A 3.51Ga bimodal volcanics-BIF-ultramafic succession from Singhbhum Craton: Implications for Palaeoarchaean geodynamic processes from the oldest greenstone succession of the Indian subcontinent

The Palaeoarchaean greenstone belt of the southern Iron Ore Group (SIOG) (3.51Ga) in the Singhbhum Craton, eastern India, includes low-strained and low-greenstone grade bimodal volcanics, Banded Iron Formation (BIF) and chromiferous ultramafics as enclaves within tonalite-trondjhemite-granodiorite (TTG) granitoids, collectively referred to as the Singhbhum Granite (3.4Ga to 3.1Ga). The succession comprises, from base to top, a lower unit of massive and pillowed basalt conformably overlain by dacitic lava and pyroclastics which in turn is overlain by a major BIF unit. The ultramafics are juxtaposed with the volcanics-BIF succession along a thrust fault. The lithological association of pillow lava, subaqueous dacitic lava and pyroclastic rocks and BIF collectively, suggests that the entire succession was deposited in a deep-marine depositional setting. The ash-poor dacitic volcanic rock succession with evidences of a transition from suppressed-volatile deep-water lava flow and pyroclastics to more evolved mass-flow deposits with increasing trend of subaqueous flow transformation, records a transition from a deep-water low-height volcanic chain to a shallower subaqueous eruption in an aggradational volcanic chain. Geochemical proxies from the bimodal volcanics and ultramafics showing enrichment of La/Nb, Th/Nb, Th/La, Ba/La, Pb/Ce, depletion in Nb-Ta relative to neighbouring REE, together with tectonic discrimination criteria using Nb, Y, Zr, Ti compositions, suggest an extending oceanic arc-forearc geodynamic setting similar to many of the Phanerozoic supra-subduction zone ophiolites where ophiolite development in the extending upper plate in a relatively short time span is facilitated by slab rollback processes. The positive Eu-anomaly together with high Y/Ho values from the BIFs also suggests their deposition in close proximity to spreading centres that might have developed over a rifted arc. The bimodal volcanic rock-BIF-ultramafic succession of the SIOG with evidence of a convergent margin geodynamic setting is an important example for Palaeoarchaean plate tectonic processes operating on Earth. © 2011 John Wiley & Sons, Ltd..