Minyahl Teferi Desta

This study investigates the petrogenetic evolution and geochemistry of volcanic rocks from the Anferfira section on the western margin of the Main Ethiopian Rift, a critical area for understanding magma generation and differentiation processes during continental rifting. The research employs detailed field investigation, spatial and systematic sampling, petrographic examination, and whole‐rock major and trace element geochemistry to characterise the volcanic suite and model its petrogenesis. The key findings of this research include that the Anferfira volcanic section in the northwestern Ethiopian plateau consists of compositionally bimodal rocks and a smaller proportion of intermediate rocks (trachyandesite) that occurred as an intrusion. Three successions of basaltic rocks overlain by rhyolitic and trachytic rock units were categorised as typical lithological formations. Petrographically, the volcanic rock types present in the study area include plagioclase phyric basalt, pyroxene phyric basalt, olivine phyric basalt, rhyolitic lava flow unit, rhyolitic ignimbrites and trachytes. The basaltic rocks are composed of plagioclase, pyroxene, and olivine phenocrysts with minor Fe‐Ti oxides. The common phenocrysts of trachyte rocks are sanidine. Rhyolites also contain sanidine minerals. Geochemically, the volcanic rocks are classified into basanite, tephrite, transitional basalt, trachyandesite, trachyte and rhyolites. Felsic rocks (rhyolites and trachytes) are the product of both crystal fractionation and crustal contamination processes. The geochemical parameters imply that the majority of the study area's felsic rocks share a common mantle source with basaltic magma sources that were generated by partial melting in a garnet‐peridotite mantle, whereas a few trachytic rock samples have a source similar to the partial melting spinel peridotite mantle. Petrogenetic models show that the Anferfira volcanic section evolved from the Afar mantle plume upwelling. The novelty of this work lies in demonstrating how variable mantle melting depths and subsequent crustal‐level differentiation processes can be reconciled within a unified petrogenetic model for rift‐related bimodal volcanism, providing crucial insights into the evolution of magma plumbing systems in continental rifts.

Ferropicrite (FeO* = 14.0 wt% and MgO = 13.9 wt%) and picritic ferrobasalt (FeO* = 14.7 wt% and MgO = 10.8 wt%) lava flows are found near Lalibela in the Oligocene (∼ 30 Ma) Ethiopian large igneous province (LIP) in association with ultratitaniferous transitional basalt and picrite of the second high–Ti (HT2) series. The dominant phenocryst in the studied samples is Mg–rich olivine (up to Fo88.9) with high CaO contents (to 0.42 wt%) without any kink band structure, indicating that the olivines are crystallized from a magnesian melt. Spinel microphenocrysts and inclusions in olivine are characterized by extremely high Cr# (79–84), moderate Mg# (18–51), moderate Fe3+# (11–39) and high TiO2 (3.6–14.8 wt%). The clinopyroxene phenocrysts are Mg#=74–88, TiO2 = 0.84–1.82 wt%, and Al2O3 = 1.2–3.2 wt% in the cores. The REE contents of clinopyroxenes display enrichment in LREE (LaN/YbN = 1.2–1.9) and MREE (EuN/YbN = 3.3–4.3) relative to HREE. Relative depletion of HFSE (e.g., Nb and Zr) is also observed. In general, these trace element characteristics of clinopyroxene are similar to those of the HT2 basalts. Some of the clinopyroxene crystals show strong reverse zoning with abrupt increase in Cr, Ni and Mg# from core to rim, which may be resulted from reaction of melt with mantle peridotite during magma ascent. Bulk rock chemistry of the studied samples exhibits very low Al2O3/TiO2 (1.8–2.2) and high Zr/Y (8.2–10.2) ratios. These may indicate the important role for garnet during melting and that the lavas were formed by a small degree of partial melting which apparently contradicts to the high Cr# of spinel. In view of low Cr/Al ratio of the bulk rock, the high Cr# of spinel suggests very high temperature of the magma (and the mantle plume). The origin of these ferropicrite and picritic ferrobasalt could be attributed to high pressure partial melting of peridotite–eclogite (pyroxenite) mixture that possibly incorporated recycled oceanic crust components.