Geochemical constraints on the origin of some intrusive igneous rocks from the Lower Benue rift, Southeastern Nigeria

Abstract

Major elements oxides, trace elements and REE data on some dolerites and dioritic rocks from the Lower Benue rift indicate that they are mainly basic in composition and belong to the alkali basalt magma series. They are characterized by enrichments in incompatible elements and high fractionation indices, (La/Yb) N, 5.7–14.72 which reflect LREE enrichment and indicate the presence of garnet in the source of the rocks. The ratios (Tb/Yb) N (1.63–2.6)and Dy/Yb (2.37–3.34) also indicate garnet-bearing lherzolite sources. The HFSE ratios Zr/Nb (0.88–2.6) and Y/Nb (0.57–1.57) suggest that the rocks were derived from enriched source(s) with dominant HIMU signature. The rocks were most likely formed in a within-plate setting of the intracontinental rift-type, similar to the Kenyan rift. Crustal contamination and interactions of the rocks with aqueous fluids are suggested by depletions in the more mobile LILE (Ba, Rb, K and Sr). The major element (K), some of the trace elements (Th, Co) and the REE (La, Ce) were heavily mobilized in some of the rocks (highly altered dolerites and dioritic rocks) during a low-grade metamorphic alteration. This is thought to have been caused by an intense hydrothermal activity involving a CO 2-rich hydrous phase contained in the crustal materials. This hydrous phase is possibly related to the saline groundwater common in the area with high concentrations of Ca 2+, Na +, Cl −, Sr +, Ba + and K +, thought to have been pressed out from the calcareous marine sediments during their burial.