The Palaeoarchaean greenstone belt of the southern Iron Ore Group (SIOG) (3.51 Ga) in the Singhbhum Craton, eastern India, includes lowstrained and low-greenstone grade bimodal volcanics, Banded Iron Formation (BIF) and chromiferous ultramafics as enclaves within tonalitetrondjhemite-granodiorite (TTG) granitoids, collectively referred to as the Singhbhum Granite (3.4 Ga to 3.1 Ga). 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.

a b s t r a c tThe southern boundary of the Singhbhum Craton witnessed multiple orogenies that juxtaposed thin slice of granulite suite of the Rengali Province against the low-grade granite-greenstone belt of the craton along the EeW trending Sukinda Thrust. The strong southerly dipping mylonitic foliation within the granulites along with the prominent down-dip mineral lineation, suggest a northerly-verging thrusting. Mylonitized charnockite at the contact zone contains enclaves of mafic and ultramafic granulite, whereas granitoid gneiss contains enclaves of pelitic granulite. Mafic granulite enclaves preserve an early (S 1M ) foliation that formed during D 1M deformation. This rock, along with the host charnockite, were intensely deformed by the D 2M thrusting event and resulting S 2M foliation development in both rock suites. Geothermobarometric and pseudosection analyses show that the garnet-clinopyroxene-plagioclaseorthopyroxene-ilmenite-quartz assemblage in mafic granulite was stabilized at high-pressure and temperature conditions (10À12 kbar, 860 C) and was overprinted by a fine-grained assemblage of clinopyroxene-plagioclase AE hornblende that developed during decompression (down to 5.5e7.5 kbar). Matrix hornblende shows incipient breakdown to garnet-clinopyroxene-quartz intergrowth due to a granulite facies reworking. A contrasting P-T history is preserved in the pelitic granulite. The peak assemblage garnet-orthopyroxene-cordierite-quartz-rutile was stabilized at w6.0 kbar, 730 C which resulted from heating of the mid crust magma during the D 2M thrusting. The contrasting P-T histories could result from the tectonic juxtaposition of lower-and mid-crustal section during the D 2M event. Evidences of an early orogenic imprint within the mafic granulite imply involvement of deep continental crust during southward growth of the Singhbhum Craton.Ó 2014, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/3.0/).

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