Geochemical characterization and regional correlation of Youngest Toba Tuff (YTT, 75 ka) glass shards, Purna alluvial basin, Central India

Abstract

Geochemistry of glass shards belonging to Youngest Toba Tuff (YTT) ash from eight different localities of Purna alluvial basin, Central India have been carried out to interpret magmatic and postdepositional processes led variations. Inter- and intra-basinal correlations of analytical data have also been made. The ash comprise of abundant glass shards, minor quantity of minerals, and fine-grained admixture. EPMA analysis of the glass shards showing high SiO2 (76.6-80.3 wt%) along with Al2O3 (11.7-13 wt%), K2O (3.2-4.4 wt%), and Na2O (1.9-2.7 wt%) reveals their rhyolitic composition. ICP-MS analysis of the same depicts enriched LREE and depleted HREE along with strong negative Eu anomaly; elements like Ba, Sr, Rb, Nb, Y, and Ce are also on higher side. Geochemical variations in major, minor, and trace elements exhibit fractional crystallization and post-depositional processes led heterogeneities in the magma constituents and glass shards, respectively. The same have been interpreted through the ratios of immobile elements i.e., FeO/TiO2 vs. Al2O3/TiO2, Sr/Nb vs. Ce/Y and mobile elements i.e., SiO2/Al2O3 vs. Na2O/K2O, FeO/Al2O3 vs. Na2O/K2O,Nb vs. Rb. Higher and lower concentrations of LREE and HREE respectively are due to fractional crystallization of magma. Concentrations of major, minor, and rare earth elements including trace are in good agreement with the glass compositions of Toba Caldera and other YTT bearing sites in India and Malaysia.