Geochemical Pattern and the Evolution of the Mafic Rocks of Southwest Obudu Plateau, Bamenda Massif, Nigeria

Abstract

The mafic and ultramafic rocks of southwest Obudu, Nigeria are located within the Paleoproterozoic granulites facies rocks of the Bamenda Massif. These mafic and ultramafic rocks were not subjected to high level deformation and metamorphism which the surrounding rocks experienced. Since there are no pervasive signatures of the Pan-African Orogeny, these rocks were probably emplaced as later stage intrusive into these Pan-African rocks. The unmetamorphosed dolerite and diorite occur as boulders and veins cross-cutting the charnockitic rocks of Ukwortung and the environs. The ultramafic rock massively outcrops in Ukwortung and in other localities as mostly dykes in the granulite facies rocks. The Mg# (magnesian number) for the mafic and the ultramafic rocks of southwest Obudu, southeastern Nigeria are high (Mg# ranges from 19.95 to 65.15) and the FeOT is equally high (FeOT is 9.87–13.20). The Mg number of the ultramafic rock on the average is higher than those for partial melts from the lower crust (basaltic magma, Mg# <40) lower than values for Qingbulake intrusion in China (59–81) and those for metaperidotite in ophiolites (89 – 91), suggesting that its derivation was not directly from the partial melting of the lower crust. Its formation may have involved derivation interaction with mantle peridotite. However, the Mg numbers for the dolerites and the diorites are less than 40, lower than those of partial melts from the lower crust. This precludes their formation from interaction peridotite. The LILE and LREE enrichment, considerable depletion in HSFE with low La/Nb ratios are indicative of subduction related activities. The Zr/Y (>4) and Nb/Y (>2) are suggestive of materials derived from enriched mantle source which may have been metasomatised by slab melts during subduction/collision. Variable Ce/Y ratios, the elevated Sr/Nd ratios at low Th/Yb ratios reflect the addition of fluids from a subducting slab. However the low Th/Yb ratios of the mafic and ultramafic rocks preclude importance of sediment melts from the subducting slab. Therefore, hydrous melting of the metasomatised mantle material may have introduced significant LILE into the source.