Cambrian mafic magmatism in the Kékem area, NW edge of the Adamawa-Yadé domain, Central African Fold Belt: Implications for Western Gondwana dynamics

The tectonic evolution of the Central African Fold Belt (CAFB) has been intensely studied, but the timing of the end of the Pan-African orogenic cycle is poorly constrained, leading to contrasting tectonic models. In order to address the nature of magmatic products associated with and the timing of the end Pan-African orogenic cycle, we use mineral chemistry, whole-rock geochemistry and Sr − Nd isotopes, and zircon trace element and U − Pb geochronology of northern and southern gabbro that cut the post-collisional Central Cameroon Shear Zone; results are discussed in the tectonic framework of the CAFB. Clinopyroxene and whole-rock chemistry show that the northern and southern gabbros are mainly subalkaline, with a tholeiitic/transitional affinity. Northern and southern gabbros have REE and trace element patterns, characterized by enrichment in LREE, and depletion in HREE, Th, U, Nb, Ta, and Pb. Their Sr − Nd isotope data show low (⁸⁷Sr/⁸⁶Sr)i of 0.703793–0.704608 and slightly negative ɛNd(t) values ranging from −3.20 to −0.66 with Paleoproterozoic Nd model ages of 2149–1612 Ma. Moreover, the northern and southern gabbros yield U − Pb zircon ages of 486 ± 15 Ma and 497 ± 13 Ma, respectively. Our combined data is consistent with the northern and southern gabbroic magma having originated at (or near) a deep, probably asthenospheric mantle source and may have interacted with Paleoproterozoic subduction metasomatized lithospheric mantle during ascent as a result of lithospheric extension associated with lithospheric scale shearing along the shear zone. In concert with previous studies, we suggest that the evolution of the CAFB was characterized by subduction and eventual collision at 660–600 Ma followed by a 600–570 Ma post-collisional stage and final extension from 560 − 486 Ma. The extensional regime was associated with mafic to felsic magmatism with geochemical signatures evolving towards a tholeiitic/transitional or alkaline affinity. Correlation of the CAFB with the Borborema Province of the Brasiliano fold belt suggests that both shared a similar tectonic evolution and orogenic cycle characterized by early crustal thickening followed by transcurrent tectonics and ultimate lithospheric extension.