Deformation, Metamorphism and Chemical Stratigraphy of Neoproterozoic Rocks in Koraro-Gelebeda Area, Northern Ethiopia
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Date
2016-06
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Addis Ababa Universty
Abstract
An integrated study aiming at understanding metamorphism and deformation history, and chemostratigraphy of the basement rocks in Koraro-Gelebeda has been done. The exposed rocks represent Tambien Group exposures which are equivalents of Werii, Assem and Tsedia formations. Protolith mineral remnants and uncompleted reactions, primary bedding, fine grained textures and structures exhibited by the rocks such as pencil structure indicate a low to very low grades of metamorphism. The mineral assemblages are also typical of lower greenschist facies assemblage. Microstructures such as stylolites, fringe structures and indented grain contacts accompany the deformation at microscopic scale with the action of fluids. Mantled as well as fractured porphyroblasts and porphyroclasts resulting from non coaxial deformations show both dextral and sinistral sense. No deflection of foliation, random distribution of porphyroblasts but having strain shadow and fringe structure parallel to the foliations is interpreted to limit their development in the range of late- very late tectonic activity. Intertectonic ones without these microstructures are also common. Superimposed structures show foliation development due to first deformation phase; gently dipping folds, kink bands, crenulation cleavages and lineations due to second phase deformation; and joints and faults resulting from the third phase of deformation. The foliations generally trend NNE to SSW, and most of the lineation are gently plunging north. δ13C values of the collected metalimestone samples fluctuate in the range of -7.6‰ -2.7‰. This fluctuation might be due to small level changes in the amount of primary organic carbon burial, interbedded siliciclastic materials associated with the influx of fresh water from continents during deposition or diagenetic effects. The metalimestones didn’t show any glacial feature but all the δ13C values are negative which is the same case elsewhere in the world as in Little Dal/Coates Lake Groups and Fifteen Mile Group (Canada), Bitter Springs Formation (Australia) and Hecla Hoek Group (Svalbard). These values are correlated to the Bitter Springs stage which is proposed to result from true polar wandering event.
Key Words: Metamorphism; Deformation; Porphyroblasts; Superimposition; Diffusive Mass Transfer; Chemostratigraphy; Bitter Springs Stage; True Polar Wander
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Keywords
Metamorphism, Deformation, Porphyroblasts, Superimposition, Diffusive Mass Transfer, Chemostratigraphy, Bitter Springs Stage, True Polar Wander