GEOLOGICAL, GEOCHEMICAL AND STRUCTURAL ANALYSIS OF THE PRECAMBRIAN ROCKS OF ADI NEBRI’ED AREA, TIGRAY, NORTHERN ETHIOPIA

Abstract

The Adi Nebri’ed structural domain is located at the northern tip of Ethiopia, bordering with Eritrea and form the southern part of Arabian Nubian Shield. It is covered by widely distributed low-grade metavolcano-sedimentary Neoproterozoic basement rocks, syn-to late-tectonic granitoids that belong to the Tsaliet and Tambien Groups of the northern Ethiopian stratigraphy.. The geotectonic and litho-stratigraphy of the metamorphic terrain in northern Ethiopia has been examined, frequently on a review-type. However, this area is exceptionally important, because it links the Nakfa Terrain in Eritrea and the Nubian Terrain in Eastern Sudan. It is also a major tectonic domain in Tigray, separating the Shiraro and Adi-Hageray blocks from West with that of the Chilla and Adwa blocks from East. The aim of the present research is to study geology, geochemical characteristics; and to constrain the geological structures and their deformation phases of study area; and to understand the tectonic history of the region. To achieve the aim of this study, review of previous work and searching maps of northern Ethiopia, and Eritrea were the prior works before the field campaign. Then, fieldwork follows to identify and describe the lithologic units, shear fabrics, structural measurement and mapping. 14 representative rock samples were taken for petrographic and geochemical analyses. Rock thin sections were prepared at the Geological Survey of Ethiopia for petrographic investigations and studied using petrological Leica Microscope. Furthermore, major, minor and trace elements data was generated using X-ray Fluorescence Spectrophotometer (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in the Geochemical Laboratories of the Australian Laboratory Sciences. Findings of this study indicate that Adi Nebri’ed is southern extension of Nakfa Terrain where it comprises of low-grade rocks. The geologic studies indicate that both foliated and non-foliated rocks are prevalent in the study area. These include phyllitic, graphitic- talc schist, micaceous slate, ferruginous sandstone, metalimestone, metavolcaniclastic and metavolcanic rocks displaying well-developed slaty cleavage, recrystallized fabrics, porphyroblastic,porphyroclast and xenoblast textures. The studied rocks demonstrate well-defined index mineral assemblages that record their metamorphic development and grade. These assemblages comprise minerals such as chlorite and biotite occurring in different combinations according to the degree of metamorphism. Chlorite-dominated assemblages typically indicate low-grade conditions, whereas the appearance of biotite signifies progression to medium-grade metamorphism Furthermore, the Adi Nebri’ed mafic and ultra- mafic rocks are present in appreciable qualitities and show significant strike continuation even outside the study area, perhaps as far to southwest of Eritrea, and western Ethiopia. A number of primary (bedding) and secondary structures like normal, inferred, and thrust faults, NE to SW striking foliations and shear zones, joints, quartz veins, and dykes are present in the study area. These structures share the same trend with the regional structural extensions of Northern Ethiopia and Eritrea. Geologically, these rocks represent volcano-sedimentary sequences that have undergone regional metamorphism and deformation during the Pan-African Orogeny. Geochemically, metavolcanic rocks typically show tholeiitic to calc-alkaline affinities, suggesting formation in island arc to back-arc tectonic settings, while metasedimentary units reflect mixed provenance and depositional environments. Structurally, the rocks exhibit well-developed foliations, folds, and shear zones, with multiple phases of deformation (D₁, D₂, and D₃) recorded through features such as slaty cleavage, and crenulation cleavage. The deformation history indicates progressive tectonic evolution, where early compressional events formed regional foliations and folds, followed by later transpressional to shear-related deformation. These structural and deformational features are consistent with regional N–S to NE–SW shortening recognized across the northern Ethiopian and Eritrean basement. Overall, the integration of geological, geochemical, and structural evidence demonstrates that these rocks share a common tectono-metamorphic evolution with the Arabian–Nubian Shield, reflecting arc accretion, crustal thickening, and subsequent deformation during the Pan-African orogenic cycle.