Petrogenesis of Tana Area Volcanic Rocks: Implications To Magmatic Evolution of Tana Basin
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Date
2018-05-05
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Addis Ababa University
Abstract
Lake Tana basin is a major part of the North Western Ethiopian plateau with numerous geologic
formations and structures. The Lake occupies the center of a large circular basin of at least 100–
120 km in diameter. It is situated within a faulted depression between erosional escarpment to
the west overseeing the Sudan plains and tectonic escarpment overseeing the Afar depression to
the east. Most of the previous works regarding petrology and Geochemistry on the Ethiopian
plateau are regional. Due to its complex geology Lake Tana area has been skipped for detail
works. The objective of this research is to study the Tana volcanics form part of the volcanic
succession of the regional flood basalts using a combined geological mapping, petrological and
geochemical characterization to understand their petrogenesis. The methodologies employed to
achieve the objectives includes fieldwork for mapping and sampling, petrographic investigations,
major and trace element geochemical analysis by ICP-MS and ICP-AES methods respectively.
Analysis and interpretation of data have been achieved using different software packages. The
major lithological units identified in the studied area includes: recent lacustrine sediments,
Quaternary volcanics, Pliocene to Miocene ash tuff and associated basalt, Oligocene rhyolite and
basalt. Basanite intrusions and scoria cones are also discussed under the Quaternary volcanics.
The Lake Tana area bimodal volcanic rocks have been classified as alkaline and some
transitional to tholeiitic mafic and sub-alkaline felsic magma series. The same age groups of
rocks are co-genetic and related to each other by fractional crystallization process starting from
mantle-derived basaltic magma with minimal crustal material involvement. In general, the trace
element geochemistry representing all rocks is formed in an intra-continental (within plate)
tectonic setting. Oligocene basalts are categorized under LT plateau basalt and also the
Oligocene rhyolites are comparable with plateau felsite.
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Keywords
Lake Tana Basin, Quaternary Volcanics, Oligocene Basalt, Petrological and Geochemical Characterization, Intra-Continental