ABSTRACT:
Gneisses in the upper amphibolite facies grade of metamorphism constitute the oldest basement complex exposed in south-central Nigeria. They are metasediments derived from pelites, semipelites and greywackes. Nineteen RbSr whole-rock isotopic analyses () on these gneisses indicate that they have severely disturbed RbSr systematics dating back to the Eburnean orogeny (2200-1680 Ma). This is illustrated by a six-point isochron age of 2202 ± 31 Ma with in initial 87Sr/86Sr of 0.705 (MSWD = 0.84) for the upper limit and a seven-point errorchron of 1683 ± 56 Ma for the lower age limit. These ages fit the event recorded for the stable parts of the West African craton. The rocks studied seem to be relics of older basement complex which resisted the later (Pan-African) (600 ± 150 Ma) orogeny that remobilized parts of the continent.Pan-African granitic plutons intruded the gneisses. Eight data points of the granites yield an isochron of 575 ± 84 Ma with an initial 87Sr/86Sr of 0.7194 ± 0.0045 (MSWD = 0.05). A biotite age of 516 ± 11 Ma was obtained from a granite sample. Because of the large error on the whole rock age, we can only advance an age close to 520-500 Ma for the emplacement. The age of biotites from the gneisses was reset to 500 ± 11 Ma. Apart from the rejuvenation of the mica age, Okene gneisses appear to have remained unreworked since the formation of the West African craton.
Journal of African Earth Sciences 01/1988; 7(1):121-126. DOI:10.1016/0899-5362(88)90058-9
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