Petrography and geochemistry of basic and intermediate plutonic suite of the Mayo Nielse and Mayo Balche areas, (Poli Group, Central Africa Fold Belt, Northern Cameroon): Post‐collisional mantle refertilization and magma genesis

Abstract

The study area is located in the Poli Group, Western Cameroon Domain of the Central African Fold Belt. It is constituted mainly of Pan‐African granitoids intruding Neoproterozoic volcano‐sedimentary rocks, schists and gneisses. The granitoids host dykes of gabbroic and dioritic rocks which are the focus of this study. Gabbroic and dioritic rocks are clinopyroxene‐hornblende gabbros, clinopyroxene‐hornblende melagabbro, clinopyroxene‐hornblende leucogabbro and quartz‐diorite. Geochemically, the rocks are characterized by: SiO2 (46.03–58.88 wt. %); Mg# (27.78–79.46); MgO (2.10–18.40 wt.%); total alkali (0.97–10.7 wt.%); Fe2O3 (8.42–13.19 wt.%); and CaO (4.90–16.57 wt. %), meanwhile all the rare earth elements are enriched with respect to chondrites. The rocks are mainly medium‐K calc‐alkaline with subordinate low‐K and alkaline samples. The ratios of La/Ta (4.0–76.0), La/Nb (0.8–9.88) and Zr/Ba (0.11–0.52) indicate that their source is both asthenospheric and lithospheric. This source has characteristics of NMORB to EMORB, overprinted by fluids‐enriched silicic and carbonatitic metasomes likely as a result of slab‐mantle interactions in a subduction environment. The weak to strong fractionation of La/Yb ratio (La/YbN = 0.9–13.51) and non‐fractionation of the Dy/Yb ratio (Dy/YbN = 0.6–1.5) together with the behaviour of the rocks in the La/Sm versus Sm/Yb modelization diagram are consistent with a polybaric melting which started in the asthenosphere at a small rate, yielding the limited volume of alkaline rocks. It then continued in the lithosphere where a more extensive degree of melting produced medium‐K calc‐alkaline rocks. The melting was likely induced by the adiabatic decompression of the metasomatized mantle during the post‐collisional relaxation closing the tectonic stage of the Pan‐African Orogeny in the Poli area. The magma generated was near‐primitive and evolved by assimilation‐fractional crystallization.