Petrology and geochemistry of bimodal volcanic rocks of Southern Lake Hayk area, northwestern Ethiopian plateau: implication for their petrogenesis

Abstract

This study presents and integrates field, petrological, and whole-rock geochemical (major and trace elements) data of the volcanic rocks from the Lake Hayk area to understand their petrogenesis. The study area's major lithological components include lower and upper basalt, rhyolitic lava, rhyolitic tuff, rhyolitic ignimbrite, and unwelded tuff. Petrographic analysis suggests that felsic rocks are dominated by quartz and well-developed sanidine (K-feldspar) phenocrysts with glassy groundmass, whereas mafic volcanic products are characterized by aphyric to porphyritic textures with the olivine and plagioclase dominant phenocryst. The area constitutes bimodal composition of flood basaltic to rhyolitic rock with scarce intermediate composition. Basalts have low Rb/Nb = 0.5-0.58, La/Nb = 0.88-1.06 and high TiO₂ = 2.08-3.04, basaltic andecite have higher Rb/Nb =2.7, La/Nb = 1.81 and low TiO₂= 1.96 and rhyolite Rb/Nb = 0.97-1.69, La/Nb = 0.51-1.08 and lower TiO₂ = 0.41- 0.71.The positive Ba and negative K anomalies testify amphibole mantle source. The basalts are characterized by low CaO/Al₂O₃ ratios (0.71–0.97) and relatively less fractionated and flat hree patterns with (TbN/YbN = 1.75-2.33) chondritic values. This suggests a mantle source mostly containing spinel rather than garnet. Rhyolites are characterized by a steep negative correlation in bivariate plots of MgO, Fe₂O₃, TiO, and CaO against SiO₂ and positive anomaly of Ta with slight Nb trough.  This suggests that Fractional Crystallization is the major process for the genesis of rhyolitic rocks, rather than crustal contribution, partial melting, and assimilation, producing rhyolitic rocks.