Detailed study of Golja's Ignimbrite (Main Ethiopian rift): eruptive and depositional mechanisms, characterization and reconstruction of the possible genesis of juvenile products

The Main Ethiopian Rift (MER) represents an ideal natural laboratory to analyze the evolution and dynamics of continental break and separation from the initial phase in the South to the continental break-up in the North (Hayward and Ebinger, 1996; Corti, 2009). The process that regulates the distribution of volcanic products is generally bimodal, with the absence or scarcity of products with intermediate composition, known as Daly Gap, still not fully understood (Gasparon et al., 1993). We describe here the stratigraphic, textural and compositional features of the Golja Ignimbrite (GI), a Low Aspect Ratio Ignimbrite (~1.2 Ma) which characterizes the recent volcanic succession of the Asela-Langano area. The main sequence of the GI, 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 the top. This basal portion of the sequence is followed by a weakly to partially welded ash flow, rich in lithics, obsidians, pumices, crystals and glassy fiamme, passing to a thick, unwelded deposit. We developed different types of analyses: measures of density and aspect ratio of the juvenile material to characterize the typical sequence from the sedimentological point of view, optical and SEM observations of thin sections to describe the textural characteristics of the matrix, bulk rock compositions, SEM-EDS data of glass and minerals in the different juvenile fractions, EMPA data on melt inclusions (MI) hosted in quartz and feldspar crystals. The presence of juvenile products with different characteristics (obsidian chips, black scorias, white and banded pumices) has aroused considerable interest, providing the prerequisites for hypothesizing a mixing process between magmas of various compositions. The data collected were discussed in terms of eruptive and depositional mechanisms, as well as of the possible processes of formation for the different juvenile fractions. We suggest that the evolution of these magmas was mainly driven by a complex process of fractional crystallization, which played a leading role in the transition from basaltic (MI) to trachyandesitic magmas (black scorias), while the observed trachydacytic compositions of the banded pumices could be the result of a mixing process between trachyandesitic and rhyolithic magma (white pumices and obsidians). Differently from other ignimbrites from this sector of the Rift, characterized by nearly homogeneous, peralkaline rhyolitic to pantelleritic compositions, the GI represents a clear example of an eruption that involved magmas of different compositions, generally not well represented in the products of the MER.