Chemical character of the Ngovayang gneisses in the northwestern border of the Congo craton (south Cameroon): sources and geotectonic framework

This article examines the results of chemical data of the granitoids of the Ngovayang massif at the Northwestern border of the Congo craton in South Cameroon. The aim is to identify the sources and geotectonic framework of emplacement of these formations. It is a contribution to the understanding of the geodynamic context of the Ngovayang massif in the Nyong unit. 24 samples were analyzed by XRF and ICP/MS and are constituted of orthogneiss, charnockitic gneiss, migmatites and metagranites. This study brings out the progressive variation of the nature of the gneissic basement of this massif. Orthogneisses are the principal rocks and are characterized by an enrichment in Na as opposed to an impoverishment in K. This confirms their enrichment in plagioclase and their impoverishment in alkaline feldspar, K/Na ratios < 0.5. Trace element contents are also characteristics, marked by an enrichment in light rare earths (Laavg=10.48ppm) and impoverishment in heavy rare earths (Ybavg=0.63ppm). Their spectra are much fractionated ((La/Yb)N=15.86) without any significant anomaly in Eu but a negative anomaly in Ti and Nb. These results confirm its belonging to granitoids of the TTG type of trondhjemitic affinity. The migmatites have a granodioritic composition and are calco-alkaline. They have low Y content and are comparable with the CA2 type of pluton. They originate from the melting of a source containing garnet in melting residues, similar to those generating TTGs. The migmatitic gneisses seem to be the product of sediments. Charnockitic gneisses have two different trends; the firsts are of granodioritic composition and the procedure of their emplacement is that of differentiation similar to that of TTGs. The seconds have a tonalitic composition and formed by the partial melting of a source containing no garnet and eventually no hornblende. No matter the domain, the formations of the sector of study present in a variable manner, an impoverishment in LILE (Rb, Th and sometimes K). These results show that the metamorphites of the Ngovayang massif are the products of archaean granitoids deformed during the collision of the Congo and Sao Francisco cratonic blocks during the Eburnean orogeny.