Abstract

UDUMA IBE, Chinedu. Geochemical characterization of the gneisses and schists in Ekumtak area: further evidence for a metasedimentary protolith and moderate weathering intensity for the Precambrian Basement complex of Nigeria. Ser. correl. geol. [online]. 2019, vol.35, n.2, pp.1-10. ISSN 1666-9479.

The study area is characterized by migmatitic banded gneisses and garnet mica schists. These rocks are associated with amphibolite and granitic intrusions. These migmatitic gneisses and schists are characterized by paragenetic mineral assemblages that reflect an evolutionary history that involved processes of sedimentation and upper amphibolite facies metamorphism. Geochemical data show that the rocks are silica rich with moderate to elevated concentrations of Al₂O₃ which is consistent with pelitic rocks. The gneisses show positive anomalies in Rb, K, Th, La, Sm and strong negative anomalies in Ta, Nb, Sr, Ti, Tm, Yb, Zr and Y with an overall enrichment of the large ion lithophile elements (LILE: K, Ba and Rb) and depletion of the high field strength elements (HFSE: Zr, Ti and Y). Specifically, the Eu-anomalies, expressed as (Eu/Eu*) range from 0.03 to 1.4 in the gneisses and from 0.09 to 1.1 in the schists. LaN/YbN ratios are from 7 to 93 for the gneisses and 1.9 to 46.4 for the schist. These values suggest that the protoliths of these metasedimentary rocks are predominantly shales, greywackes and arkosic sandstones with surbodinate basic to intermedíate volcanic components. Their chemical index of alteration values ranges from 57.7-71.7 in the gneisses and 66.6-74.9 in the schists with mean of 68% and standard deviation of ±3.4 suggesting a recycle process and relatively moderate chemical weathering of the protoliths. The metasedimentary origin for the rocks is consistent with results from other parts of the Nigerian Precambrian Basement complex.

Keywords : Gneisses, Metamorphism, Protolith, Precambrian, Metasedimentary, Schists.

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