Contents of Volume

Abstract

There are four Neoproterozoic ophiolitic belts in southern Ethiopia—Megado, Kenticha, Moyale-El Kur and Bulbul. The mafic rocks, which form the bulk of these belts, are dominantly subalkaline, low-K, low-Ti tholeiitic basalts (LOTI). This study, for the first time, shows occurrences of boninites in the Moyale-El Kur Belt and in the Geleba area, in addition to the already known occurrence in the Megado Belt. The mafic rocks of the Bulbul Belt consistently exhibit high-Ti, high-K calc-alkaline basaltic geochemistry, similar to that reported from the Kenticha Belt. Several samples from the Geleba area and Moyale-El Kur Belt also show high-K, high-Ti calc-alkaline geochemistry.The REE patterns of the Megado low-Ti tholeiites are similar to arc-tholeiites of the Southern Sandwich Islands, whereas the Moyale tholeiites show patterns similar to back-arc tholeiites of the Scotia Sea Rise. The most distinctive features in spider diagrams of the LOTI include the selective enrichment of Sr and Ba, and the relative lack of enrichment for K, P, Zr, Ti, Ce, Sm±Y, similar to those of oceanic basalts from supra-subduction zone (SSZ) settings, where boninitic and tholeiitic magma mixing could occur.The boninites are dominantly high-Ca boninites and are more akin to tholeiites and boninites from known marginal basins, such as the Mariana fore-arc basin. The presence of boninites in association with low-Ti tholeiites in the Moyale-El Kur Belt suggests that this belt also represents an ophiolite sequence formed in a SSZ setting similar to the Megado ophiolite. Concentrations of most immobile elements (Ti, Nb, P, Ce, Zr, Th, V) in the Megado rocks are lower than in the Moyale rocks. This may imply that the magnitude of subduction was less during the formation of the Moyale ophiolite than in the case of the Megado ophiolite.On tectonic discrimination diagrams, the Megado and Geleba mafic rocks and the boninitic rocks of the Moyale-El Kur belt consistently show fore-arc basin affinity, whereas the Moyale-El Kur tholeiites plot into fore-arc basin and back-arc basin fields. The Bulbul mafic rocks consistently have calc-alkaline basaltic affinity, suggestive of a continental-arc setting. However, high-K calc-alkaline basalts are also known from a SSZ setting, such as the New Hebrides.The geochemical interpretation presented in this study, together with discussion of lithological association and geochronological and structural data, is used to decipher the tectonic evolution of the Neoproterozoic of southern Ethiopia.