Petrogenesis and Source Rock Characterization of Volcanic Rocks from the Gidole Horst in the Southern Ethiopian Rift
No Thumbnail Available
Date
2021-09-13
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
The eastern branch of East African rift system consists of Ethiopian and Kenyan rift systems.
Ethiopian rift system start from the afar triple junction to the broaden zones of the over lapping
rift systems segmented into three parts the northern, the southern and the central rift system.
Gidole horst lies in the complex zone of the overlapping Ethiopian and Kenyan rift in the
southern part of Ethiopia. It is bounded between Chamo basin to the East and Woyto basin to the
west nearly orient in southeast-northwest direction. It has step up normal faults rise to an
elevation 2540m above mean sea level.
To constrain the petrogenetic evolution and source rock characterization of the Gidole horst
volcanic rocks; integrated field, petrographic analysis and major and trace element geochemistry
has been conducted. Thus, the study constrained petrogenesis of volcanic rocks and their
association through petrographic, major and trace element data, and the source rocks of the
basalts and the rhyolites through trace element models. The possible contamination of the basalts
and the rhyolites were also addressed.
The stratigraphic studies in the Gidole horst allowed the recognition of several phase of volcanic
activity. The lowest exposed unit is the lower basalt equivalent to the Amaro horst lying over the
metamorphic basement exposed around Gato. This unit has several phases of eruption and is
overlain by felsic tuff. Felsic tuff consists of the lower ignimbrite and the upper unwelded tuff. It
is sandwiched between the lower basalt and the middle basalts, where the middle basalts are
characterized by columnar joints. A thin trachytic flow over lays the middle basalts around
Gebele-Beno, specifically in Himbro expected to be the volcanic vent. The existence of mud
stone overlaying these units indicates the presence of hiatus between the trachytic flow and the
upper basalts. The upper basalts cover a large volume and the upper flows fall in the sub-tropical
climate condition that are highly altered.
Petrographically the rocks are classified as olivine-plagioclase phyric, aphyric, trachytic and
felsic tuff. The very early flows of lower basalts show olivine-plagioclase phenocrysts with
microphenocrysts of olivine, plagioclase and pyroxene. The groundmass of it consists of
opaque’s in addition to the above minerals. Whereas, the upper flows of the lower basalt, the
middle basalts are aphyric consists olivine, plagioclase, pyroxene and opaque minerals with less
amphibole observed in the upper basalts. The trachyte show trachytic texture with plagioclase
and plagioclase is absent in felsic tuff, instead large crystals of sanidine are observed.
Major and trace element variation of Gidole horst volcanic rocks show strong association of the
rocks and support the origin of the evolved rocks by crystal fractionation starting from the lower
basalt to the more evolved products. The separation of olivine, plagioclase, pyroxene and opaque
occurred in the basaltic rocks and addition of sodic plagioclase in the intermediate trachyte.
Furthermore, the evolution generated strong enrichment of incompatible elements and depletion
in compatible elements during the course of the fractionation. As a result the more evolved
rhyolite have higher concentration of incompatible elements (e.g. Zr, Rb, Th, Ta and REE) and
lower concentrations of compatible elements (Ni, Cr, Sr and Ba). As Ba and Sr substitute Ca in
plagioclase, they are readily compatible to the early formed mineral, which is plagioclase.
Consequently, Ba and Sr formed peak in the mantle normalized diagrams specifically in the
lower basalts. Whereas the evolved produces, produce a trough for those elements. In addition to
this, the rhyolites have a trough for P and Ti indicating the fractionation of apatite and Fe-Ti
oxide.
The strong correlation between the volcanic rocks shown on the petrographic analysis, major and
trace element variation diagrams, the volcanic rocks of Gidole horst have shared comment source
rather than displaying different sources. Thus, the lower basalt is the oldest and primitive basalt
from which the other basalts and felsic rocks are evolved. Based on the strong fractionation of
REE the model was introduced to determine the source of the rock and indicates they were
derived by relatively small degrees of partial melting of a source in which, garnet remains as a
residual. Even though they share common source there are some differences observed among the
rocks. This is due to the contamination with rocks occurred during the rout to the surface below
the upper crust.
Description
Keywords
Petrogenesis, Source Rock Characterization, Volcanic Rocks, Gidole Horst, Southern Ethiopian Rift