PALEOMAGNETIC CHARACTERISATION OF PRECAMBRIAN BASEMENT ROCKS AND THEIR ASSOCIATED OROGENIC EVENTS IN SOUTHWESTERN NIGERIA

Abstract

Paleomagnetic characteristics of iron-oxide in Precambrian Basement rocks documents orogenic events in those rocks and has been used to decipher the evolution of such rocks world-wide. Previous studies on paleomagnetism of rocks in Nigeria concentrated mainly on microstructures and Curie temperatures with little or no emphasis on continental drifts that are important for correlation of rocks globally, direction of movements and reconstruction of ancient continents. This work was aimed to determine the magnetic character, evolution, orientation of microstructures and paleomagnetic pole positions of Precambrian rocks associated with orogenic events in southwestern Nigeria. Precambrian Basement rocks samples collected from 110 locations in southwestern Nigeria were cylindrically cored into 25 mm by 22 mm specimen. Eighty-two of these rock samples were cut into fabrics and polished sections for the determination of magnetic mineralogy using Raman spectroscopy, scanning electron microscopy and Electron Probe Microanalyzer (EPMA). The Curie temperatures of the rock specimens were measured using multifunctional Kappabridge. Anisotropy of Magnetic Susceptibility (AMS) measurements were carried out to determine the microstructures in the cored samples. The cored rocks were subjected to paleomagnetic measurements using Alternating Field (AF) and thermal demagnetisers. The poles direction of the demagnetised cored samples was determined using cryogenic and JR5 spinner magnetometer. Day plots, First Order Reversal Curves (FORCs), backfield remanence and hysteresis loops of the rock samples were determined using Princeton vibrating sample magnetometer. The magnetic mineralogy of iron-oxide minerals in Precambrian Basement rocks (gneisses, schists and granites) revealed mostly maghemite, magnetite and titanomagnetite minerals with their Curie temperatures of 590-600°C, 575-585°C and 360-410°C, respectively. Iron oxide composition of maghemite which is the most predominant was 74.6%. Magnetic foliations were exhibited and striked dominantly in the NE-SW (32.9°-252.9°) direction with moderate to steep dip angles and lineation plunging 8° to 86° to the NE/ENE direction. The foliation poles defined a girdle pattern with a zone axis (52/11) close to the best line of the lineation (44/21). The microstructures of the rocks had suffered deformation from the magmatic state to the high temperature solid-state due to tectono metamorphic events. A positive inclination in the northwest direction, which corresponded to the paleomagnetic pole of Pan-African was identified. Isolated locations with paleomagnetic discrepancies were observed at >530℃, which indicated Pan-African regional remagnetisation of the granitoids in the Trans- Saharan province. These suggested high mantle activity, a true polar Wander drifts towards the equator and the amalgamation of the Rodinia supercontinent. The Day plots and FORCs showed Pseudo-single domain and multi-domain phases. The backfield remanence and hysteresis loops showed narrow-waisted loops which, generally indicated ferromagnetic to paramagnetic Precambrian Basement rocks. The ferromagnetic to paramagnetic maghemite, magnetite and titanomagnetite in the Precambrian Basement rocks of southwestern Nigeria bear evidences of tectono-metamorphic events in the Pan African. The resultant microstructural deformation and orientation, remagnetisation and True Polar Wander drifts of the paleomagnetic pole move towards the equator.