Petrogenesis of Basalts of Ghibe River Gorge, South-Western, Ethiopia.
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Date
2015-06-06
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Addis Ababa University
Abstract
Ghibe River gorge is located in Southwestern Ethiopia, in the central part of Omo-Ghibe
River Basin. The main objective of this study was to constrain the petrogenesis of Ghibe
River gorge basalts using the geochemical signature of major and trace elements and
thereby to fill the gap of geochemical data and geochemical process that control the
Ghibe River gorge basalts evolution. The ICP-OES and ICP-MS geochemical methods
were used for major and trace element analysis, respectively. The study area generally
comprises of dominantly basalt, and some trachyte and tuff, with aphyric to sparsely
porphyritic texture. Classification of the basalts on TAS variation diagram of LeBas et al.,
(1986) Ghibe River gorge samples fall in the basalt field and considered to be alkaline in
composition. The Ghibe gorge basalts have low content of MgO (3.35-6.00 wt %) and
compatible trace elements (Ni, Cr and Co), and positive correlations of Al2O3 and CaO
with MgO, suggest that fractionation of olivine, plagioclase and pyroxene. Given that the
high ratio of incompatible elements (Ce/Pb, Nb/U), which are not affected by fractional
crystallization and low ratio of Nb/Pb and Nb/U, Ghibe River gorge basalts are not
affected by crustal contamination, rather they indicate their derivation from lithospheric
mantle.
The positive anomaly of Ba in the primitive mantle normalized multi-element variation
diagram shows the presence of amphibole which is stable in lithospheric mantle.
Therefore, this signifies their derivation from lithospheric mantle. The flat HREE pattern
together with (Gd/Y)n = 1.02-1.14 suggests that Ghibe River gorge basalts were derived
from spinel peridotite, with no significant contamination by crustal rocks.
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Keywords
Petrogenesis, Basalts, Ghibe River Gorge, South-Western, Ethiopia