Evolution of the Western Ethiopian Shield revealed through U-Pb geochronology, petrogenesis, and geochemistry of syn- and post-tectonic intrusive rocks

Abstract

Ethiopian basement formed between 850 and 450 Ma during the East African Orogen and represents the junction of the two distinct basement types found in Africa and Saudi Arabia: the low-grade Arabian-Nubian Shield and the high-grade Mozambique Belt. While many other localities along the East African Orogenic belt are well studied and despite the huge potential insight it could grant to the orogen’s evolution, Ethiopian Basement is poorly understood. This study aims to understand the tectonic evolution of the East African Orogen in Ethiopia through the examination of the age, origin, and evolution of previously unmapped syn- and post-tectonic plutons intruded into the Western Ethiopian Shield’s eastern edge. Dabana granite pluton crystallization ages were determined through U-Pb geochronological dating, and structural history was revealed through field mapping and thin section petrographic analysis. Whole rock elemental composition was determined through XRF (X-ray fluorescence) and ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) methods to investigate petrogenesis and chemical evolution of the plutonic rocks. Results indicate this pluton formed during three intrusive periods under distinct tectonic domains that represent an evolving convergent margin. The oldest granite dates to 797.6 Ma and is associated with hydrous melting during volcanic arc subduction, followed by post-subduction related anhydrous magmatism at 774.6 Ma. The youngest granites date to 635–639 Ma and are evidence of late-stage crustal thickening in the final stage of collision. Two metamorphic episodes are recorded in the structure and petrography, which formed gneiss during high-grade metamorphism at 775.22 Ma and low-grade deformation around 630 Ma. This study presents the first geochronological and geochemical data from plutonic rocks in the study area, and grants valuable insight into the dynamics and evolution of the East African Orogen in Ethiopia.