GEOLOGY AND STRUCTURE OF THE NORTHERN PART OF IGARRA SCHIST BELT, SOUTHWESTERN NIGERIA

Abstract

Northern part of Igarra Schist Belt typifies the Basement Complex rocks of southwestern Nigeria, a part of a larger West African shield. It constitutes the northern closure of the Igarra Schist Belt. Geological mapping, structural analysis and geochemistry were combined for this study. The geological aspect involved: (i) the use of remotely-acquired data to preliminarily interpret geology and structure; (ii) geological mapping on a scale of 1:50000; and (iii) petrographical study of the rocks. The geochemical study comprised systematic geochemical sampling of rocks, major and trace element analyses of 45 rock samples using ICP-MS/OES methods and data interpretation using maps and charts. Integrating different methods facilitates the understanding of structural fabric, litho-petrotectonic history and possible establishment of various parameters for potential mineralization for which the southern part of this Schist Belt is known. Field and petrographic studies revealed migmatitic gneiss, biotite schist, quartz schist, granite gneiss, charnockite, calcsilicate/ marble as the rock types in the study area. Migmatites are essentially metatexites with quartz, plagioclase feldspar, microcline, muscovite and biotite as the essential minerals, while metallic ore minerals include pyrite, chalcopyrite and magnetite. Biotite schist is highly migmatized and granitized which results in the rarity of fresh outcrop; it exhibits both mineralogical and lithological banding attributable to a combination of sedimentary fabric S0 and a subsequent metamorphic fabric S1. Calc-silicate gneiss is characterized by clear, coarse banding which shows alternating white, dark green and black layers; it is associated with dolomitic marble. Absence of metaconglomerate may be attributable to two possibilities of either a plunge of folded metasedimentary units which results in increase in the thickness of overburden or a thinning-out, characteristic of sedimentary unit. Quartz-schist form

prominent ridges, resulting in impressive topographic expression, consisting of schistose (flaggy and fissile) and massive varieties. Field structural data interpretation confirmed NW-SE and NE-SW as the dominant planes of foliation which corresponds to two opposite limbs of fold. The NW-SE plane is associated with fault which is believed to be a late brittle stage of this deformational episode. Further interpretation of structures revealed that a synformal fold characterizes the northern part of the Igarra schist belt. Three episodes of deformation were recognized in the study area. The first is attributed to an episode of ductile deformation preserved in the relics of early metapellitic units. The second is associated with that which produced plane of foliation of the main phase gneiss while the third is interpreted as that which modified the earlier two. These phases of deformation correspond to those denoted by various workers as D1, D2 and D3 in parts of the Basement Complex rocks. Whole-rock concentrations of Cr, Ni, Th, Sc, and Zr in the studied rocks are consistent with derivation from post-Archean, continental sources that have undergone sedimentary maturation and recycling typical of detritus deposited in margin environments and affected by a weak to moderate weathering history. The characteristics of the major and trace elements analyses suggest felsic provenance, continental tectonic setting as well as wackes and arkosic source materials.