Geochemistry of the Precambrian Basement of the Bamenda massif of southeastern Nigeria: petrogenesis and tectonic setting

A trace and Rare Earth Element (REE) geochemical study of twenty samples of migmatitic banded gneisses, garnet biotite schists, dolerites, granites and rhyolites was carried out in a bid to determine their petrogenetic and tectonic significance in the evolution of the southeastern Basement complex of Nigeria. The data show that partial melting (crustal anatexis) of migmatitic gneisses and schists played a significant role in the evolution of the granitic intrusions. This is supported by a highly incompatible element ratio in the granitic intrusions (Rb/Sr = 0.16 to 1.31 and Ba/Sr = 0.75 to 6.21) compared with that of the migmatitic gneisses and schists (Rb/Sr = 0.051 to 0.824; Ba/Sr = 0.7 to 5). Higher ratios of Ba/Sr and Rb/Sr and lower values of the Ba/Rb ratio in some granitic intrusions suggest an increase in fractionation during anatexis. Partial melting also plays a role in the smooth REE patterns shown by most of these rocks and the negative Eu anomaly as indicated by the Eu/Eu* ratio (0.097 to 0.7). Light Rare Earth Element (LREE) enrichment is evident in the high ratio values of Ce/YbN (12.08-174.5), La/YbN (15.2-228.4) and La/SmN (2.6-7.2) in the granitic intrusions. Tectonic discrimination diagrams of the rocks indicate that basement rocks were most probably formed in a post-collision orogenic setting while dolerite and rhyolite developed in a within-plate anorogenic setting.