Petrography and Petrology of Gina Ager-Megezez Locality, Megezez Shield Volcanics North Shoa, Central Ethiopia
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Date
2018-06-02
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Addis Ababa University
Abstract
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
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Megezez Shield Volcanics, Petrography, Fractional Crystallization, Assimilation Fractional Crystallization, Crustal Contamination