La région de Bidou I au Sud-Cameroun : évolution tectono-métamorphique et âge des évènements

Abstract

Bidou I area belongs to the Nyong Group greenstone belts, which represents the NW edge of the Congo Craton in Cameroon. Petrographic, structural, geochemical and geochronological studies highlight the tectonometamorphic evolution of these rocks and age of events they have experienced and to establish correlations with other parts of the Congo Craton, as well as with its NE extension in Brazil (São-Francisco Craton). The present study show that the Bidou i area consists of metamorphic rocks made up of garnet gneisses (Grt + Px + Qz + kfs + Rt + Pl ± Bt; Ms + Op; Bt + Ms + Qz; Ms + Bt + Qz +Rt), garnet micaschists (Grt + Bt + Ms + Qz + Kfs + Pl; Ms + Qz + Pl) and garnet quartzites (Grt+ Bt + Ms + Po + Qz; Qz + Crn + Bt). These mineral associations testify a retrograde granulite to amphibolite facies plurifacial metamorphic evolution. The rocks have experienced a polyphase tectonic evolution with two main deformational phases: a D1 compressional phase with a N-S to NE-SW kinematic direction, a D2 shearing phase with NNE-SSW direction and britle D3 phase trending NE - SW. These directions are similar to that of the Kribi Campo Fault (NNE-SSW). The Bidou I rocks are metasediments with felsic composition. The garnet gneisses and garnet quartzites derived from arkoses while the garnet micaschists are from greywackes. These protoliths are thought to originate from a felsic continental crust and are the result of partial melting and/or fractional crystallisation in a subduction-collision regime. Garnet gneiss and garnet quartzites were deposited in a passive margin, while micaschists are from arc domain. Such fluctuation of tectonic settings highlights a progressive change of the basin geometry from a passive to an active continental margin and the development of a continental island arc. LA-ICP-MS U-Pb analyses on zircon of garnet gneisses give an average age of 2067 Ma corresponding to a metamorphic event that affected these gneisses. The garnet micaschists which are from Archean materials (2786 Ma), underwent a major depositional phase in the Palaeoproterozoic (2400 Ma), and were affected by metamorphism around 2002 Ma. This Paleoproterozoic metamorphic event is contemporaneous with the eburnian/transamazonian orogeny (~ 2100 - 2000 Ma) widespread in the Nyong Group and the São-Francisco craton in Brazil. These results confirm that the Nyong Group corresponds to a Paleoproterozoic suture zone between the Congo and the São-Francisco cratons in Brazil; and that the Bidou I locality is part of the northern part of the link connecting the North Equatorial Fold Belt to western Congo,responsible of the separation of congo and São-Francisco cratons.