The Gondwanaland was assembled during the Neoproterozoic time from two fragments, the East and West Gondwana along the Pan-African Mozambique Belt of East Africa (McWilliams 1981). This idea is supported by the lithologic association’s characteristic of the Mozambique Belt and the Arabian- Nubian Shield. The Pan African Mozambique Belt of East Africa is also called East African Orogen (EAO) (Stern, 1994). It consists of metamorphosed and deformed rocks of the higher grade Mozambique Belt (MB) in the south and low grade Arabian-Nubian Shield (ANS) in the northern part (Stern, 1994; Kroner and Stern, 2005). The fragments of Neoproterozoic rocks are now exposed in the African, Madagascar, Arabian and Indian Plates, once they were intact until the dispersal of Gondwanaland (Johnson et al., 2011). The Precambrian rocks of northeast Africa and western Arabia is called the Arabian Nubian Shield (ANS). The ANS is one of the two major terranes in northern and eastern Africa that developed during the Neoproterozoic time (Kroner, 1985; Stern, 1994). It extends from Jordan and Israel in the north, through to Ethiopia and Sudan in the south (Berhe, 1990; Kroner et al., 1991; Stern, 1994; Teklay et al., 1998, 2001; Kroner and Stern, 2005). But the Mozambique Belt is exposed in the south part and comprises mostly pre-Neoproterozoic crust with a Neoproterozoic early Cambrian tectonothermal overprint (Fritz et al., 2005; Collins, 2006).
The Ethiopia Neoproterozoic basement rocks are exposed in northern, western, southwestern, southern and eastern parts of the country. They are part of southern ANS extensions which are juvenile Neoproterozoic crust formed during the Mozambique Ocean closure accompanying greater Gondwana formation (Asrat et al., 2001; Stern, 1994, 2008; Fritz et al., 2013) (Fig.1.1). The low grade basement rocks of northern Ethiopia or Tigrai region are subdivided into Tsaliet and Tambien Groups (Beyth, 1972). The former Group is older, mainly contains metavolcanic/metavolcano clastic sequence, agglomerates, tuffs and mafic to felsic volcanic flows while, Tambien Group is a younger unit, mainly exposed in a series of synclinal inliers overlying the Tsaliet Group. It consists of metasedimentary rocks of argillite and carbonate composition.
Figure 1.1. Geological map of Ethiopia. The white lines and white irregular shapes are rivers and lakes, respectively. The map modified from Tefera et al. (1996) cited in Stern et al. (2012).
The Tsaliet and Tambien Groups are recorded in various tectonic structures that developed during the East African Orogen (Stern, 1994; Meert, 2003; Johnson and Woldehaimanot, 2003). There are two phases of deformation in particular are commonly recognized in these rocks as D1 and D2 deformations (Alene, 1998; Alene and Sacchi, 2000). According to Sifeta et al. (2005), the provenance, weathering condition of the source area and tectonic setting of the sediment is determine by studies the geochemical of the metasedimentary rocks where as the tectonic setting of the volcanic rocks and its magma type are also determine and understand by studies the geochemistry of metavolcanic and micrograntic intrusion rocks (Alene et al., 2000). The present study area of Tahtai Logomti area consists of both Tsaliet and Tambien groups of the northern Ethiopia rocks. The structural, petrography and geochemical studies of these rocks are important to evaluate the grade of metamorphism, phase of deformation and understand the tectonic evolution of the area.
1.2. Description of the study area
1.2.1. Location and accessibility of the study area
The study area is located around Tahtai Logomti area, Central Tigrai, northern Ethiopia which is far about 930 km from Addis Ababa. Geographically, it is bounded by UTM coordinates of 490000m to 497000m E, and 1548000 m to 1555000 m N latitude and longitude, respectively with areal coverage about 49.24 km2 (Fig. 1.2). It is accessed by the main asphalt road running from Mekelle to Adwa through Mai-Kenetal to the study area and it is accessed by different trails (Fig.1.2B).
Figure ?1.2. Location and accessibility map of the study area extracted from DEM of Ethiopia using Global Mapper-11, extracted as map by Surfer 10 software (with contour interval 50m).
1.2.2. Drainage pattern and physiography
The study area is characterized by sub parallel, radial and dendritic drainage patterns (Fig.1.3A). But the main river shows meandering like flow. The area has minimum and maximum elevation of 1620 m and 2020 m above sea level respectively (Fig 1.3B)
Figure ?1.3. A) Drainage map of the study area. B) Physiographic map of the study area extracted from DEM of Ethiopia using Global Mapper-11 and changed to 3-D using Surfer software.
