Geology, Petrology and Geochemistry of Volcanic Rocks Around Arba Minch, Southern Ethiopia

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.


Introduction
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. Within the EARS, the Main Ethiopian Rift represents the link region between the Afar triple junction and the Kenya Rift regions where extensional deformation nucleated as early as late Oligocene-early Miocene times [2,6,8,11]. Volcanic rocks of Pliocene and Pleistocene age such as pantelleritic rhyolites, trachytes and ignimbrites are abundant within the rift floor and on the adjoining plateau [9].
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 NNE-SSW trending southern domain of main Ethiopian rift belonging to the East African Rift system. Different authors and researchers have written documents at various scales about the Geology of Arba Minch area, southern Ethiopia. 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].
Irrespective of these works, detailed geological and geochemical investigations of volcanic rocks is not conducted yet. Therefore, this research studies the geological, petrographic and geochemical properties of volcanic rocks around Arba Minch, Southern Ethiopia.

Description of the Study Area
The study area is subpart of the East African Rift system which lies at the southern margin of the main Ethiopian Rift. The area is geographically bounded between UTM coordinates of 662000m-669000m latitude and 336000m-341000m longitude. It lies at the eastern and western sides of Arba Minch city ( Figure 1). The physiography of the study area is a result of volcanism, tectonic effect and erosion processes which resulted in the formation of rugged topographies with steep valleys and high ridges. The climate of the study area is characterized by arid climatic condition with high temperature ranges. It can be characterized by two rainy seasons that is in autumn and spring. In winter, there is much less rainfall in Arba Minch than in summer.

Methodologies
Different sets of methodologies were employed to achieve the objective of this research work. The methodology adopted for data acquisition, analysis, processing and the procedures followed while planning and carrying out the geological, petrographic and geochemical investigations are described as follows.

Pre-Field Activities
The first phase of this research study gave emphasis on collection and review of different related journals and reports which are done by different companies and researchers.

Field Mapping and Sampling
Detail description of different outcrops was done based on the lithological and mineralogical variations. A total of fourteen samples are collected during the field work and among them eight best representative samples are selected for thin section and whole rock geochemical analysis.

Sample Preparation for Thin Section
Eight selected samples were resized or reduced by hammer in Arba Minch University Geological laboratory and were further resized by using cutting machine. First the rock samples were cut and trimmed into rectangular slab and further cutting was then followed till it reduced to the required size to be polished by DAP-U polishing machine, not greater than 4 cm since the mould has a maximum diameter of 4cm. The rock samples were cut-off and ground mechanically to transparent thickness using carbide abrasive until the slice reaches the conventional thickness of 30 microns. This is carried out using petro discs with 400-grit aluminum oxide powder. Eight (8) thin sections were prepared at Geological Survey of Ethiopia (GSE), Addis Ababa central Laboratory.

Sample Preparation for Geochemical Analysis
Five selected samples were resized and crushed to rock powder with crushing machine at Addis Ababa to be sent to Ireland for analysis. Inductively Coupled Plasma Mass Spectrometry is the analytical instrument used for geochemical analysis. Major, minor and trace elements analytical results for five rock samples are received from ALS (Australian Laboratory Services) center from Ireland (Table 1).

Data Processing
Mineralogical properties of rocks are identified from petrographic studies of thin section samples. The analytical results of major, minor and trace elements were used to study the geochemical nature of volcanic rocks of the area. Field based naming of the rock types are further proven from petrographic studies and geochemical classifications made by using Geochemical Data Toolkit (GCDkit) software. Different plots of major, minor and trace elements were used in petrological interpretations. Tectonic setting of the study area is distinguished by using Zr-Ti [13] and Zr-Zr/Y [14] plots. The rare earth and trace element spider plots and ratios are used for studying the petrogenesis of the rock units.

Geology and Petrography
The study area is covered by volcanic rocks such as aphanitic basalt, porphyritic basalt, vesicular basalt, rhyolite and dolerite and an alluvial deposit.

Aphanitic Basalt
It is a fine-grained extrusive igneous rock characterized by highly packed crystals and dominantly contains black colored minerals with some visible quartz grains (Figure 2a). It is formed on the earth's surface under faster rate of cooling from the basic lava. Small irregular oreinted fractures has affected the rock unit on most part of the exposures ( Figure  2b). The fine-grained basalt shows small elongated plagioclase feldspar using a hand lense. Petrographically, the thin section sample shows dominant distribution of microphenocrysts of plagioclase feldspar ( Figure 3). Small amount of opaque minerals that remains dark under both plane and cross polarized light observations are found associated together with plagioclase feldspar. This unit has intergranular texture in which glassy and opaque minerals distributed between the plagioclase grains. The mineral forming this rock unit has subhedral to euhedral shape. Fine-grained olivine minerals are changed to epidote.

Porphyritic Basalt
Relatively large crystals of plagioclase feldspar and olivine are clearly visible within the fine-grained groundmass. Besides to these major minerals, minor amounts of quartz and secondary calcite minerals are also observed ( Figure 4).

Vesicular Basalt
This rock unit is characterized by open spaces called vesicles. It is fine-grained, has black to grey color. The rock unit has significant porosity due to escaping of gas bubbles during rapid cooling of the magma.

Rhyolite
The lithology has fine-grained texture and formed due to rapid cooling of acidic lavas. It is characterized by flow banding and leucocratic color (Figure 8a). It formed a ridge and steep sided cone may be due to high viscosity ( Figure  8b). Under thin section, this rock unit consists of phenocrysts of K-feldspar within the fine-grained ground mass. The sample shows porphyritic fluidal texture and contains quartz and tiny plagioclase. It contains phenocrysts of feldspar and quartz in euhedral forms in a groundmass which is glassy but is full of tiny laths of alkali feldspar.

