Abstract
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.
Highlights
McCurry (1976) recognized two classes of metamorphic rocks of sedimentary origin in the Nigerian Basement complex: the “Older Metasediments” was used to describe metasedimentary rocks deposited some 2,500Ma ago surviving in gneisses and migmatites
It was earlier thought that the Newer Metasediments occupy N-S trending belts, that they are isoclinally infolded into the older metasediments and that they are restricted to the western half of Nigeria
Zircons separated from the schist in Kokona area of northwest Akwanga are broadly subhedral, sometimes with preserved pyramidal faces
Summary
McCurry (1976) recognized two classes of metamorphic rocks of sedimentary origin in the Nigerian Basement complex: the “Older Metasediments” was used to describe metasedimentary rocks deposited some 2,500Ma ago surviving in gneisses and migmatites. Oyawoye(1964) and Rahaman (1976) referred to these rocks as older metasedimentary series or the gneissmigmatite-quartzite complex. “Younger metasedimantary belts” were considered to have been deposited some 1000-800Ma ago. This group of rocks is referred to as the Newer metasedimentary series and comprise pelites and semi-pelites that have been metamorphosed to form migmatized to non-migmatized paraschists in the Basement Complex of Nigeria (Oyawoye 1972; Rahaman 1976; Ajibade 1979). Extensive geologic mapping and detailed geochronological studies have shown that both the Older and Newer Metasediments occur in every part of Nigeria (Ekwueme and Onyeagocha 1985; Ekwueme and Kroner 1997; Kroner et al 2001)
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