Petrography and geochemistry of the Sanaga river sediments, central Cameroon: Constraints on weathering, provenance, and tectonic setting

The Sanaga River (SR), central Cameroon, flows predominantly over metasediments (2.1 Ga), Paleoproterozoic amphibolites and orthogneisses, granitoids from the Adamawa-Yade batholith (AYB), sedimentary clastic rocks of Cretaceous age, and Cenozoic rocks of the Cameroon Volcanic Line. To infer the origin of alluviums and tectonic setting of source rocks, the texture, heavy mineral, major, trace, and rare earth element concentrations of stream sediments were investigated. The very high SiO2/Al2O3 ratios and the ZTR (Zircon-Tourmaline-Rutile) index of SR sediments suggested a high maturity. This interpretation was additionally confirmed by the Th/Sc vs Zr/Sc plot. The Plagioclase- and Chemical Index of Alteration (PIA and CIA) values, the (A-K)-C-N and A-CN-K triangular plots show an intense degree of weathering in the source area. Numerous discrimination diagrams, based on trace elements, indicate that sediments were derived from felsic source rocks mainly granodiorites, granites, and biotite granitoids exposed in the SR watershed. This shows that the metamorphic and volcanic rocks do not significantly contribute to the stream sediments. Paleotectonic investigations using multi-element discrimination plots indicate passive and rift settings for the SR drainage basin. Rift settings were also inferred from downstream sediments of the SR. They are probably related to Proterozoic events including opening of large oceanic basins. The passive and rifted margin settings are supported by the occurrence of rare ophiolitic rocks, which indicate an old oceanic crust.