Geology and Geochemistry of Guna Volcanic Massif Northwestern Ethiopian Plateau

Adise, Mekonnen (2006) Geology and Geochemistry of Guna Volcanic Massif Northwestern Ethiopian Plateau. Masters thesis, Addis Ababa University.

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Abstract

Guna volcanic massif is found in Northwestern Ethiopia plateau. It has an average basal diameter of about 40 km and a height of 1553 m with elevation of 4135m at the peak. It covers an estimated area of about 760 m2. A recent work of kieffer et al., 2004, provided an Ar/Ar age of 10.7 ma. The general objective of this thesis research work is to study the geology and the petrogenetic processes responsible to produce the rock suites of the volcanic massif. The methodologies employed to achieve the objectives includes fieldwork for mapping and sampling, petrographic investigations, electro microprobe analysis on major phenocryst phases of phonolte lavas, major and trace element geochemical analysis by ICPMS method, analysis and interpretation of data using different computer soft wares. Geologically the massif overlying the flood basalt mainly consists of rhyolite lava flow, pyroclastic flow deposit and phonolite lava flow from bottom to top. The rhyolites occur as layered lava flows, glassy with columnar jointing and huge domes. In thin section they have vitrophyric, perlithic and porphyrithic textures with essential mineral composition of alkali fieldspars, plagioclase and quartz. The pyroclastic flow deposit includes ash, tuffs, ignimbrite and trachyte flows. The phonolite occurs as thick viscous lava flow. They show porphrithic and trachythic textures with phenocrysts of alkali feldspar, feldesphathoids (nepheline, leucite and nosean) and alkali pyroxenes. The lavas are rich in silica and alkalis. Major element variation plots displays features agreeable with fractionation of major phenocryst phases, Fe-Ti oxides and apatites. The chondrite normalized Rare earth element (REE) patterns shows enriched light rare earth element LREE, strong negative Eu anomaly and fairly flat heavy rare earth (HREE) pattern that indicates fractionation processes. In the Primitive mantle normalized profiles (spider diagrams) Ba , Sr, Ti and P shows strong negative anomaly which verifies fractionation. In most samples, Nb shows a remarkable positive anomaly. The lavas have low Rb/Nb (0.31-1.31) and La/Nb (0.34-1.03) ratios high La/Lu ratios that increase from rhyolite through trachyte to phonolite. The field observations, petrography, mineral chemistry and whole rock geo chemical data of this work indicates that Guna is not a basaltic shield volcano; instead it is a huge felsic centre mainly consisting of pyroclastic flow deposits, rhyolite, phonolite and trachyte lava flows. The lavas are co genetic and derived from mantle origin basaltic magma through low-pressure extensive fractional crystallization and limited crustal contamination.

Item Type:	Thesis (Masters)
Subjects:	G Geography. Anthropology. Recreation > G Geography (General) Q Science > Q Science (General) Q Science > QD Chemistry Q Science > QE Geology
Divisions:	Africana
Depositing User:	Selom Ghislain
Date Deposited:	25 Jun 2018 11:54
Last Modified:	25 Jun 2018 11:54
URI:	http://thesisbank.jhia.ac.ke/id/eprint/4504

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