Dolerite
This rock unit has dark grey color with small specks of white and also it has medium grain size. This sub intrusive crystalline igneous rock has typical spheroidal weathering (exfoliation weathering) which is the splitting of rocks into a series of concentric shells by subsequent weathering processes. Much of the dolerite found in these places has a greenish tinge because of the presence of the mineral olivine.  Petrographically, the shape of most of the minerals is subhedral. Plagioclase feldspar is the only major mineral in the thin section. This plagioclase feldspar shows polysynthetic twinning under XPL. Also plagioclase feldspar is found as phenocryst surrounded by the other fine-grained minerals and this phenocryst form of plagioclase feldspar shows contact/simple twinning under XPL. The given thin section of the rock contains other minor minerals such as biotite and accessory minerals such as iron oxides and opaque. Finegrained groundmass of feldspar aligned sub parallel along their flow directions. Phenocrysts of alkali feldspars Oxidations of iron ores may be hematite or magnetite minerals.  T1-S1 T1-S2 T1-S3 T2-S1 T3-S2 T1-S1  T1-S2  T1-S3  T2-S1  T3-S2  Sr  716  823  195  405  631  Zr  257  224  1165  218  272  V  111  192  11

Trace and Rare Earth Elements Geochemistry
The analyzed samples from different volcanic suites exhibited different trace and rare earth element contents. Samples have trace and rare earth element contents of (195-823) ppm of Sr, (218-1165) ppm of Zr,

Geochemical Classifications
Different schemes of classifications were employed by petrologists for classifying various types of volcanic rocks. By utilizing the analyzed major, minor and trace elements data, a number of classification systems were conducted to identify compositional variations, tectonic settings and etc. Geochemical classification techniques such as TAS [4] and R1-R2 [5] are used to identify the chemical nature of rock samples.

I. R1-R2 classification
Based on the R1-R2 classifications same results are obtained where most of the samples are plotted on the basalt field and one sample lies on the alkali rhyolite field.

Tectonic Setting
Trace elements data is used for the discrimination of tectonic settings.Based on the Zr-Ti [13] plot of samples all lie on the with in plate tectonic settings.

Trace Rare Earth Elements Pattern
The analysed samples are characterized with 10-100 fold enrichment of most large ion lithophile elements (Rb, Ba, K and Pb) and high field strength elements, uranium and thorium while the other high field strength elements; Y, Zr, and Ti were relatively flat and show slight depletions under primitive mantle normalizations.  [15].
Where as the rare earth element spider diagram shows more than hundered fold enrichments in LREE and slight enrichement to depletions in HREEs.   Figure 17. Spider plot-REE Chondrite [3].

Discussion
Besides to the geological mapping data, petrographic and geochemical results confirmed that the study area is covered by rocks units such as aphanitic basalt, Porphyritic basalt, vesicular basalt, rhyolite and dolerite.
Petrographic studies revealed that the dominant minerals for most of the thin section samples are mafic in nature (olivine, pyroxene and plagioclase) except one sample taken from the rhyolite outcrop which consists of higher proportion of quartz and K-feldspar.
Based on the primitive mantle [15], trace elements spider diagrams show enrichment in LILE's (Large Ion Lithophile Elements) relative to the HFSE's (High Field Strength Elements) and this could be resulted from a low-degrees partial melting in mantle origin. Among all, sample T1-S3 (Rhyolite) and T1-S1 (porphyritic basalt) developed a maximum titanium negative anomaly that can be due to crustal contaminations.
La/Ta ratios have been used as an indicator of crustal contamination [7,10]. Leat PT.et al. [10] sugested that rocks with La/Ta ratios of <22 are derived from an asthenospheric source and have undergone little to no contamination from the continental crust or mantle lithosphere. Out of the analysed five rock samples two samples, T1-S3 and T2-S1 shows La/Ta ratios of greater than 22 which may indicate that rhyolite and dolerite rocks have been affected by crustal contaminations. As cited in [11], La/Nb of >1.5 indicates crustal contamination. Likewise, La/Nb ratios of the analysed rocks ranged from 0.64 to 1.28 (Table 1), implying further evidence of less crustal contamination. Only one sample from the rhyolite field has 1.72 ratio value due to it's crustal contamination.
The Eu/Eu* ratio of most samples are less than one (Table  1) wich further proves the rocks to have little crustal contamination and the source to be asthenospheric mantle. From the REE spider diagram its clearly visible that Eu is developing a little trough wich is due to small contaminations.

Conclusion
The study area is covered by rocks units such as aphanitic basalt, porphyritic basalt, vesicular basalt, rhyolite and dolerite. Mineralogically, most of the samples in the study area are mafic in nature containing minerals like olivine, pyroxene and plagioclase. The rhyolite outcrop consists of higher proportion of quartz and K-feldspar. Except one sample which lies on the rhyolite field, all the remaining samples lie on basic type of Igneous rocks (Hawwaiite, Mugearite and Basalt fields). All the outcrops in the study area lie on the with in plate tectonic setting.
The rock units in the study area are geochemically enriched in large ion lithophile elements relative to the high field strength elements that could be resulted from the low degree partial melting of the mantle. Except two samples (T1-S3 and T2-S1), all samples are derived from an asthenospheric source and have undergone little to no contamination from the continental crust but rhyolite and dolerite rocks units are strongly affected by crustal contaminations. Europium is developing a little trough wich