Geology and Geochemistry of Zungeru Amphibolites, North Central Nigeria

Abstract

Basement rocks of Zungeru area consist of gneisses, schists, migmatites and intercalations of amphibolites and quartzites .This work involves detailed geological and geochemical investigation of Zungeru amphibolites so as to reveal their possible protolith. Field investigations show two petrographic types of the amphibolites on the basis of their textures, structures and mineralogy. They include the banded and massive types. The banded type is medium grained, strongly foliated and dark coloured rock while the massive type is fine to medium grained, dark-grey to black coloured with very weak foliations. Both types have been fractured due to deformation resulting in the development of joints and faults. Twelve representative samples of the amphibolites in the study area were collected and analyzed for major and trace elements using the Energy Dispersive X-Ray Fluorescence (EDX-RF) Spectrometer, Model: Minipal 4 at the laboratory of the National Geological Agency, Kaduna. However, only the major elements data was used for determination of the protolith of the amphibolites. The analytical results show that the Zungeru amphibolites contain on average 50.15% SiO2,12% Al2O3, 0.39% Na2O, 0.02% K2O, 9.41% CaO, 9.90% MgO, 0.02% MnO, 12.24% Fe2O3, 4.89% FeO, 0.5% TiO2, 0.10% P2O5 and 0.78% H2O. Various bivariate plots indicate that the precursors of the amphibolites are sedimentary. They are inferred to have been formed from metamorphism of decarbonated mixtures of pelitic sediments and calcite or dolomite. The occurrence of the amphibolites in association with quartzites further strengthened the fact that their origin is sedimentary and therefore referred to as para-amphibolites. African (600 ma) granitic series commonly called older granite suites. The most pervasive tectonic fabric of igneous reactivation of this suite is attributed to Pan-African event. This Precambrian crystalline basement complex covers nearly 60% of land surface of Nigeria and extends to neighboring countries. The migmatite-gneiss complex makes up about 60% of surface area of Nigeria basement. These rocks recorded at least four major tectonic cycles of deformation, metamorphism and remobilization in the Liberian, Eburnean, Kibarian and Pan-Africa, thus, confirming Russ's view of age of determination from various part of the Nigeria basement. The migmatite-gneiss complex rocks are believed to be basement rocks in real sense.