Browsing by Author "Asrat, Asfawossen (Professer)"
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Item Characterization of Selen Wuha Mafic-Ultramafic Massif of Raya Marginal Graben, Northern Ethiopia(Addis Ababa University, 2018-05-03) Redae, Senay; Yirgu, Gezahegn (Professer); Asrat, Asfawossen (Professer)Ultramafic intrusion is found in Selen Wuha area of Raya marginal graben. Selen Wuha area is located 600km away from Addis Ababa to the northern Ethiopia. Some of the previous researchers were considered it as basement low grade metamorphic unit while others have not conformed to this idea and considered is as young aged un-metamorphosed igneous intrusion. Thus, the target of this study is to resolve this gap by characterizing the intrusion using field observation, petrography and geochemical data. The study area covers about 20km2 and it has mapped with the scale of 1:25,000. Accordingly, ultramafic unit, plagioclase-olivine phyric basalt, pyroclastic material and alluvial sediment deposit are recognized major mappable units. Moreover, small intrusions (dykes), normal faults, lineaments, joints and veins are identified geologic structures of the area. Hence, the contact of ultramafic with adjacent units is controlled by tectonic contacts. Minerals in the ultramafic are coarse grained Cpx, Opx, Ol and Fe-Ti oxide. Similarly, minerals in gabbroic units are coarse grained plag, Cpx, Ol and Fe-Ti oxides. Texturally, both units are porphyritic-phaneritic in which the size of grains is range from medium pheneritic to coarse pheneritic. Plagioclase-olivine phyric basalt has porphyritic texture in which Ol and plag are the dominant phenocryst minerals. Based on the proportion of Ol, Pxn, and concentration of MgO, the studied ultramafic unit is classified into pyroxenite and peridotite. Since any sign of metamorphism is not found in the unit, it is an igneous intrusion which has intruded in early Cenozoic formations and Mesozoic sedimentary successions. Accordingly, the studied ultramafic unit is not Neoproterozoic in age however it is Cenozoic aged intrusion. From the geochemistry point of view, the ultramafic unit is derived from depleted mantle source since it shows high concentration of MgO (22.31 wt. % to 37.08 wt. %), Mg # (72.63 to 81.94), Cr (1690ppm to 3000ppm), Ni (1030ppm to 1980ppm) and Co (96ppm-143ppm). It shows also slight depleted in LREE and very low concentration of the highly incompatible trace elements. Furthermore, the samples are cogenetic, sourced from homogeneous source and related by fractional crystallization since they show good correlation in the selected major oxide and trace element variation diagrams.Item Environmental Impact Assessment of Quarrying Activities on the Downstream Section of the Big Akaki Catchment. Addis Ababa, Ethiopia(Addis Ababa University, 2020-06-05) Yimame, Samrawit; Asrat, Asfawossen (Professer)This study focused on the assessment of the impact of quarrying on the natural environment (water resources and land use land cover), and the vulnerability of the community downstream of the quarry sites within the Big Akaki River catchment, using an integrated geological, hydrogeochemical, land use/land cover and social impact assessment methods. Initial reconnaissance survey to select water sampling areas and assessment of overall operations of quarrying in the area was followed by surface water sampling at the quarry sites and upstream and downstream of the quarry sites, along three major rivers in the catchment. These water samples were analyzed for major physical and hydrogeochemical parameters. The results show that the turbidity, TDS, Temperature of the river waters and most of the ions increased their concentration from upstream to downstream of the quarry sites considered, indicating the clear impact of the quarrying activity on the rivers and directly affecting the community using these river waters for domestic and other related activities. However, it should be noted that this might not suggest chemical pollution only because of quarrying. The land use/land cover change assessment was done by using spatiotemporal data of the area between 2000 and 2020, which enabled to assess~20 years of quarry expansion rate, calculated from the classified image by using standard remote sensing and GIS methods and tools. Landsat_7 ETM+, SPOT-5, SPOT-7 and Quickbird raster data for the years 2000, 2006, 2016 and 2020 have been used land use land cover maps for the years considered, and the net change between 2000 and 2020 have been produced and the change has been analyzed. The results indicate that quarrying sites have been expanded by about 692.68 ha, mainly at the expense of the cultivated (agricultural area) which has decreased by 3941.19 ha during the same period. The social impact assessment conducted by a systematic, structured open and closed-ended questionnaire survey of the quarry workers and residents within the immediate vicinity of the quarry sites indicate multiple problems including dust, noise, vibration, and accumulated water in the abandoned quarry sites caused problem on residents and quarry workers. The study Environmental Impact Assessment of Quarrying Activities on the Downstream Section of the Big Akaki Catchment Environmental Geology and Geohazards Stream Page v indicates that unless properly mitigated by all responsible bodies, the quarrying activity in the area will continue to cause serious damages to the natural environment and the community.Item Petrogenetic Evolution of the Basalt and Associated Rhyolite Suiti Volcanics (“Alaje Formation Volcanic Rocks‟‟) From Aiba Area, Tigray, Northern Ethiopia.(Addis Ababa University, 2018-05-30) Hiluf, Hagos; Asrat, Asfawossen (Professer)Continental flood basalts (Trap and shield basalts) of very large area and thickness, which occur in Ethiopia and Yemen are related to the Afro-Arabian continental break up. The current study deals with constraining the petrogenetic evolution of the mafic-felsic association of Alaje Formation in the Northwestern Ethiopian plateau, particularly at Aiba, Tigray, using detailed field mapping, petrographic analysis, as well as major and trace element geochemistry. The Alaje Formation is a succession of: rhyolitic tuff, olivine phyric basalt, rhyolitic ignimbrite, plagioclase phyric basalt, aphyric basalt. The basaltic rocks comprise phenocryst of plagioclase, clinopyroxene, Fe-Ti oxides and rare olivine phenocrysts. On average, the basaltic suite is plagioclase dominated, except the older basalt where olivine is dominant, and the younger basalt at Tsibet section where Fe-Ti oxide is dominant. The common phenocrysts of the silicic rocks are quartz and sanidine, though the banded rhyolitic lava is particularly quartz dominated. Although the basaltic rocks and the associated rhyolitic units have different phenocryst assemblages and textural relationship in between different unit of basalt and rhyolite, they do not show significant geochemical variation, while there is close genetic relationship between basalt and rhyolite. This is supported by the consistently constant similar trace element ratio Nb/Zr (0.09-0.14), Ce/Zr (0.20-0.29), La/Zr (0.10-0.012), Rb/Zr (0.05-0.14), Ta/Nb (0.05-0.07), La/Nb (0.85-1.05),Hf/La (0.21 0.26) and linear trend of two high incompatible trace element. Besides the geochemical data suggest that the basaltic units are more primitive than the associated rhyolites unites. However, the lower contents of MgO wt% (5.41 - 8.11) and compatible trace element content like Ni (19 – 151 ppm) and Cr (100-300 ppm) indicates that the basaltic units have undergone some fractionation except anomalous high Cr (670), Ni (421) of the lower basaltic unit. Furthermore, the rhyolites of the Alaji Formation have peralkaline affinity and show consistently similar geochemistry to the well-constrained rhyolitic rocks of Wegel Tena and Lima limo, which have been shown to be formed by low pressure crystal fractionation process. It is concluded that the rhyolites are derived by low pressure crystal fractionation of basaltic magma derived from enriched source component of the mantle.