Volcanological and petrological characterization of the Golja Ignimbrite (Main Ethiopian rift)

The Main Ethiopian Rift (MER) is an ideal natural laboratory to study magmatism related to rifting. The MER is an active magmatic rift that records variations in rift evolution from mature rifting in the north to less evolved rifting southwards. The Plio-Pleistocene volcanism in the MER is mainly characterized by eruptions of mafic products (i.e., transitional basalts) associated with cinder cones and lava flows, alternating with ignimbritic eruptions emplacing large volume of felsic products (i.e., peralkaline rhyolitic and trachytic pyroclastics) with the formation of large calderas. After widespread Miocene-Pliocene volcanism, the Quaternary magmatic activity became mostly localized in the MER axis, with products showing a typical bimodal composition (i.e., dominantly basaltic and rhyolitic composition) with a notable compositional gap, also known as the Daly gap, which remains a poorly understood aspect of the rift-related magmatism. In this context, we present the first volcanological and petrological characterization of the Golja Ignimbrite, a crystal-poor (<5% crystals of qtz+K-feld+pl+cpx+aen), low aspect ratio ignimbrite sourced from the MER floor, that crops out over ~400 km2 and has an estimated bulk-tephra volume of ~100 km3. The Golja Ignimbrite pyroclastic sequence is characterized from bottom to top by: 1) a coarsening upward basal fallout layer; 2) an obsidian vitrophyre with rare, scattered fiammae; 3) a weakly to partially welded, lithic-rich PDC deposit and 4) a thick, unwelded PDC deposit containing white and banded pumices together with black scorias. Both density and welding degree are relatively low at the transition between the basal fallout and the vitrophyre (1.32 ± 0.03 g /cm3) and reach their maximum in the vitrophyre (2.38 ± 0.02 g/cm3), where most of the glass shards are not distinguishable from the groundmass glass. Subsequently, density tends to decrease up-sequence (down to 1.20 ± 0.01 g/cm3 at the top), as indicated by the increasing occurrence of glass shards without a preferential orientation, glass spherules and vesicular juvenile clasts. 40Ar/39Ar dating of single K-feldspars from a white pumice and a black scoria yielded and age of 1.159 ± 0.006 Ma relative to the Fish Canyon Tuff (FCs) sanidine age 28.02 ± 0.28 Ma (Renne et al., 1998). The ignimbrite composition plots in a transitional field between the subalkaline and peralkaline series, which is typical of magmas associated with continental rifts. Compositional differences have been observed between the various juvenile clasts with rhyolitic white pumices and, trachy-basaltic to trachy-andesitic black scorias. Mingled, streaky pumices range between trachytes and rhyolites, while truly basaltic compositions were only found in melt inclusions contained in bytownitic plagioclase crystals. The presence of intermediate compositions is an uncommon feature for MER magmas and our data support that they are likely due to two different processes: fractional crystallization and mingling between magmas with strong compositional differences (i.e, basalts and rhyolites) occurring before eruption. Such evidence places important constraints on the evolution of large magma reservoirs in this peculiar geodynamic setting.