Silicate and Sulphide Mineralogy, and Conditions of Equilibration of Ultramafic Rocks of the Indo-Myanmar Ophiolite Belt between Tusom, Manipur and Shomra Village, Myanmar

Abstract

The northeast-India ophiolite complex is exposed in many parts of Nagaland and Manipur, with particularly well-preserved mantle-sequence peridotite present in its southern section in Manipur state. We present the results of an investigation into the petrology and constituent mineral chemistry of this peridotite, which is associated with the Indo-Myanmar ophiolite located between Tusom, Ukhrul District (Manipur) and Shomra village (Myanmar). Mineral compositions indicate that it is an abyssal peridotite that has undergone minimal partial melting (1-5%). Equilibration probably occurred in the upper mantle at a temperature of 1080-1240C, a pressure of 23-24 kbar, and an oxygen fugacity between 0.303 and 0.580 log units above the FMQ buffer. Primary and secondary sulphide phases in the studied peridotites have also been analysed. Pentlandite ((Fe,Ni) 9 S 8 ) was found to be the primary sulphide mineral, which occurred in association with orthopyroxene and awaruite (Ni 3 Fe). Cu-rich alloys and magnetite occur as secondary minerals that developed during later serpentinization.Keywords: Abyssal peridotite, equilibrium temperature, pentlandite, metal alloys, oxygen fugacity, partial melting.THE compositions of phases in ophiolitic, mantle-derived ultramafic rocks depend upon the degree and pressuretemperature conditions of partial melting, and the effects of magma-rock interaction 1 . Mineral compositions in mantle peridotites are considered to be reliable petrogenetic indicators. Although a few studies of the ophiolitic mantle section of Ukhrul district (Manipur side) are available 2-4 , studies of its continuation onto the Myanmar side are lacking. In this study, we analysed primary silicate mineral phases within mantle rocks exposed between Tusom village, Manipur, and Shomra village, Myanmar, to elucidate their equilibrium conditions of formation and related intensive parameters. We also analysed metal sulphides and metallic alloys in the ultramafic rock and present an interpretation of the probable formation of these phases.The Indo-Myanmar Ophiolite belt, approximately 200 km long, extends from Nagaland in the north to Manipur in the south. It is a NNE-SSW trending, westerly convex linear belt and is mostly restricted to the eastern section of Indo-Myanmar Range. The belt has an average width of ~15 km in its northern sector (Nagaland) but gradually narrows to a width of a few metres at its southern end (Manipur). The ophiolite belt consists of lensoid bodies interbedded with Disang flyschoids, which are highly tectonized and dismembered. Despite having undergone varying degrees of serpentinization, relict primary minerals are preserved in some rocks. A traverse map showing the study area is shown in Figure 1. Majorelement compositions of mineral phases were determined by electron probe microanalysis (EPMA) using a Jeol JXA 8900 RL in wavelength dispersive mode. Silicate phases were analysed using an accelerating potential of 20 kV, a beam current of 12 nA, and a spot size of 2 m. Natural minerals and oxides (fo...