Structures, Petrography and Geochemistry of Deccan Basalts at Anantagiri Hills, Andhra Pradesh

The Deccan basalts of Anantagiri hills near Vikarabad are characterized by the occurrence of a complete and continuous sequence of colonnade structures below the laterites. These structures have enabled division of flow units 2, 3, 4, 5, 6 from the bottom upwards, into three well defined zones - Lower Colonnade Zone (LCZ); Middle Entablature Zone (MEZ) and Upper Colonnade Zone (UCZ) underlying a 5-10 m thick laterite cover. These fine to medium grained basalts composed of plagioclase, clinopyroxene, + olivine, Ti-magnetite, ilmenite, glass, cholorophaeite and zeolites commonly show intersertal, glomeroporphyritic and rare sub-ophitic and porphyritic textures. On the A-F-M ternary plot these basalts plot in the iron rich tholeiitic field. REE plots of these rocks define patterns with mild LREE enrichment and negative Eu anomaly. Their primitive mantle normalized multi-element patterns show mild positive LREE coupled with slight LILE depletion, positive Ba, Ta and Pb peaks. The TiO2/Yb vs Nb/Yb ratios in these tholeiites suggest a slightly enriched source with an MORB affinity. The major and trace element signatures of these tholeiitic basalts occurring at Anantagiri hills and the adjacent low lying areas in south-eastern Deccan Volcanic Province, have broad affinities with the basalts of Ambenali Formation with an overlap on basalts of Poladpur Formation, though with minor variations suggesting geochemical similarity with these two type area basalt formations.