Petrogenesis of the peralkaline Golja Ignimbrite (Main Ethiopian Rift)

The volcanic activity in the Main Ethiopian Rift, Ethiopia, displays a bimodal distribution of volcanic rocks, with abundant mafic and felsic compositions and rare or absent intermediate magmas. This characteristic, common to other volcanic settings (e.g., the Yellowstone caldera), is known as the Daly Gap. In the Main Ethiopian Rift, mafic and felsic magmas are rarely found together in the same volcanic system, except for few cases (e.g., Erta Ale). The most evolved magmas typically feed large explosive eruptions producing widely dispersed ignimbritic deposits and consequent caldera collapses of, while the mafic compositions are more commonly associated with fissural eruptions, monogenetic cones and lava flows. Consequently, the genetic relationship between the felsic and mafic magmas still represents a matter of debate with interpretations ranging from pure fractional crystallization to more complex scenarios involving magma mixing and crustal melting. In this contribution, we present the first volcanological, petrological and geochemical characterization of a pyroclastic sequence erupted from an uncertain location in the Main Ethiopian Rift at ~1.3 Ma, the Golja Ignimbrite (GI). The GI was emplaced during a large explosive eruption involving about ~100 km3 of crystalpoor magma. Unlike other eruptions from the region, the GI encompasses basaltic and intermediate compositions (trachydacites and trachyandesites) in addition to peralkaline rhyolites, which represent the main component. Therefore, the GI represents an ideal case study to investigate the genesis of the rhyolitic products and their relationship with the more mafic compositions. To achieve this goal, we conducted in-situ and bulk 87Sr/86Sr analyses of feldspar crystals as well as 87Sr/86Sr and 143Nd/144Nd of matrix glasses within the different units of the GI. Our results show that Ca-rich plagioclase crystals hosting basaltic melt inclusions display Sr isotopic compositions akin to the Main Ethiopian Rift basalts and slightly more radiogenic than those of the Afar Plume. Bulk 87Sr/86Sr of K-feldspars and matrix glasses exhibit notable variations ranging from mantle-like signatures to more crustal-like ones. Bulk 143Nd/144Nd of matrix glasses do not show significant variations between mafic and evolved compositions within the ignimbrite, but are significantly lower than typical Afar Plume mantle values. Overall, our data point towards a complex geochemical and isotopic evolution, where the most evolved and intermediate magmas are genetically linked to the mafic compositions via fractional crystallization with assimilation of crustal material mostly occurring in the deep part of the plumbing system. Repeated isotopic analyses of matrix glasses before and after leaching show differences in the 87Sr/86Sr ratios, with the leached samples being less radiogenic than their unleached equivalents, despite the absence of textural or geochemical signs of post-eruptive alteration. Notably, the amplitude of the shift in Sr isotopic compositions decreases from matrix glasses within pumiceous samples to those from the vitrophyre indicating that syn- or post-eruptive fluid circulation might have modified the original Sr isotopic signatures. Although these results do not significantly affect our interpretation, we suggest that caution should be taken when analysing Sr isotopes even in fresh-looking vesicular glasses.