Geochemistry of Mafic-Intermediate Intrusive Rocks from the Twangiza-Namoya Gold Belt, Eastern Democratic Republic of Congo: Trace Element Constraints on their Origin, Petrogenesis and Tectonic Setting

Abstract

Bulk geochemical data for the Twangiza-Namoya Gold Belt, Democratic Republic of Congo, are presented in order to classify the rock assemblages, elucidate their petrogenesis and tectonic settings. Nb/Yb and Zr/Ti constraints reveal two suites for the rocks: sub-alkaline to andesitic basalts (Suite-1); and alkaline basalts (Suite-2).  Ratios of Ti/Zr = 22–70; Zr/Hf = 35–42; Nb/Ta = 12–13; Nb/Th = 1–2 and La/Nb = 2.4–3.8 suggest arc-generated mantle melts with crustal inputs and arc-fluids metasomatism for Suite-1. The samples have positive LILE and negative Nb, Ta, Ti anomalies, enrichments in the LREE over middle and HREE (La/Yb)_CN = 3.6–8.9 with negative Eu anomalies (0.7–0.9) in Chondrite-normalized diagrams. Yb-Th/Ta and Ta/Yb-Th/Yb ratios suggest active continental margin settings, whereas Ta/Hf-Th/Ta and Nb/Yb-Th/Yb diagrams suggest a depleted MORB source. Ti/Zr, Zr/Hf, Nb/Ta, Nb/Th and La/Nb constraints for Suite-2 suggest MORB and oceanic plateau basaltic sources. REE patterns show enrichments in the LREE with negative to positive Eu anomalies (0.7– 1.3). Yb-Th/Ta and Ta/Yb-Th/Yb ratios suggest a within plate tectonic setting. It is suggested that Suite-2 rocks were sourced from mantle-derived melts with a composition between an enriched MORB to OIB.

Keywords: Twangiza-Namoya; Petrogenesis; Tectonic setting; Trace element ratios