Petrogenesis and Source Rock Characterization of Volcanic Rocks from the Gidole Horst in the Southern Ethiopian Rift

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Addis Ababa University


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.



Petrogenesis, Source Rock Characterization, Volcanic Rocks, Gidole Horst, Southern Ethiopian Rift