Abstract

Dabbahu is a composite volcano at the north end of the active Manda-Hararo segment of the Afar Rift in northern Ethiopia. We present 93 new whole-rock analyses, mineral analyses from 65 samples, and 9 new 40Ar39Ar dates for rocks ranging in composition from mildly alkaline basalt through trachyandesite to peralkaline rhyolite (comendite and pantellerite) erupted from Dabbahu. These data, supplemented by a new geological map, are used to provide insights into the evolution of the volcano. We show that Dabbahu has been active for over 67 k.y., but an apparent hiatus occurred between the eruption of comendite (29 ka) and pantellerite (ca. 8 ka) lavas. Mineral data for olivine, clinopyroxene, plagioclase, and alkali feldspar show a remarkably extensive range of solid solution across the rock suite consistent with protracted fractionation from basalt to rhyolite. The parental basalt is compositionally similar to recent rift-related basalts in the Manda-Hararo rift, with low initial H2O contents (<1 wt%). Closed-system fractionation increased H2O contents of residual liquids sufficiently for some rhyolites to erupt both explosively and effusively. The diverse magma types were erupted from a relatively closely spaced network of vents and fissures. Field evidence indicates that magmas were not erupted in a simple fractionation sequence. Some mixing occurred between magmas of less-evolved compositions and more-evolved compositions shortly prior to, or during, eruption. The differentiation of basalt to rhyolite must have occurred on time scales that were relatively short compared to the lifetime of the volcano, probably due to the small volumes of basalt intruded into the crust and consequently enhanced cooling and crystallization rates. A network of stacked sills or closely spaced dikes in the shallow to midcrust represents the most plausible configuration of the subvolcanic plumbing system. Input of new magma batches into such a system may serve as a key eruption trigger at Dabbahu.

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