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Title: The geology of the area around Okene, south-western Nigeria
Author: Annor, A. E.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1981
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An area of the Basement Complex of Nigeria exposed around Okene, was mapped at the scale of 1:50,000; and rock samples were collected for chemical analysis. The mapping and subsequent petrological study yielded evidence for three thermo-tectonic episodes. The earliest episode is distinguished in the banded gneiss (Early gneiss suite) by its metamorphic-tectonite fabric (D1) which pre-dates that in a swarm of minor meta-intermediate dykes. Outcrops of the gneiss occur as 'undeformed' inclusions interleaved between the foliation planes of an intrusive member of a later suite (Main phase gneiss) whose earliest metamorphic-tectonite fabric (D2) is concordant to that in the dykes. The banded gneiss preserves a high grade amphibolite facies metamorphic assemblage. The Main phase gneiss which distinguishes the second episode, consists of an early metasediment and later meta-intrusives. Metamorphic assemblages (M2) in the suite vary from high grade amphibolite facies in the earliest member to medium grade amphibolite facies in the last member; suggesting waning PT conditions and syn-D2 emplacement of the intrusives. Folds (D3) defined by D2 fabricjand intrusive diorites and granites (Late Intrusive suite) characterise the last episode. These intrusives serve as D3 strain-markers; developing metamorphic-tectonite fabric which reflect medium grade amphibolite facies conditions in the most intensely deformed parts of the folds, but preserving their original igneous microstructures in the other parts. The origin(s) and/or mode of evolution of the Okene banded gneiss suite is not revealed by comparative major element geochemistry. The compositions of all the other rock samples fit those of their unmetamorphosed equivalents, suggesting isochemical conditions of metamorphism throughout the evolution of the rocks. Correlation of the rocks with structurally similar lithologies of known ages from other parts of the Basement Complex suggests that the tectonic episodes are probably of Archaean, Early Proterozoic, and Late Proterozoic-Early Paleozoic ages respectively.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available