Petrography and geochemistry of the granodiorite and dolerite of the Bingo complex, eastern DRC

Abstract

We have investigated granodiorite and dolerite dykes around the Bingo carbonatite cropping out in the western branch of the East African Rift System (EARS), eastern Democratic Republic of Congo (DRC). These dominantly N-S fracture-controlled dykes are emplaced in an older orthogneiss basement surrounding a Neoproterozoic carbonatite-bearing alkaline intrusive complex. Petrographic and geochemical studies of the granodiorite and dolerite intrusions were carried out to constrain the petrogenesis and tectonic setting of the rocks. Granodiorite is medium to coarse-grained, contain plagioclase of oligoclase to andesine composition, quartz, K-feldspar, hornblende, biotite and accessory magnetite, titanite, ilmenite, and zircon. Dolerite exhibits an ophitic texture with lath-shaped, randomly oriented plagioclase of andesine to labradorite composition embedded in large orthopyroxenes and clinopyroxenes. Accessory minerals include serpentine, altered olivine, amphiboles, magnetite, and ilmenite. The granodiorite is a metaluminous to weakly peraluminous, calc-alkaline. It is enriched in LREE (La_N/Yb_N = 10.91–22.28) and depleted in HREE with insignificant Eu (0.9–0.95) and negative anomalies in some high field strength elements (HFSEs) notably Ta, Nb, and Ti, which suggest melts generated in volcanic arc tectonic setting. The LREE (La_N/Yb_N = 1.97–2.92) for dolerite and the Eu anomaly ((Eu/Eu*)_N 0.85–0.92) suggest a mantle-derived MORB melt probably contaminated by crustal materials during ascent. Emplacement of the dykes was aided by crustal extension and associated fracturing to develop dyke swarms of basic, calc-alkaline, and alkaline rocks.