Journal Geolollical Society of India. Vol· 22. Dec. 1981. pp. 593 to 596 Mafic and ultramafic volcanism of Ongarbira greenstone belt, Singhbhum, Bihar ANUPENDU GUPTA, ANIRUDDHA BASU AND DEEPAK SRIVASTAVA Geological Survey of India, Bihar Circle (E) Abstract The • Ongarbira Traps' within the Archean Iron-Ore Group metasediments in Singhbhum is characterised by the presence of mafic-ultramafic lavas, gabbro-pyroxenite with subordinate tuffaceous derivatives. The occurrence of pillow structures, intricate interlayering with sediments and association of cherts indicate subaqueous eruptive environment. Introduction In the course of a recent study to elucidate the tectonic history of the Ongarbira traps, a major volcanic assemblage in the Archean Iron-Ore Group of South Singh- bhum (Fig. 1), with interesting volcanic structures was identified indicative of an Figure I. Key geological map of Eastern India. I) Tertiary gravel; 2) Chota- nagpur granite; 3) Younger granites of Chakradharpur and Mayurbhanj; 4) Volcanics (greenstone belts); 5) Dhalbhum phyllites; 6) Kolhan Formation metasediments; 7) Chaibasa Formation metasediments; 8) Quartzite; 9) [ron Ore Formation metasediments; 10) Singhbhum granite with Newer Dolerites. DM - Dalma greenstone belt; CB - Chaibasa Formation; SZ-Shear wne; DJ -Dhanjori basin; OB - Ongarbira volcanics. eruptive environment. Though the Ongarbira trap has cursorily been described by earlier workers (Dunn, 1929; Sarkar and Saha, 1962,1977) as doleritic in composi- tion and basaltic flows within the Iron-Ore Group (3200 Ma - 2900 Ma; Sarkar and Saha, 1977) in South Singhbhum, its geological history, structure, petrographic nomenclature and relationship with other volcanic belts surrounding the Singh bhum craton remained far from being clearly understood. Geological setting In the dominantly northerly dipping sequence of low-grade greenschist facies rocks comprising phyllites, shales and quartzites, the' Ongarbira traps' are identified as a igneous suite (Fig. 2) that include ultramafic and basaltic lavas (pillow lava at 3*