Neoproterozoic syn-collision magmatism in the Nkondjock region at the northern border of the Congo craton in Cameroon: Geodynamic implications for the Central African orogenic belt

Abstract

The Nkondjock area located in Western Cameroon is part of the Neoproterozoic Central African orogenic belt, at the northern border of the Congo craton. Rocks of the Nkondjock area are: two-mica orthogneisses, two-pyroxene orthogneisses and mylonitic orthogneisses. The protolith of two-mica, two-pyroxene and mylonitic orthogneisses crystallized at 607 ± 4, 624.4 ± 3.5 and 621 ± 9.1 Ma, respectively, and are thus consistent with the Central African Orogenic belt. Inherited zircons (2043 ± 27 Ma) of mylonitic orthogneisses are interpreted as an inherited age of the metacratonization of the northern border of the Congo craton. S-type affinity of two-mica orthogneisses together with two-pyroxene orthogneisses (Mabombé) display geochemical affinities typical of adakite, including high La/YbN (29.7–55.6) and low Y (3.43–6.69 ppm) with low Mg# (30.8–55), εNd(t) (−8.6 to −6.5), high 87Sr/86Sri (0.70684–0.71063) and positive εHf(t) (+3 to +4.9), similar to those adakites formed by partial melting of thickened lower crust with some mantle input. Two-pyroxene orthogneisses (Ndogboni) and mylonitic orthogneisses display I-type granitoids affinity with A/CNK (0.93–1.06) and are depleted in Nb, Ta, Sr, P, Ti and enriched in Rb, Th, and Pb. They also have low εNd(t) (−10.3 to −11), high 87Sr/86Sri (0.71017–0.71063) and negative εHf(t) (−19.1 to −1) similar to melts derived from mature continental crust. Sr-Nd-Hf isotopes confirm heterogeneous source for these orthogneisses with bimodal distribution on εHf(t) and εNd(t) values, suggesting mixing between juvenile mafic melt and a mature continental crust during the evolution of the Central African Orogenic belt. We suggest that the protolith of these orthogneisses (S- and I-type granitoids) were formed during syn-collisional setting between the Congo and the West African craton which followed a subduction through partial melting of a thickened lower continental crust due to a thermal anomaly induced by upwelling of asthenosphere through slab break-off.