Abstract
The study area is subpart of the southern main Ethiopian Rift and geographically bounded between UTM (Universal Transverse Mercator) coordinates of 662000m-669000m latitude and 336000m-341000m longitude. The main aim of this study is to investigate the geological, petrographic and geochemical properties of volcanic rocks around Arba Minch, southern Ethiopia. Both field and laboratory analytical techniques are employed to characterize minerals and the rock types. Eight thin section samples are prepared to study petrographic properties of minerals and five best representative samples are analyzed for whole rock chemistry using ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). Petrographically, samples from the porphyritic basalt show phenocrysts of olivine and plagioclase within the fine-grained groundmass. There are rounded to subrounded holes or vesicles that appear white under plane polarized light observations and dark under cross polarized light observations which further confirms vesicular nature of the basalt. Thin section sample from rhyolite outcrop shows phenocrysts of K-feldspar and quartz within tiny fine-grained groundmass of feldspar minerals. Aphanitic basalt, Porphyritic basalt, vesicular basalt, rhyolite and dolerite are the rock units found in the study area. The geochemical results of the analyzed samples indicate silica ranges of (48.5-60.1)%. Based geochemical classifications made, rock units are classified into rhyolite, basalt, hawaiite and mugherite rock types. Trace element analytical results show all samples lie on the within plate tectonic setting. The rare earth and trace element ratios and spider diagrams confirmed that most samples are derived from magmas of asthenospheric source with little crustal contamination.
Highlights
The East African Rift (EARS) is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary where rift tectonics accompanied by intense volcanism has taken place from the Tertiary to Recent
The study area is located at the western margin of NNESSW trending southern domain of main Ethiopian rift belonging to the East African Rift system
The study area is covered by rocks units such as aphanitic basalt, porphyritic basalt, vesicular basalt, rhyolite and dolerite
Summary
The East African Rift (EARS) is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary where rift tectonics accompanied by intense volcanism has taken place from the Tertiary to Recent. Magmatic activity in East Africa began in the Eocene and recent reviews suggest multiple pulses of magmatism since ∼45 Ma, with the most volumetrically-significant flood basalt and silicic eruptions taking place in the Oligocene ∼33.9 to 27 Ma as cited in a study [17]. The study area is located at the western margin of NNESSW trending southern domain of main Ethiopian rift belonging to the East African Rift system. The northern part of Lake Abaya, the extensional axis of the MER became the locus of volcanic activity with bimodal basalt – rhyolite extrusion in the rift floor during Quaternary period [16]
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