, of the thickened domain to isostatic equilibrium. This was accompanied by the emplacement of late-orogenic granite (2040–1920 Ma) and the development of folds and strike-slip faults. Comparing the Congo–São Francisco craton with the Ubendian, Luiza and Transamazonian orogenic belts shows similarities in the successions and distributions (with time) of the sedimentary, tectonic, metamorphic and magmatic events, revealing a period (2500–2000 Ma) of major displacement (break up, extension, convergence) of the Archean continental blocks (São Francisco, Congo, Tanzanian) and leading one to consider a vast orogenic domain comprising the West Central African and Transamazonian belts. This period of continental displacement includes an early stage (2500–2400 Ma) corresponding to the Archean–Proterozoic transition which is characterized by dislocation of the margins of the Congo (WCAB, Luiza belt) and São Francisco (Transamazonian belt) cratons and development of oceanic crust in an intercontinental setting (Atlantic Coast Complex, Salvador–Curaça belt and early basic magmatism of the Luiza basin). A second stage of extension occurred between 2300 and 2145 Ma, with the opening of new pre-oceanic basins: the early Booué; Franceville; and Sembé-Ouesso (Francevillian A) and Luiza basins (Archean Congo block); Rio Itapicuru and Rio Jacare greenstone belts (Archean São Francisco block). At the scale of the WCAB, and probably also of the Transamazonian belt, this second extension occurred between two stages of crustal thickening (E1 and E2) each characterized by metamorphism with complete P– T cycles (i.e. a prograde stage followed by a retrograde stage). This alternation of extensional and compressive tectonic phases leads us to propose a two-phase (if not a two-cycle) orogenic model.