Native iron in the Palaeoproterozoic Chaibasa Formation: primary or secondary?
Native Fe in the Chaibasa Formation may be the oldest native Fe ever found. The Chaibasa Formation contains pre-1.6 Ga offshore shelf sediments from the Singhbhum Craton in India. The sediments are light green in colour, consistent with a low content of organic matter and a metamorphic grade of greenschist–lower amphibolite facies. The native Fe grains are enclosed in fibrous mica. The native Fe occurs with magnetite crystals, evidence of relatively oxidizing conditions in the Chaibasa Formation during diagenesis. (Mn,Fe)S, a compound characteristic of reducing conditions, is found within the native Fe. The native Fe contains small amounts of native Si. Most grains of native Fe have rough edges. One bulbous grain has smooth edges and an internal wavy structure that is suggestive of a drop of melted iron. The melting temperature of pure Fe is 1535 °C. We also found native Fe with small amounts of carbon in it that appears to have a plate-like, pearlitic structure. Pearlite typically forms at temperatures of 723–500 °C. All of these temperatures are too high to be achieved during greenschist-facies metamorphism. As a result, we infer that the native Fe is primary rather than secondary. Our work and that of others suggests a primary, impact origin for the native Fe.
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Palaeoproterozoic of India
The Indian shield represents a vast repository of the Palaeoproterozoic geological record. Built over the four large amalgamated Archaean nuclei (Dharwar, Bastar, Singhbhum and Aravalli–Bundelkhand) the major and minor Palaeoproterozoic sedimentary basins and supracrustal sequences in India are comparable in scale, and perhaps also in development, to those of North America, Africa, Australia and Brazil. The deformation of these supracrustal sequences, attendant metamorphism and emplacement of plutonic bodies hold important clues to their connection with major orogenies. Research in these areas has led to investigations into global correlation, which in turn has had a direct bearing on refining models of Palaeoproterozoic supercontinent assembly and break-up. This book covers various aspects of regional geology as well as broader issues of the Indian Palaeoproterozoic geology and its global context. It is an outcome of the UNESCO-IGCP 509 Palaeoproterozoic Supercontinents and Global Evolution research project.