Petrography and Geochemistry of Basement Rocks from Abay (Blue Nile) River Gorge, Around Agemsa Town Western Ethiopia
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Date
2018-06-04
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Addis Ababa University
Abstract
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.
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Keywords
Abay (Blue Nile) River Gorge, Palaeotectonic Setting, Protolith, Metamorphism, Granite and Orthogneiss