Geochemistry of the metavolcanic rocks from the Archean Bikélélé greenstone belt (Chaillu Massif, Republic of the Congo): Petrogenesis and tectonic implications

Abstract

The Bikélélé greenstone belt belongs to the Chaillu Massif, which corresponds to the northwestern part of the Congo Craton outcropping in the southwestern Republic of the Congo. It mainly consists of metavolcanic lithological sequences composed of amphibolites, clinopyroxene amphibolites, epidote amphibolites, amphibole gneisses and amphibole-chlorite schists. These rocks show granoblastic to grano-nematoblastic textures, and contain amphibole, plagioclase, alkali feldspar, quartz, chlorite, clinopyroxene, biotite, sericite, sphene and opaque minerals. They have high contents in SiO₂, Fe₂O₃, Al₂O₃, CaO and MgO, with subordinate Na₂O and K₂O. In the REE chondrite-normalised diagrams, the metavolcanic rocks show flat to LREE-enriched and flat HREE patterns. However, in the primitive mantle-normalised diagrams, most of the samples show depletion in Nb, Ta and Ti. Trace and rare earth element (REE) data suggest that their protoliths correspond to basalts and basaltic andesites of tholeiitic and calc-alkaline affinity. The metavolcanic rocks likely formed by 2.5–4 % partial melting of a subcontinental lithospheric mantle source that was enriched through metasomatism involving sediment-derived melts and subduction-related fluids. The results of this study suggest that the protoliths of the Bikélélé metavolcanic rocks were emplaced in a subduction-related back-arc environment, during the second magmatic episode recorded in the Chaillu Massif and underwent metamorphic grade up to amphibolite facies during the Neoarchean tectono-metamorphic event.