Petrology and geochemistry of the metamorphic rocks from the Bikélélé area, the Chaillu Massif, southwestern Republic of the Congo

Abstract

This study presents the petrographical and geochemical data of the metamorphic rocks from the Bikélélé area in order to constrain their petrogenetic evolution, provenance source and tectonic setting. These rocks include biotite gneisses, amphibolites, hornblende gneisses, clinopyroxene amphibolites, epidote amphibolites, actinolite-chlorite schists, quartzites and mica schists. The biotite gneisses exhibit analogous geochemical features with sanukitoids, as well as negative Eu anomalies (Eu/Eu*)_N = 0.4–0.61 in their REE chondrite-normalised patterns. The metavolcanic rocks have high SiO₂, Al₂O₃, Fe₂O₃, MgO, CaO, Mg#, low to high fractionation (La/Yb)_N = 0.89–39.45 and negative to no Eu anomalies (0.43–1.07). Both the biotite gneisses and metavolcanic rocks show depletion in Nb, Ta and Ti in their primitive mantle-normalised diagrams. The metasedimentary rocks exhibit high SiO₂, Al₂O₃, Fe₂O₃ and enriched light rare earth element (LREE) with depleted heavy rare earth element (HREE) patterns. Based on the geochemical characteristics, we suggest that the melts of the biotite gneiss protoliths formed by the partial melting of a high-K rich mantle that was metasomatised by melts derived from subduction-related terrigenous sediments associated with oceanic slab. The metavolcanic rocks likely formed at shallow depths, by 2.5–4% partial melting of a subcontinental lithospheric mantle source that was enriched through metasomatism involving sediments, melts and subduction-related fluids. However, the metasedimentary rocks originated from the Archean granitoids of the Chaillu Massif that underwent an intermediate tropical weathering. Based on the literature and data of this study, a subduction-arc environment associated with a back-arc extensional basin was suggested as the potential tectonic setting of these rocks.