A Note on the Occurrence of Ultra-Potassic Volcanic Rocks in the Proterozoic Cuddapah Basin

This note records for the first time the occurrence of ultra-potassic volcanic rocks in the Proterozoic Cuddapah basin. The ultra-potassic rocks occur within the tuffaceous sequence of the Pullampet Formation of Nallamalai Group of Cuddapah Supergroup in the area around Naiduvaripalle in Cheyyeru valley in the Cuddapah district of Andhra Pradesh. Petrographic studies reveal that the rock is aphanitic with phenocrysts of alkali feldspar set in a finely crystalline groundmass of slender feldspar microlites and chlorite. The rock displays typical ultra-potassic chemical characters in having high K₂O>8%, K₂O:Na₂O >25% and MgO >3%.