Browsing by Author "Ayalew, Dereje (Professer)"

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    Geochemical and Petrographic Characteristics of Crystal Rich Volcanic Rocks From Lalibela Area, Northwestern Ethiopia.
    (Addis Ababa University, 2018-06-03) Hailu, Girma; Ayalew, Dereje (Professer)
    The Bilbala magmatic complex is situated in northwestern Ethiopian plateau near to northern main Ethiopia rift. The aim of this work was to investigate and identify the sources involved in the petrogenesis of the pre –rift basalts of the Bilbala area around Lalibela wich located with approximate distance of 730 Km from Addis Ababa. Field observation, ptrographic analysis and geochemical data analysis are the main method used to meet the aim or objective of this study. Based on field observation and description the study area is covered by basaltic rocks. The paper presents the results of a comprehensive major element, trace element, and petrographic study of Oligocene volcanic sequences from the northwestern Ethiopian flood basalt province around Lalibela. A petrographic analysis result with field observation indicates the presence of clinopyroxene, olivine and plagioclase as phenocrysts and groundmass with Fe-Ti oxides, opaque minerals and volcanic glass. The major and trace element compositions with mineral assemblage of crystal rich basaltic rocks in the area show the HT basalts characteristics of the Ethiopian continental flood basalt. The analyzed samples result displays high contents of TiO2 (3.36-6.87%), FeO* (11.59-15.71%), CaO/Al2O3 (0.89-1.989%) and enrichment of incompatible trace elements and LREE. On primitive mantle normalized diagram of the studied samples there is positive Ta and Nb anomalies, but Sr with Pb shows negative anomalies.Enrichment of LREE and a relative depletion of HREE in the studied samplesis observed and this suggest the presence of residual garnet in the source, and from trace elements concentration and ratios there is little evidence for contamination during the magmatic ascent.The Ce/Pb, Nb/Th and Nb /U ratios of the study area basaltic rocks display 21.3-51.7, 9.5-13.8 and 33.1-45.2 respectively. These elemental ratios have nearly OIB type character. The studied samples have low Zr/Nb ratios (< 10). The low Zr/Nb value of this mafic lava might imply a dominant mantle plum contribution during the genesis of the study area lavas. .
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    Geology and Stability of Opal From Wegel Tena Area, Ethiopia
    (Addis Ababa University, 2017-11-01) Dagnachew, Selamawit; Ayalew, Dereje (Professer); Getaneh, Worash (PhD); Rondeau, Benjamin (Professer)
    Crazing (cracking) is a common opal destabilization which has not been studied in detail prior to this work. It is a major economic issue for the opal gemologic market and the industrial synthesis. The aim of this research is to understand cracking (crazing) phenomena of opals from Wegel Tena locality. To achieve this, opal samples are directly extracted from three different local mining sites of Wegel Tena opal deposit (Koke Wuha to the northeast of Wegel Tena town, Anset to west of Wegel Tena town and Chegen to the north of Wegel Tena town) from the host rocks of rhyolitic ignimbrites. The opal samples have been characterized under natural light, crossed polarizers and Raman spectra data. The opal samples were heated in order to accelerate the evacuation of the water. Hydrophanes opals (KOK01, CH01 bleach zone and AN02) have shown no development of cracks after heating. While non-hydrophanes opals (KOK04, KOK05, internal zone of CH01) develop crack pattern after heating regardless their transparency. All non-hydrophane samples at low Raman spectral resolution (3cm-1) showed significant variation at 780cm-1 peak (up to 13cm-1), which is an indicator for strong strain into the Si-O-Si bond. The average value for this shift is about 3.3cm-1 down after crazing (cracking) reflecting extensive stresses impact for crack development. Birefringence patterns (produce extensive local constraint) observed around the cracks between crossed polarizers. Abnormal birefringence figures appear with crazing: anisotropy is created around breaks. Opal stress distribution is dependent to opal surface irregularities and breaks distribution. Water loss triggers crazing into non-hydrophane opals. Crazing reorganizes stress distribution according to the new surface distributions. Generally hydrophanes opals (KOK01, CH01 bleach and AN02) have better resistance to cracking than non-hydrophanes opal (KOK04, KOK05, internal zone of CH01) even at high temperature. In Hydrophane opals water exist in molecular form.
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    Petrogenesis of Nech Sar Volcanics at Southern Part of Arba Minch Area in Gamo Gofa Zone from South Main Ethiopian Rift
    (Addis Ababa University, 2018-06-12) Mamo, Workinesh; Ayalew, Dereje (Professer)
    The Nech Sar volcanics, are locate on the southern Ethiopia within the floor of south Main Ethiopian Rift. The study area is found locate about 521 km south of Addis Ababa. The main objective of this thesis is to understand the petrogenesis of volcanic rocks that are exposed on south of Arba Minch town. Field investigations, petrographic and whole rocks geochemical analysis methods have been applied in order to meet the main and specific objectives of this work. The studied volcanics are all of Quaternary age and include basaltic lava flow and scoria, rhyolite lava, unweldded tuff and intermediates rocks. The basaltic rocks range from aphyric to porphyritic and mildly porphyritic basalt. The acidic rocks are characterized as alkali feldspar and quartz rich rocks on the basis of field and petrographi descriptions. The geochemical data indicate that the Nech Sar volcanic products vary from basalts through basaltic tarchyandisite, trachyandesite, trachydacite and rhyolite. The mafic rocks comprise a suite of transitional to weakly sub alkaline basalt and are characterized by enrichment of highly incompatible trace elements e.g. high La/Yb ratio (11-27). The Nech Sar volcanic rocks are genetically inter related by the process of fractional crystallization starting from mantle derived basaltic magma with limited involvement of crustal contamination. - This is shown by major oxide (MgO, CaO, Fe2O3, TiO2 Na2O and K2O) and selected trace element (Ni, Co, Cr, V, Sc, and Rb, Zr, Y, Nb) versus SiO2 Harker diagrams which exhibit either decreasing or increasing correlation that define continuous and regular trends. Similarly plots of selected (HFSE) incompatible trace elements (e.g. Nb versus Zr, Rb versus Th, Ta versus Th and Nb versus La) form moderately_ positive_ linear relations. The plots of (Tb/Yb)N (0.92-1.7) versus (La/Sm)N and La/Sm ratio versus Sm/Yb (2-2.5), suggest that the Nech Sar basaltic rocks are derived from garnet_ free_mantle source, i.e is spinel peridotite mantle.
