Microstructural and Mineralogical Evolution of the Oke Awon Shear Zone in the Jebba Area, S.W. Nigeria
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Date
2014
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Akamia University
Abstract
The Jebba area of southwestern Nigeria is underlain by metasedimentary and metaigneous rocks which have been intruded by probable Pan-African (ca.600 Ma) granitic rocks. Locally, in the west, these rocks have been tectonised within two N – S trending brittle-ductile shear zones. Granitic mylonites contain elongate quartz ribbons surrounded by finer grained groundmass of microcline, quartz and plagioclase. Metabasic mylonites contain fine-grained hornblende and plagioclase defining the mylonitic, S2, fabric. Locally, the assemblage is epidote, actinolite, albite, and quartz indicating a marked retrogression. Semi-pelitic rocks, the mineralogy contain syntectonically rotated garnets with sigmoidal inclusion trails, fractured and elongated garnet, fine-grained biotite and muscovite, late porphyroblastic muscovite, and locally, minor epidote and chlorite resulting from partial retrogression. The quartzites are marked by microstructures ranging from cataclasites to ultracataclasites.
These observations indicate two generations of shearing, an earlier one under amphibolites facies conditions at deeper structural levels which was locally overprinted by brittle deformation under greenschist facies conditions following exhumation.
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Jebba area, southwest Nigeria, Oke Awon shear zone, microstructures, mineralogy