Article révisé par les pairs
Résumé : The West-Congo orogenic belt is a Late Proterozoic chain that extends parallel to the western coast of central Africa from Gabon to Angola for a length of more than 1000 km. Its origin is still debated and two contrasting viewpoints have been proposed: plate collision (Vellutini et al.) versus ensialic thermal orogeny (Schermerhorn). The question is reinvestigated in the light of new field work and structural, petrological and geochemical studies carried out in Zaïre. We focus on the Zadinian Group that outcrops between Boma and Matadi. We compare it with the Mayumbian Group occurring to the east of Matadi, and we have come to the conclusion that they are stratigraphically equivalent. The metavolcanic rocks ranging in composition from basalt to rhyolite have trace element signatures comparable with those of Mesozoic and Cenozoic continental tholeiites that erupted in rift zones. The most primitive metabasalts have a Nb anomaly relative to Th-La, which suggests a 'supra subduction' setting for the postulated spreading centre. The rift system developed inside the 2100 Ma old Eburnean basement, which is recognized here and there at the base of the Zadinian Group. The presence of this continental crust is also deduced from the thick sequence of Mayumbian rhyolites, which could not have been produced without crustal melting due to uprise of mantle-derived basaltic magmas. Structural investigations reveal two compressive structural types F1 and F2, different in trend but similar in style. The F1 type, trending E-W, is restricted to the Boma area, while the F2 type affected the whole West-Congo belt. The structural and metamorphic asymmetry of this orogenic belt results from: (1) the strong eastward vergence of F2 folds and N-S tectonic thrust faults; (2) the eastward decreasing metamorphic grade (lower part of the amphibolite facies to greenschist facies). The P-T paths of regional metamorphism have been estimated using the parageneses of metapelites, calc-silicate rocks and metabasalts. They characterize a low-P/high-T facies series with a probable average thermal gradient of 60° C km-1. The syntectonic single-phase metamorphism took place before the amplacement of the 733 Ma old peralkaline Noqui granite. The various geological constraints are integrated in a geotectonic model: the West-Congo orogenic belt is likely to have evolved from a wide intracratonic marginal basin that has been subsequently deformed in response to the collision between the Brazilian and the Congo plates. The suture zone is located farther west than the amphibolitic migmatites of Boma, probably near the present continental margin of central Africa or its Brazilian counterpart. © 1988.