Archean Metallogeny and Crustal Evolution in the East Indian Shield

Abstract

East Indian Shield bears the evidences of Archean metallogenic and crustal evolution like the other major shield areas of the world. The primordial continental crust in the form of more silicic bodies were floating like metallurgical slags over the hotter and denser ultramafic-mafic lithosphere (UM-ML) before 3800 Ma. Earliest sediments, now represented by the Older Metamorphic Group (OMG) , were deposited in small basins the basement of which was made of UM-ML containing rafts of primitive silicic bodies. Possibly the earliest greenstone rocks (some banded iron formation, associated high-Mg basalts and ultramafics) formed in the OMG with deformation and metamorphism continued upto about 3500 Ma. Partial melting of the lower amphibolitic crust due to underplating and asthenospheric sagging formed the tonalitic magma around 3500 Ma (Older Metamorphic Tonalitic Gneiss or OMTG) and subsequently Singhbhum Granite Type-A (Phase-I and Phase-II) around 3300 Ma both of which intruded the folded and metamorphosed OMG rocks. The major event of greenstone belt formation took place during 3500-3200 Ma both in the eastern part and in the western part of the Singhbhum granitic craton. The general trend of these two greenstone belts is NNE-SSW and they were the repositories of the Iron Ore Group (IOG) sediments, volcanics and ultrmafic-mafic rocks. The IOG rocks formed prolific mineralization of Fe,Mn,Cr,Ti,Cu,Ni,Au and platinum group elements(PGE). Multiple phases of tectonism and partial melting of crustal materials ultimately led to the formation of Singhbhun Granite Type-B (Phase-III) around 3100 Ma which later intruded the folded and metamorphosed IOG rocks. From 3000 to 2500 Ma cratonization of the East Indian Shield formed the batholitic mass of Singhbhum granitic complex at the central part with the greenstone belts on either side. Around 2500 Ma the Singhbhum craton became tectonically active again with the formation of three mobile belts – Dalma in the north, Dhanjori-Simlipal in the east and Jagannathpur-Malangtoli in the west.The sediments and lavas of these mobile belts are the major resources of Proterozoic Fe, Ti, Au, U, P, Cu, Pb, Mo, W and Ni mineralization