Petrography and Geochemical Classification of the Saigbe Granites, North Central Nigeria

Abstract

This work involves detailed field, petrographic and geochemical studies of Saigbe granites to bring forth their possible petrogenesis. Geological field mapping reveals the basement rocks of the Saigbe area consist of schist, amphibolite, and granite with minor intrusion of pegmatitic veins. Structural features were also discerned with principal joint direction as plotted on rose diagrams showing NWW – SEE for the granites. A total of ten granitic rock samples were collected and described based on their field relationships. Three fresh representative rock samples were subjected to petrographic analysis while five others were later subjected to geochemical analysis using the X-ray Fluorescence (XRF) technique. Petrography of the investigated rock samples showed an average mineralogical composition of plagioclase feldspar, quartz, biotite, microcline, muscovite and a trace of opaque minerals for the granites. The result of the geochemical analysis revealed the average percentage composition of major oxides as follows; SiO2 (71.30 wt.%), Al2O3 (11.13 wt.%), Fe2O3 (3.37 wt.%), CaO (4.44 wt.%), K2O (2.18 wt.%), Na2O (1.58 wt.%), MgO (2.09 wt.%), P2O5 (0.003 wt.%), TiO2 (1.04 wt.%) and MnO (0.62 wt.%). The geochemical classification of the granitic rocks revealed that the rocks fall within the Calc-alkaline/Tholeiitic series (AFM Plot), which supports a mixed and evolved magma origin during the evolution of the granitic rock. They were also classified into both I-Type and S-Type granite based on the Al₂O3/(Na₂O+K₂O+CaO) versus SiO2 plot. Ternary plot of the granitic samples sho.ws continent.al setting for the granitic rocks in the area. Therefore, the granitic rocks in the Saigbe area can then be said to have been formed from the fractionation of magma within the continental setting as the granitic rocks range in composition from Diorite and Granodiorite to Granite.