Sm-Nd and Rb-Sr Datings, Petrogenesis and Thermometry of the Ngovayang Area (South-West Cameroon): Isotopic Data Insight of Recycling Crust and Convergence Orogen

Abstract

The Ngovayang trondhjemitic gneisses belong to the Nyong Complex at the Northwest boundary of the Ntem Complex. A combined study of whole-rock major, trace elements and isotopic data, Sm-Nd and Rb-Sr indicate that trondhjemitic gneisses are peraluminous and range from weakly I-type to S-type with nearly constant A/CNK values between 1.04 and 1.24. Their I-and S-type characters suggest that juvenile materials have been partially remobilized or recycled. These trondhjemitic gneisses have slightly to moderate MgO, Cr and Ni contents, ISr (0.703677 -0.741911) and low εNd(t) (from -16.48 to -10.6) values. Such geochemical features suggest a small mantle-source contribution, coupled with assimilation of some upper and lower crustal materials and indicate the implication of old crust probably with both Archean and Early eburnean origin (2597 -2318 Ma). They also exhibit geochemical features typical of calc-alkaline, crustal contaminated calcalkaline rocks, volcanic arc and trondhjemitic nature. These results show that old crust has recycled and trondhjemitic gneisses are linked to convergent geodynamic system. The petrogenetic diagrams of AFM vs CFM, MgO vs SiO 2 and Rb/Ba vs Rb/Sr and lower K 2 O/Na 2 O ratio values (0.38-0.51), indicate that the magmatic source materials are mainly composed of plagioclase-rich sources such as metagreywackes -metabasic mixtures, metagreywackes and metabasics and basalt or igneous rocks. These sources materials occurring in the melt originated from both slab melting and assimilation of upper and lower crust at ca. 732.4 -928.7°C and at 2597 -2318 Ma. This study shows that Archean crust may have a North-West extension beyond the actual boundary with Nyong Complex.