Petrology of the meta-mafic rocks from the Lolodorf area, Nyong complex (Southwest Cameroon): implication for the origin and emplacement conditions

Abstract

This study presents the new petrology and geochemistry data for the comprehension of the origin and emplacement conditions of the Lolodorf mafic rocks whitin the Nyong Complex. The meta-mafic rocks consist of fine-grained garnet meta-mafic (FGM) and coarse-grained garnet meta-mafic (CGM) rocks presenting nearly similar mineral composition. They are made up of quartz, plagioclase, garnet orthopyroxene, clinopyroxene, amphibole, opaques, apatite and rutile. They also present recrystallization features such as quartz ribbons, corroded grain mineral, cuspate plagioclase boundary and necklace garnets in the matrix. These mineral assemblages suggest that the recrystallization took place under granulite facies condition. The meta-mafic rocks have basaltic and tholeiitic composition like the magmas (SiO2 FGM = 48.20–50.70 wt% and SiO2CGM = 46.68–50.27 wt% and MgFGM = 43.24–62.27 and MgCGM = 40.85–69.70) emplaced in the active continental margin context. The low La/Th ratio observed in the FGM and the high La/Th ratio in the CGM together with low Hf content imply that the FGM are reworked plutonic rocks that recorded both high grade partial melting and metamorphism whereas CGM recorded only metamorphism imprints. Thermobarometry results suggest that the partial melting and metamorphic peak occurred around the amphibolite-granulite facies transition. The fine-grained garnet meta-mafic rocks emplaced under pressure estimated at around 2.7–14.89 kbar with temperature interval between 672 and 952 °C while the coarse-grained garnet meta-mafics settled at around 1.5–16.09 kbar and 631–909 °C. The selected amphiboles display low water content (1.92–2.10 wt%) and plot between NNO and NNO + 2 buffers indicating that they crystallized under high oxidizing conditions. These results coupled with previous geochronological study suggest that the FGM may have occurred at the Mesoarchean and have been reworked during Eburnean whereas the CGM may have been formed at the Eburnean.