1.2.3. Climate condition and soil
The climate condition of the study areas is mainly dominated by semi arid to arid environments. The wet season commonly prevails in the months from May-August with total monthly rain fall 95.7-219.2mm. However, there is no rain fall during September, October, November, December, January and March. Therefore, the dry season lasts from September to March with the maximum temperature varying from 26.2 up to 30.6 C0 (From: National Metrological Agency of Tigrai (NMAT)). The study area is covered by thin and poor fertile soil but at the margin of the main stream it is relatively thick and fertile and serves as agricultural crop production for the local people.
Figure ?1?.4.A) A bar graph represents the average monthly rain fall and B) is also show the minimum and maximum temperatures of Adwa and its surrounding area (Source: NMAT, 2017).
1.2.4. Vegetation and wildlife
The study area is characterized by sparsely distributed vegetation coverage. It consists of acacia, bushes and shrubs types of vegetation. The vegetation is mostly occur nearly the streams and in the lowland areas, it composed of big and small trees and they serve as invaluable shading for the local communities and animals during the hottest period.
Large settlements are located either outside or near to the margin of the study area. Tahtai Logomti and its surrounding area is mostly sparsely populated. However, in the central and southeast part of the study area, it is densely populated than the southwestern and western parts. The livelihood of the people is entirely based on mixed and traditional agricultural practice that includes both crop cultivation and animal husbandry. Generally, the people commonly grow or cultivate different types of crops and cereals including sorghum, barley teffe, maize and millet.
1.3. Problem of the statement
The Precambrian geology of northern Ethiopia is defined by the presence of low grade metamorphic rocks (Johnson and Woldehaimanot, 2003), which consists series of thick heterogeneous volcano-sedimentary rocks (Asrat et al., 2003). These rocks have good correlation with the low grade metamorphic terrane of southern and western Ethiopia (Beyth, 1972; Kazmin, 1972) and they are comprises of metavolcanic/metavolcano clasts, slate and carbonate (Beyth, 1972). The metavolcanics mineral assemblage represents peak regional metamorphism (pumpellyite–actinolite to lower greenschist facies) that was attained during the D1 deformation (Alene, 1998; Alene and Sacchi, 2000). The previous researchers have worked on this region basement regarding geology and geochemistry of the rocks (Tadesse, 1997; Alene 1998; Tadesse et al., 1999, 2000; Alene et al., 2000, 2006; Miller et al., 2003, 2009; Ayele and Gangadharan, 2016). These researchers studied the geology and geochemistry of the Neoproterozoic rocks on small scale (1:50,000). But the present study focuses on detailed geological and structural mapping of lithological units at larger scale (1:25,000) and more specific petrographic studies to determine the metamorphic mineral assemblages, facies, metamorphic grade, metamorphic event and deformation phases in order to better understand the geological evolution of the area.
1.4.1. General objectives
The main objective of the study is to describe and understand the metamorphic mineral assemblages, deformation history and geochemistry of Tahtai Logomti area, Tigrai northern Ethiopia in line with the evolution of the ANS.
1.4.2. Specific objectives
? To produce detailed geological and structural map of the study area (at 1:25,000 scale).
? To understand/determine the degree of metamorphism based on the mineral assemblages and differentiate the phases of deformation by examining the macro/micro structures in petrographic study.
? To understand the tectonic setting and petrogenesis based on the geochemistry (major, minor and trace elements) of the rocks in order to understand the evolution of the area.
In order to achieve the above mentioned objectives, the following methodology has been used.
1.5.1. Data collection
Data from the literature including geological and topographic maps, satellite imagery, relevant published and unpublished literatures related to the study area were collected. Lithological and structural data were also collected from the field. The GPS recording of each lithological unit was collected in order to compile the geological map of the area. These different exposures were described in terms of mineral composition, texture, rock fabric and outcrop-scale geological structures (folds, faults). Representative fresh rock samples were also collected from each lithological unit for thin section and geochemical analysis. Outcrop photographs and location were taken by using GPS and camera at each sample station. A topographic map with 1:25,000 scale has been used as a base map during field traverses. The topomap has been supplemented with a satellite imagery, which is downloaded from website www.golvis.USGS.gov.
