Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria

Authors: ANTHONY CHUKWU, SMART CHIKA OBIORA

Abstract: Petrographic studies on the intrusive rocks from the Ishiagu area show that they are gabbro, dolerites, and diorite. The gabbro and diorite occur as stocks while the dolerites occur as sills in folded sedimentary sequence predominated by shale. Geochemical data confirmed the rocks as basic (gabbro and dolerites) and intermediate (diorite) rocks with silica contents ranging from 42.45 to 50.75 wt.% and 55.03 to 60.24 wt.%, respectively. They are predominantly alkaline (gabbro, dolerites, and diorites) with sparse tholeiites (few gabbro). The basic rocks are made up of calcic plagioclase (An_{49-54}), clinopyroxene, olivine (altered), nepheline, and opaques, while the intermediate rocks consist of plagioclase (An_{41-49}), hornblende, and clinopyroxene. Calcite occurs as a secondary mineral in both groups. Although many samples are moderately to heavily altered (loss on ignition: 2.12–10.46 wt.%), a consistent decrease in magnesium number (Mg#) (gabbro, 0.47; dolerites, 0.37; dolerites, 0.27) and immobile elements (Y, Zr, Hf, Ti, Nb, and Ta), which are more enriched in the intermediate than the basic rocks and similar patterns of spidergrams and rare earth elements (REEs), show that the rocks (gabbro, dolerites, and diorites) are genetically related. The rocks show stronger enrichment in incompatible elements and REEs [(La/Yb)_n, 4.83–8.15 for the gabbro, 7.01–7.79 for the dolerites, and 8.85–9.63 for the diorites]. Primitive-mantle normalized trace-element patterns and trace-element ratios of both basic and intermediate rocks indicate similarity to ocean island basalts, which suggests a garnet-lherzolite mantle source. The ratios of La/Nb, Th/La, and Th/Nb and the high ratio of Nb/La (1.18–1.50 for gabbro, 1.37–1.95 for dolerites, and 1.21–1.32 for diorites) indicate a high ^{238}U/^{204}Pb (HIMU) mantle source region. There is no evidence of significant crustal contamination on the basic and intermediate rocks, which may be related to the rapid ascent of the magmas in an intracontinental rift setting similar to the East African rift.

Keywords: Intrusive rocks, alkaline affinity, HIMU mantle source, intracontinental rift setting, Ishiagu area, Lower Benue rift, Nigeria

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