Geochemistry and Geochronology of Phonolitic and Trachytic Source Rocks of the Axum Obelisks and Other Stone Artifacts, Axum, Ethiopia

Abstract

© 2016 The European Association for Conservation of the Geological Heritage. The northern Ethiopian Kingdom of Axum (ca. 100–700 AD) is known for its underground royal tombs and monolithic obelisks, which were probably carved and erected before/during the fourth century A.D. Most or almost all of these structures have been sculpted or made from only the phonolitic and trachytic rocks, which are part of the Ethiopian post-trap volcanic sequences. The post-trap volcanic suites have two end-members, the basalts and the nephelinites. Based on geochemistry, the nephelinites cover a wide spectrum of alkaline rocks. Our new 40Ar/39Ar dates for samples of phonolite and trachyte sampled at various levels of the sequence show that the Axum phonolites and trachytes erupted in the mid-Miocene, between 19.5 and 15.0 Ma. The rocks from which the monuments were sculpted correspond to the 19.29 ± 0.22 Ma old Gobo-Dura phonolites. Other stone-made artifacts have geochemical signatures similar to the 15.04 ± 0.53 Ma Adi-Tsehafi phonolitic trachyte. Despite the large numbers of quarry sites in the Axum area, most of the Axum relics were mined from three local quarries: two of the quarry sites are found at the flanks of the Gobo-Dura ridge, whereas the third quarry site is located at the Adi-Tsehafi village.