1.5.2. Petrographic analysis
Total of 19 samples were sliced into two parts by using rock cutter at thin section laboratory of Geological Survey of Ethiopia (GSE), Addis Ababa central laboratory (one piece each for thin section, one piece for geochemical analysis and the remaining is placed as reserve). Nineteen samples were prepared for thin sections at this laboratory however; ten samples were used for geochemical analysis at Australia laboratory science (ALS). The rock samples prepared for thin section are, described using transmitted light microscope in petrographic laboratory of School of Earth Science, Addis Ababa University.
1.5.3. Structural data analysis
All the structural data that have been collected for different structural elements such as foliation, folds and fractures are plotted and analyzed using stereographic projection software and Arc GIS software. The geological and structural maps are generated using Arc GIS software, which show different lithological units and structures.
1.5.4. Geochemical Analysis
Ten samples were selected for geochemical analysis to determine the concentration of the major, rare earth element and trace elements. These samples are submitted to the Australian Laboratory Science (ALS) to determine the concentration elements.
1.5.5. Data processing
The Enhanced thematic satellite images were digitally processed by using ENVI-4.5, SURFER-10, ArcGIS-10 software packages to increase the resolution and to produce true ground color in order to determine the lithological contacts and structures. The Geological map and cross-section were produced using Surfer-10, Global mapper-11and ArcGIS-10 at 1:25,000 scale.
1.6. Significance of the research
As mentioned earlier, the Neoproterozoic basement rocks of northern Ethiopia have been studied by many researchers at smaller scales. However, the present study focuses on the detailed large scale geological and structural mapping (1:25,000), understanding of the phases of deformation, metamorphic events and geochemical data important to fill the existing gap regarding the area. Mostly, this present study is important to determine the metavolcanic and metasedimentary rocks of the western limb of the Mai Kenetal syncline. This new result of geological and structural mapping and stratigraphic section of the area make a contribution to determine the tectonic history of the Precambrian rocks of the Tigrai region, specially the study area. In addition to that, this study is important for future individual researchers and organization.
1.7. Review of previous work
Many geological investigation have been carried out in northern Ethiopia by several authors (e.g. Beyth, 1971,1972; Kazmin, 1975; Kazmin et al., 1978; Garland, 1980; Tefera et al., 1996;Alene, 1998, Alene et al., 2000, 2006; Tadesse,1997;Tadesse et al., 1999, 2000; Sifeta et al., 2005; Miller et al., 2009, Swanson-Hysell et al., 2015). According those researchers, most Precambrian rocks of northern Ethiopia are belongs to two major stratigraphic groups. These are Tsaliet and Tambien groups with minor, locally important and younger stratigraphic formations, including Didikama, Sheraro and Matheows formations in younging stratigraphic sequences (e.g. Beyth, 1971; Kazmin, 1972; Kazmin et al., 1978; Garland 1980; Tefera et al., 1996). The Tsaliet Group is characterized by greenschist facies metamorphism of originally volcanic and volcaniclastic rocks.
The Tambien Group is mainly exposed in several synclinal inliers: Mai-Kenetal, Tsedia, Chehmit and Negash synclines (Alene et al., 2006, Garland, 1980). It consists of metamorphosed argillite and carbonate units. The former two groups of rocks are recorded as tectonic structures that are developed due to rifting, arc accretion and terrain amalgamation processes in the East African Orogen during Neoproterozoic time (Stern, 1994; Meert, 2003; Johnson and Woldehaimanot, 2003). According to Alene (1998), and Alene and Sacchi (2000), these groups of rocks show D1 and D2 phase of deformation and it correlates with post-accretion structures of the ANS (Abdelsalam and Stern, 1996).
Six tectono-stratigraphic blocks were also identified in west-central Tigrai such as Sheraro, Adi-Hageray, Adi-Nebrid, Chila, Adwa and Mai-Kenetal blocks (Tadesse, 1997). Shiraro and Mai-Kenetal blocks are dominated by weakly metamorphosed and deformed, post accretionary clastic and carbonate deposits. The Adi-Hageray block situated sharp topographic break of eastern Shiraro block and it consists of extensively deformed, greenschist facies, metasedimentary and metavolcanic rocks. The chila block contains mainly sedimentary in origin rocks but Adi Nebrid block is metavolcanic (intermediate to acidic lava, pyroclastics) and metasedimentary rocks. Whereas Adwa block comprises low grade, intermediate to acidic metavolcanic, chert, pyroclastic and metasedimentary rocks. A series of NE-SW synclinal basins filled with weakly metamorphosed and deformed shale and limestone (Beyth, 1972) unconformably overlie Adwa rocks. The succession in this block is intruded by composite granitoids of batholitic size (Kazmin, 1972; Beyth, 1972).