Geochemical characteristics of the Jos-Plateau Basalts, North-Central Nigeria

Abstract

The Jos Plateau basalts are subdivided into two subtypes based on their textures and mode of occurrence viz, Newer and Older Basalts. The Newer Basalts occur as cones and lava flows and are mainly built of basaltic scoria and pyroclastics. The Older Basalts are characterized by small eroded, partly decomposed remnants, and with a number of plugs or dome-like outcrops. These two sub-types, however, present the same mineralogical composition (phenocrysts of both olivine, plagioclase (bytwonite-labradorite) and rarely pyroxene (diopside-augite) set in a groundmass of labradorite laths, magnetite, ilmenite, biotite, minor k-feldpars, nepheline and volcanic glass.
Geochemical data show that the basalts are nepheline normative and have relatively low SiO₂ contents (39.8-46.49 wt%). These compositional characteristics are typical of alkaline olivine basalts and are consistent with their mineralogy (biotite, k-feldspar and nepheline), their deficiency in silica (understaturated) and their enrichment in incompatible elements (V, Y, Ti, Zr, Sr, Nb, to Th). This alkaline nature is further attested to by the positive Nb anomalies exhibited by these basalts. The narrow difference in the ratios AlSub>2O₂/TiO₂ (4.44-6.13); CaO/TiO₂ (3.04-4.30); Al₂O₃/CaO (1.07-1.61) and the very similar chondrite normalized distribution exhibited by these basalts are in support of their formation by partial melting process. The Jos Plateau basalts, present Zr/Nb ratios (2.4-3.0) comparable to those of the alkali basalts of the lower Benue valley, and of the Cameroon volcanic line, suggesting that they were possibly derived from the same mantle source.

Keywords: Jos Plateau, alkali basalt, mantle, partial melting, incompatible elements.

Global Journal of Geological Sciences Vol. 3(2) 2005: 187-193