The eastern branch of East African rift system consists of Ethiopian and Kenyan rift systems.rnEthiopian rift system start from the afar triple junction to the broaden zones of the over lappingrnrift systems segmented into three parts the northern, the southern and the central rift system.rnGidole horst lies in the complex zone of the overlapping Ethiopian and Kenyan rift in thernsouthern part of Ethiopia. It is bounded between Chamo basin to the East and Woyto basin to thernwest nearly orient in southeast-northwest direction. It has step up normal faults rise to anrnelevation 2540m above mean sea level.rnTo constrain the petrogenetic evolution and source rock characterization of the Gidole horstrnvolcanic rocks; integrated field, petrographic analysis and major and trace element geochemistryrnhas been conducted. Thus, the study constrained petrogenesis of volcanic rocks and theirrnassociation through petrographic, major and trace element data, and the source rocks of thernbasalts and the rhyolites through trace element models. The possible contamination of the basaltsrnand the rhyolites were also addressed.rnThe stratigraphic studies in the Gidole horst allowed the recognition of several phase of volcanicrnactivity. The lowest exposed unit is the lower basalt equivalent to the Amaro horst lying over thernmetamorphic basement exposed around Gato. This unit has several phases of eruption and isrnoverlain by felsic tuff. Felsic tuff consists of the lower ignimbrite and the upper unwelded tuff. Itrnis sandwiched between the lower basalt and the middle basalts, where the middle basalts arerncharacterized by columnar joints. A thin trachytic flow over lays the middle basalts aroundrnGebele-Beno, specifically in Himbro expected to be the volcanic vent. The existence of mudrnstone overlaying these units indicates the presence of hiatus between the trachytic flow and thernupper basalts. The upper basalts cover a large volume and the upper flows fall in the sub-tropicalrnclimate condition that are highly altered.rnPetrographically the rocks are classified as olivine-plagioclase phyric, aphyric, trachytic andrnfelsic tuff. The very early flows of lower basalts show olivine-plagioclase phenocrysts withrnmicrophenocrysts of olivine, plagioclase and pyroxene. The groundmass of it consists ofrnopaqueâ€™s in addition to the above minerals. Whereas, the upper flows of the lower basalt, thernmiddle basalts are aphyric consists olivine, plagioclase, pyroxene and opaque minerals with lessrnamphibole observed in the upper basalts. The trachyte show trachytic texture with plagioclasernand plagioclase is absent in felsic tuff, instead large crystals of sanidine are observed.rnMajor and trace element variation of Gidole horst volcanic rocks show strong association of thernrocks and support the origin of the evolved rocks by crystal fractionation starting from the lowerrnbasalt to the more evolved products. The separation of olivine, plagioclase, pyroxene and opaquernoccurred in the basaltic rocks and addition of sodic plagioclase in the intermediate trachyte.rnFurthermore, the evolution generated strong enrichment of incompatible elements and depletionrnin compatible elements during the course of the fractionation. As a result the more evolvedrnrhyolite have higher concentration of incompatible elements (e.g. Zr, Rb, Th, Ta and REE) andrnlower concentrations of compatible elements (Ni, Cr, Sr and Ba). As Ba and Sr substitute Ca inrnplagioclase, they are readily compatible to the early formed mineral, which is plagioclase.rnConsequently, Ba and Sr formed peak in the mantle normalized diagrams specifically in thernlower basalts. Whereas the evolved produces, produce a trough for those elements. In addition tornthis, the rhyolites have a trough for P and Ti indicating the fractionation of apatite and Fe-Tirnoxide.rnThe strong correlation between the volcanic rocks shown on the petrographic analysis, major andrntrace element variation diagrams, the volcanic rocks of Gidole horst have shared comment sourcernrather than displaying different sources. Thus, the lower basalt is the oldest and primitive basaltrnfrom which the other basalts and felsic rocks are evolved. Based on the strong fractionation ofrnREE the model was introduced to determine the source of the rock and indicates they werernderived by relatively small degrees of partial melting of a source in which, garnet remains as arnresidual. Even though they share common source there are some differences observed among thernrocks. This is due to the contamination with rocks occurred during the rout to the surface belowrnthe upper crust.