Petrography and Geochemistry of Basement Rocks from Abay (Blue Nile) River Gorge, Around Agemsa Town Western Ethiopia

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Addis Ababa University


The geology Western Ethiopian Precambrian shield (WES) is known by low-medium grade metavolcano-sedimentary rocks, high grade gneiss and migmatites and ophiolite belts. The study area is a part of high grade metamorphic terrain of Western Ethiopian Precambrian shield that consists of both high grade Precambrian metamorphic and intrusive rocks. The high grade metamorphic rock is represented by orthogneiss, whereas the intrusive rock is represented by granite. The integration of field investigation, petrographic and whole rock geochemical analysis has been carried out to understand the petrogenic evolution and tectonic setting of the area. The presence of segregated mafic and felsic rich compositional bands both at the mesoscale and in the thin sections and the abundance of certain minerals such as hornblende and; plagioclase with lesser amount of clinopyroxine indicates that the orthogneiss of the study area has been subjected to high grade metamorphism that belong to upper amphibolite facies. Geochemicaly, the studied orthogneiss rock show calk-alkaline magma affinity. Except the granitic gneiss, all the remaining samples have relativelly high concentrations of FeO, MgO and CaO and low concentration of SiO2 content. Their REE abundance pattern shows nearly parallel trend in a negative general slope. The negative slope and calc-alkaline affinity combined with tectonic setting infer that the original rock of orthogneiss rock is originated from lower enriched mantle reservoir. The discrimination diagram Zr/117-Th-Nb/16 suggests that the original rock of orthogneiss of the study area were developed in the volcanic arc tectonic setting. The overall geochemical characteristic of granite shows high SiO2 and Al2O3 contents 68.6-77.3 wt % ; 12.04-17.5 wt% and lower content of other major oxides. Such as, Fe2O3 1.13-3.59 wt. %, MgO 0.02-0.62 wt % and CaO 0.83-1.69 wt.%. It display High K-calk-alkaline and peraluminous magma affinity. Their REE pattern shows only slight variation between the abundances of LREE and HREE elements which indicates moderate percentage melting. The discrimination diagrams Rb- (Y+Nb), Ta-Yb, Rb-(Y+Nb) and Nb-Y, suggested that the granite were developed in volcanic arc and syn-COLG environment.



Abay (Blue Nile) River Gorge, Palaeotectonic Setting, Protolith, Metamorphism, Granite and Orthogneiss