Petrogenesis of Nech Sar Volcanics at Southern Part of Arba Minch Area in Gamo Gofa Zone from South Main Ethiopian Rift
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2018-06-12
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Addis Ababa University
Abstract
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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Keywords
Petro Genesis, Fractional Crystallization, Mantle Source and Nech Sar Volcanic Rocks