Petrology, Geochemistry and Structural Attributes of the Irruan Basement Rocks, Southern Obudu Plateau, Southeastern Nigeria

Abstract

Very little research has been carried out and reported on the Irruan granitoids. Geological field mapping and laboratory analyses were carried out to ascertain the petrological, geochemical and structural attributes of the Irruan Basement Rocks in southern Obudu Plateau, southeastern Nigeria. Five petrological units: granite gneiss, banded gneiss, migmatite gneiss, granodiorite and biotite granite were identified in the area. The rocks are generally siliceous and quartzofeldspathic, recording ≥70.11 wt.% of SiO2 except the biotite granite that records 61.10 wt.% of SiO2; quartz ranges from 25 – 35%, 30%, 20 – 27%, 22 – 35% and 30% for granite gneiss, banded gneiss, migmatite gneiss, granodiorite and biotite granite respectively. Feldspar ranges from 20 – 35%, 22 – 25%, 20 – 27%, 20 – 30% and 25% respectively for the granite gneiss, banded gneiss, migmatite gneiss, granodiorite and biotite granite. Relative to plagioclase, orthoclase is more dominant in many of the samples. Biotite and muscovite range from 5 – 15%. The metamorphosed rocks are predominantly peraluminous, showing aluminum saturation index (ASI) ranging from 1.05 to 3.37 and (Na2O + CaO)/Al2O3 ratios of 0.27 – 0.95 and average A/CNK of 1.13. Generally, there are marked variations in the distribution of trace elements with Rb/Nb, Rb/Zr and Rb/Sr ratios of ≤25,000, ≤0.5319 and ≤0.7377 respectively. The granitoids have resulted from the partial melting of the crust with possible contribution from mantle-derived, crust-contaminated magma.  On the outcrop scale, fractures occur in NW-SE, NE-SW, NNE-SSW, E-W and N-S orientations. Evidences of primary and secondary structures were observed. The dominant NE-SW fractures were developed contemporaneously with the foliations and lineaments. The N-S and fairly weak E-W fracture sets were developed from a later episode.