U-Pb geochronology of metasedimentary schists in Akwanga area of north central Nigeria and its implications for the evolution of the Nigerian basement complex
The geology of Akwnaga area of northcentral Nigeria is dominated by schists, metaigneous gneisses and weakly to nonfoliated granites. The Schist shows grades up to the sillimanite zone of the Barrovian type regional metamorphism and occurs as xenoliths in other rocks of the area. The zircons separated from a sample of the Schists show growth zoning and a number of grains display distinct cores and rims. Out of 131 analyses 117 yielded useful geochronological data. 67 of these gave 90-110% concordant results. Most ages scatter between 600 and 1100Ma with distinct peaks at 700, 850 and 1000Ma. A few Archaean Zircons are also present but a minority of grains yielded Palaeoproterozoic ages (1700-2200Ma). These first-ever geochronological data from rocks of Akwanga area are further evidence of the polycyclic nature of the Nigerian basement. They confirm that the Pan-African orogeny, though pervasive, did not obliterate traces of earlier events in the Akwanga area.
KEYWORDS: Akwanga, Zircon, Archaean, Palaeoproterozoic, Polycyclic.