Stratigraphy, textural and compositional features of the Golja Ignimbrite (Main Ethiopian Rift)

The Plio-Pleistocene volcanism of the Main Ethiopian Rift is mainly characterized by large eruptions of mafic products (transitional basalts) associated with cinder cones and lava flows alternating with greater quantities of felsic products (peralkaline rhyolitic and trachytic pyroclastics), marked by the formation of large calderas in the Rift floor. The distribution of products is generally bimodal, with a limited occurrence of intermediate products. The processes at the base of this bimodality, known as Daly Gap, are still debated. We have characterized the Golja Ignimbrite (~1.2 Ma) in stratigraphic, textural and compositional terms. The main sequence of the ignimbrite, sourced from an uncertain position within the Rift, is characterized by a coarsening upward basal fallout, overlain by an obsidian fiamme-bearing vitrophyre, brecciated at top. This basal portion of the sequence is followed by a weakly to partially welded ash flow, rich in lithics, obsidians, pumice, crystals and glassy fiamme, passing to a thick, unwelded deposit with white pumice and black scorias. Density of the deposit has its maximum value in the basal vitrophyre and progressively decreases, paralleling the variation observed in the aspect ratio of fiamme and black scoria. The microtextural study of the matrix allowed us to describe different eutaxitic structures along the sequence. Preliminary results obtained from microanalysis of glass and minerals suggest the contemporaneous presence of three different types of juvenile glass: black, low-SiO2 trachyandesite, microcrystalline glass; light-coloured, microlite-free, comenditic to pantelleritic glass; black, locally banded, intermediate high-SiO2 trachyandesite glass. Basaltic glass is also sometimes found in plagioclase-hosted melt inclusions. The finding of partially evolved melts, together with textural evidences of magma mixing, represent a not common feature for the products of the Main Ethiopian Rift.