Genesis of Acid Volcanism in Continental Rift Environments: A Case in the Ethiopian Rift Valley

Abstract

Intracontinental regions affected by extensional tectonics are often associated with widespread magmatic activity. In most cases, acid volcanics are by far the most abundant lithologies, followed by minor amounts of basalts and scarce intermediate volcanics (Kampunzu and Mohr, 1991). The presence of a Daly gap and the huge volumes of salic rocks have made the genesis of acid magmas in continental rift areas a much debated issue (see Davies and MacDonald, 1987). One class of hypothesis suggests that acid magmas are the products of partial melting of the lower crust, a hypothesis which explains the large predominance of acid over basic and intermediate rocks. Other hypotheses suggest a derivation from marie parents by extensive fractional crystallization or AFC. The Ethiopian Rift Valley is a key place where these problems can be investigated. In this zone extensive peralkaline rhyolitic and trachytic volcanism is associated with minor basalts and scarce intermediate rocks (Di Paola, 1972). In this paper we report on petrological and geochemical data on a suite of basic to acid rocks coming from Gedemsa, a composite volcano sited in the norther part of the Ethiopian rift. The bearing of these data on the genesis of acid magmas in rift environments is discussed.