Geodynamics of Precambrian Rocks of Southwestern Nigeria

Abstract

The geodynamics of the Southwestern Nigeria Precambrian Basement Rocks were studied with aim of understanding the evolution of rocks globally. Magnetic carriers of Precambrian Basement rocks samples collected from 110 locations were prepared for rock magnetism, optical microscopy and Scanning Electron Microscopy (SEM). The Natural Remanent Magnetisation (NRM) of the remagnetised and unmagnetised rocks are strong (0.3–1.7 A/m -< 0.5 A/m) showed northwesterly direction with moderate inclination and weak NRM with westerly shallow direction respectively. Primary and secondary NRMs are carried by maghemite, and the remagnetised and unmagnetised rocks revealed a higher coercivity for alternating field demagnetisation (<20 mT – < 10 mT median destructive field). Optical microscopy revealed maghemite, poor titanomagnetite, titanomaghemite lamellae >30 pm and finer maghemite/magnetite grains finer than 10 pm. X-ray Diffratometry (XRD) and SEM results implied NW remanence in the remagnetised rocks reside in the fine poor-maghemite during the alteration of hornblende to actinolite while the coarse-grained maghemite in both rocks carries the W remanence of a thermoremanent magnetisation acquired in the Pan – African times. Global cold collision geodynamics resulted in the generation of ultra-high pressure metamorphic complexes and remagnetisation and True Polar Wander drifts of the paleomagnetic pole move towards the equator.