New U–Pb and Sm–Nd data from north-central Cameroon and its bearing on the pre-Pan African history of central Africa

Abstract

North-central Cameroon is part of the Central African Fold Belt (CAFB) to the north of the Congo craton. In pre-drift reconstructions of Pangea, the CAFB matches with the Borborema province (BP) of NE Brazil. U–Pb analyses of zircons and Sm–Nd analyses of minerals and whole rock for samples from north-central Cameroon show a long and complex crustal evolution beginning in the late Archean and extending to the late Neoproterozoic. This crustal structure can be correlated with crustal boundaries in Brazil. The region north of the Congo craton in Cameroon was primarily the locus of two successive orogenies, Eburnian at 2100 Ma and Pan African at 600 Ma. The Eburnian event fused the Congo craton with the São Francisco craton of Brazil to form a larger, middle Paleoproterozoic Congo–São Francisco craton. Archean ages (2900–3000 Ma) north of the Congo craton have been obtained primarily as inheritance (xenocrystic zircons and Nd isotopic signatures) in high-grade metamorphic rocks of the Adamaoua region, although no exposed Archean units have been recognized. The Eburnian event is recorded as metamorphic U–Pb ages in both metasedimentary and metaplutonic rocks. Some of the Paleoproterozoic rocks were derived from juvenile crustal growth and others show Sm–Nd isotopic signatures indicating derivation from reworking or melting of Archean crust. Mesoproterozoic Sm–Nd model ages of 1600–1000 Ma occur in northern Cameroon, northwest of the Tcholliré-Banyo fault system and correlate with similar terranes in NE Brazil. These model ages are probably the result of mixing of Neoproterozoic material with older crust, as found in Brazil. Early Neoproterozoic Sm–Nd model ages of 1000–700 Ma on metasedimentary and metavolcanic rocks may correspond to a period of Neoproterozoic crustal rifting and basin formation dominated by more juvenile sources. The Poli basin is characterized by important development of juvenile rocks while the Yaoundé and Lom basins have substantial amounts of older crustal material. Late Neoproterozoic U–Pb ages (620–580 Ma) correspond to the period of West Gondwana assembly with calc-alkaline magmatism, convergent tectonism, metamorphism and anatexis, development of regional strike-slip faults, and finally the uplift of the belt associated with alkaline subcircular massifs. The results from north-central Cameroon are dominated by the effects of Eburnian and Pan African orogenesis; there is little evidence for the widespread ca. 1000 Ma Cariris Velhos activity found in NE Brazil. Some Sm–Nd data and limited zircon data suggest that central Africa may have had a limited middle to early Neoproterozoic tectonic history, similar to that recently documented for the BP of NE Brazil, but more detailed studies will be needed to explore this possibility.