The Adamawa-Yadé domain, a piece of Archaean crust in the Neoproterozoic Central African Orogenic belt (Bafia area, Cameroon)

Abstract

New field observations, ages and geochemical data are presented for the Adamawa-Yadé domain, forming part of Central African Orogenic Belt in Cameroon. This belt delineates the northern margin of the Congo craton and is part of the Braziliano/Pan-African orogenic system, which was formed during amalgamation of the West Gondwana continent at the end of the Neoproterozoic.U–Pb dating indicates that the protoliths of some orthogneisses from the Adamawa-Yadé domain were emplaced during three distinct periods. Tonalite-Trondhjemite-Granodiorite (TTG) suites in the Makénéné area intruded at ca. 3.0–2.5Ga, and were affected by partial melting at ca. 2.08–2.07Ga, during the Eburnean orogeny, and by a magmatic event with a metamorphic overprint at 0.64–0.61Ga, during the Pan-African orogeny. A dioritic orthogneiss from the Maham area, north of Makénéné, was emplaced at 2.07Ga and also reworked at 0.62–0.61Ga. The youngest granitic orthogneisses, sampled in the vicinity of Bafia, Bep and Ngaa-Bapé, yield intrusion ages of 0.64–0.63Ga. Some of these Neoproterozoic plutonic rocks show shoshonitic affinity and reveal εNd(630Ma) values ranging from −1.2 to −6.9 (1.7>TDM>1.1Ga), and 87Sr/86Sr initial ratios of 0.706 and 0.704, providing evidence for crustal reworking during the Pan-African collision. The isotopic geochemical characteristics provide evidence that potassic magmas result from the interaction between the Archaean TTG continental crust and magmas derived from a mantle enriched in silica and incompatible elements during subduction underneath a continental margin. The Bafia group appears as a complex geological entity comprised of an Archaean/Palaeoproterozoic basement and its Neoproterozoic sedimentary cover, both deformed and recrystallized at ca. 0.61Ga.The crustal evolution of the Adamawa-Yadé domain is very similar to that inferred for the northern margin of the Congo craton (the Ntem complex and Nyong series). This and the sedimentary record suggest that the Adamawa-Yadé domain represents an Archaean/Palaeoproterozoic microcontinent, which was detached from the northern margin of the Congo craton in the early Neoproterozoic, but became re-accreted together with the Mayo Kebbi (magmatic) arc during the Pan-African orogeny.