Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Tapan Pal ¹, Biswajit Ghosh ², Sujit Chakraborty ³

Affiliations
1 Petrology Division, ER, Geological Survey of India, Kolkata, IN
2 Project: Andaman and Nicobar, Op: WSA, ER, Geological Survey of India, Kolkata, IN
3 Monitoring Division, CHQ, Geological Survey of India, Kolkata, IN

Abstract

The chromite bearing ultramafic rocks of Roro-Jojohatu areas in the Singhbhum Craton within the metasediments of the Iron Ore Group (IOG) is a part of Archaean greenstone terrain of Eastern India. In this area since all the primary silicates are altered, chromite has been used for host rock petrogenesis. Chromite occurs in different forms viz. (a) mainly cumulus phases in chromitite layers, (b) intercumulus phases forming banded/net textured chromite and (c) fine disseminations. Chemically all the chromites are aluminian chromite. The massive chromite and banded/net-textured chromites show high Cr-number (79-83) and intermediate Mg-number (51-60), but disseminated chromites have consistently low Cr-number (76-79) and low Mg-number (43-52). The plots of TiO₂ versus Cr-number, Al₂O₃ versus TiO₂, and Mg-number versus Cr-number reflect boninitic parental melt for the chromites. Al₂O₃ contents in liquid ranging from 9.67-11.03wt% and TiO₂ contents in liquid 0.42-0.75wt% also correspond for boninitic parentage. The FeO/MgO ratio in liquid ranging from 1.15 to 1.29 for massive chromites and 0.86-1.23 for banded/net-textured chromite, also suggest for boninitic source. Al₂O₃ versus TiO₂ plots indicate that chromites plot in the field of supra-subduction zone peridotites. Boninitic magma was produced from hydrous mantle melting in supra-subduction zone. At supra-subduction setting an oxidizing hydrous fluid derived from subducting slab facilitated the formation of chromitite layers at high fO₂ conditions.

Keywords

Chromites, Boninite, Supra-Subduction Zone, Roro-Jojohatu, Singhbhum Craton.

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