Characterisation of chromites, chromite hosted inclusions of silicates and metal alloys in chromitites from the Indo-Myanmar ophiolite belt of Northeastern India

Abstract

Chromite is commonly used to decipher the petrogenesis and geodynamic setting of ultramafic rocks. Podiform chromitites associated with the mantle sequence of the Indo-Myanmar ophiolite belt are studied here to characterise the major and trace element concentrations of the chromites. The molar ratios Cr# (100×Cr/(Cr+Al)) and Mg# (100×Mg/(Mg+Fe2+)) range from 67 to 82 and 66 to 71, respectively, in the chromites. Calculated equilibrium conditions for chromite crystallisation are 19–25kbar pressure, 1080–1240°C temperature and oxygen fugacity ranging between 0.30 and 0.58 log units above the FMQ buffer, showing that the samples formed in an upper mantle environment. Minor and trace element concentrations in the chromites (Ga 23–33ppm, Ni 1203–1843ppm, V 585–780ppm, Sc 2–5ppm) are comparable to other podiform chromitites hosted in the mantle sections of ophiolite complexes. Using chrome spinel compositions, the parental magma for the chromitite is identified as boninite. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting for the Manipur ophiolite chromitites. The chromites host inclusions of silicates, base metal sulphides and base metal alloys. The diversity of these inclusions reflects the important role played by hydrothermal fluids. Inclusions (base metal sulphide and alloys) of linear trails with enriched Cr2O3 (63wt.%, Cr# ∼80) in the chromite grains (Cr2O3, Cr# 70) indicate mobility of this component in chromites.