Geochemical Investigation of the Bimodal Volcanic Suite at the Western Afar Margin: the Case of Shewa Robit Marginal Graben, Northern Shewa, Central Ethiopia
| dc.contributor.advisor | Ayalew, Dereje (Professor) | |
| dc.contributor.author | Mulu, Kiflom | |
| dc.date.accessioned | 2021-07-30T11:36:45Z | |
| dc.date.accessioned | 2023-11-09T14:12:55Z | |
| dc.date.available | 2021-07-30T11:36:45Z | |
| dc.date.available | 2023-11-09T14:12:55Z | |
| dc.date.issued | 2020-12-01 | |
| dc.description.abstract | The study area, Shewa Robit, is situated in the southern end section of the Western Afar Margin. It is located approximately 222 Km northeast of Addis Ababa. In this study, field, petrological and geochemical (major and trace elements) data of the volcanic rocks from the Shewa Robit area are presented and integrated to understand their petrogenesis. The main lithological units of the study area are lower basalt, glassy rhyolite, porphyritic rhyolite, rhyolitic ignimbrite, unwelded tuff and upper basalt. The petrographic study shows that the mafic volcanic products are characterized by aphyric to porphyritic textures with the plagioclase dominant phenocryst, whereas the felsic rocks are dominated by quartz and K-feldspar (sanidine) phenocrysts and glassy groundmass. The geochemical analysis results indicate that the Shewa Robit volcanic rocks are bimodal in composition with a significant compositional gap (between 52.75 and 69.8 wt. % SiO2) between the mafic and felsic members. It comprises a rock type, including basalts, trachy-basalts (Hawaiite), basaltic-andesite and rhyolites. The mafic volcanic rocks are olivine to quartz tholeiitic whereas the silicic rocks are metaluminous and peralkaline rhyolites. Trace element ratios of the mafic samples show some crustal contamination on samples from the lower basalts. Moreover, the investigated mafic samples from the study area exhibit trace element ratios such as Nb/U (36- 48.3), Zr/Nb (6-9.69), Ba/La (11.39-18.31), La/Nb (0.93-1.13) and Ba/Nb (12.3-20.7) and suggest that these lavas are derived from mantle source similar to OIB-type sources. Furthermore, mafic rocks of the volcanic suite are characterized by low CaO/Al2O3 ratios (0.47-0.72), enrichments at Ba, depletions at K and flat HREE patterns (TbN/YbN = 1.55-2.08). These major and trace element data indicate that basalts from the Shewa Robit area were derived from the melting of amphibole-bearing spinel peridotite source. Moreover, mafic and felsic rocks of the studied area are proved to be genetically related by the fractional crystallization process. The variations in major and trace element concentrations and petrographic studies indicate that fractionation of olivine, clinopyroxene, plagioclase, K-feldspar, apatite and Fe-Ti oxide or opaque minerals occurred during the evolution of the Shewa Robit bimodal basalt - rhyolite volcanic suite. | en_US |
| dc.identifier.uri | http://10.90.10.223:4000/handle/123456789/27503 | |
| dc.language.iso | en | en_US |
| dc.publisher | Addis Ababa University | en_US |
| dc.subject | Petrogenesis | en_US |
| dc.subject | Western Afar Margin | en_US |
| dc.subject | Bimodal Volcanic Rocks | en_US |
| dc.subject | Fractional Crystallization | en_US |
| dc.title | Geochemical Investigation of the Bimodal Volcanic Suite at the Western Afar Margin: the Case of Shewa Robit Marginal Graben, Northern Shewa, Central Ethiopia | en_US |
| dc.type | Thesis | en_US |