Petrography and Deformation History of Bila (Gidano King) Area, Wollega, Western Ethiopia

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Addis Ababa Universty


Bila area is located in the metamorphic terrains of West Ethiopian Shield. It covers an area of about 56 Km2. The main objective of this study is to understand lithology, petrography, structures, deformation history and geochemistry of the basement rocks. To accomplish the objectives; an integrated descriptions of field work, petrographic investigations, structural and geochemical data analysis have been carried out. The area consists varieties of rock units; slate, marble, phyllite, quartzite, graphite schist, metabasalt, chlorite schist, serpentinite, talc schist and actinolite-tremolite schist. The metasedimentary rock group contains chlorite, muscovite, biotite, sericite, quartz and feldspar whereas the metaultramafic and metavolcanic rocks composed the minerals; sericitized plagioclase, chlorite, epidote, actinolite, feldspar, opaque, quartz, calcite and talc with some relict of igneous origin. Based on their mineral assemblages, relict igneous texture and the existence of primary bedding the rocks of the area experienced low grade green schist facies metamorphism. Field structural data and petrographic investigations from this area demonstrate that, at least three deformational phases are recorded in the area. D1 and D2 phases are mostly responsible for the formations of ductile deformations whereas, post D2 deformational phases may be responsible for the formations of the latter brittle deformations during which the faults, veins and joints were developed. The major oxides and trace elements Harker variation diagrams, REE and multi element patterns of Bila metavolcanic rocks demonstrate the rocks are linked with fractional crystallization process. The Zr-Y-Nb and Th- Ta- Hf triangular discrimination diagrams revealed that most of the Bila rocks are within the tectonic environment of volcanic arc basalts. Key words: Bila area, metavolcanic rocks, meta-ultramafic rocks, metasedimentary rocks, deformation history, metamorphism, fractional crystallization.



Bila area, Metavolcanic rocks, Meta-ultramafic rocks, Metasedimentary rocks, Deformation history, Metamorphism, Fractional crystallization