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    Petrogenesis of Volcanic Plugs and Volcanic Rocks of the Infranz Area, Northwestern Ethiopia
    (2018-06-01) Tadele, Gedefaw; Ayalew, Dereje (Professer)
    By: Gedefaw Tadele Vi Aau, Cncs,School Of Earth Sciences The Bi-Modal Basalt-Rhyolite Ethiopian Continental Flood Basalt, Formed By The Impinge-Ment Of The Afar Mantle Plume Beneath The Ethiopian Lithosphere, Contains A Number Of Vertically Sided, Thought To Be Feeders Of Volcanic Rocks At The Time Of Their Formation, Volcanic Plugs. Whole-Rock Major- And Trace-Element Data Including Rees Are Presented For Volcanic Plugs Of The Infranz Area, Northwestern Ethiopia In Order To Investigate The Petrogenesis Of Volcanic Plugs And To Understand Petrogenetic Inter-Plug Relationships. The Geochemically Phonolite-Trachyte-Rhyolite Alkaline Volcanic Plugs Of The Infranz Area, Are Mostly Characterized By Nepheline, Alkali Feldspar And Quartz Phenocrysts Respective-Ly, Set In Dominantly Alkali Feldspar Microlites Of Parallely Aligned Groundmass That Form Typical Trachytic Texture. The Infranz Volcanic Plugs Have Almost Similar Major- And Trace-Element Concentrations On The Major-Element Oxide And Trace-Element Versus Silica Varia-Tion Diagrams Hence, Formed By The Little Role Of Fractional Crystallization Between Them. These Volcanic Plugs On Chondrite-Normalized Ree Patterns Show Highly Fractionated [La/Y]N ( 8.97-21.66) Rees With Negative Eu (Eu/Eu*= 0.43-0.83) Anomalies; On The Primitive Mantle-Normalized Multi-Element Plots They Illustrate Negative Anomalies In: Ba, Sr, P, And Ti. They Are Formed From The Same, Explained By The Well- Defined Trends In-Compatible Elements Form, By The Almost Similar Zr/Nb, Zr/Th Ratios They Have, Garnet Free, Shown By The Greater Than 10 X Chondrites Of Sample/Chondrite Ratios, Source Formed By The Same, Illustrated By Parallel To Subparallel Chondrite-Normalized Ree Patterns, Petrogenetic Processes. The Variations In Concentration Of Major Elements (Tio2, Fe2o3, Al2o3, Na2o), Trace Elements (Nb, Th, Y) And Incompatible Trace Element Ratios (Zr/Th, Zr/Nb) Between The Infranz Volcanic Plugs And Lima Limo Rhyolites Demonstrates The Heterogeneity of Their Source. This Implies That The Lima Limo Rhyolites Are Not The Fed Products of The Infranz Volcanic Plugs.
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    Petrography and Geochemistry of Basement Rocks from Abay (Blue Nile) River Gorge, Around Agemsa Town Western Ethiopia
    (Addis Ababa University, 2018-06-04) Abdissa, Selamawit; Ayalew, Dereje (Professer)
    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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    Petrography and Petrology of Gina Ager-Megezez Locality, Megezez Shield Volcanics North Shoa, Central Ethiopia
    (Addis Ababa University, 2018-06-02) Mengistie, Tenaw; Ayalew, Dereje (Professer)
    Megezez shield volcanics is a mountain area with elevation ~3593 meter above mean sea level forming gentle slope of (~50); situated on the north western margin of main Ethiopian rift, central Ethiopia. The area is located 125 kms northeast of Addis Ababa. It is characterized by fissural volcanics with the pre-rift phase of eruption of mid-Miocene age (10.5 Ma). The main objective of this thesis work is to understand the petrography and geochemical compositions of Megezez shield volcanic products and finally to assess their petrogenesis. The present study is aimed to address geological data on Megezez shield volcanics via: 1) detailed geological map; 2) composite stratigraphic section; and 3) detailed petrographic and geochemical data. Twenty samples for petrographic analysis; nine samples for geochemical analysis and structural analysis has been utilized to achieve objectives of the research. Basalt, rhyolite, trachydacite, trachybasalt, and trachyandesite are major volcanic units of Megezez shield volcanics. The studied volcanic products are affected by tectonics of crustal extension. Major geological structures include joints, NE-SW strike and SE dipping normal faults imitating the general orientation of border faults of the Main Ethiopian Rift. Megezez volcanic products show wide variation in chemical composition (45.5-72.2 wt% SiO2) as mafic and intermediate-felsic suite. Basalt and intermediate rocks are transitional type; whereas, the rhyolite rock unite is peralkaline. Selected variation and spider diagrams of major and trace elements show parallel trends indicating the volcanic rocks are co-genetic with the major role of fractional crystallization. Assimilation fractional crystallization has some role to form evolved (intermediate) rocks observed from Rb Vs Rb/Nb discriminating diagram