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Characteristics of Cr-spinel in mantle peridotite-chromitite from Manipur Ophiolite Complex, India: Implication for magmatic processes in mantle and discrimination of tectonic settings.
  • Kshetrimayum Premi Devi,
  • Amit Kumar Sen,
  • A Krishnakanta Singh
Kshetrimayum Premi Devi
Indian Institute of Technology Roorkee

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Amit Kumar Sen
Indian Institute of Technology Roorkee
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A Krishnakanta Singh
Wadia Institute of Himalayan Geology
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As far the ophiolites sequence study has concerned, the characteristics of chromian spinel (Cr-spinel) from mantle rocks (peridotite) and associated chromitite has been a ubiquitous petrogenetic indicator with regard to three tectonic settings (mid-oceanic ridges, subduction zone and arc) in terms of Cr# (= Cr/Cr+Al atomic ratio), Mg# and Ti content. In this study, we attempt to understand the tectonic environment and genetic relationship of mantle peridotites associated chromitites using chromian spinel characteristics from the southern Manipur Ophiolite Complex (MOC) which occupied an integral part of the Indo-Myanmar Orogenic Belt (IMOB), India. The study area mainly consists of peridotites (clinopyroxene-bearing harzburgite and lherzolite) with small pods of podiform chromitite, mafic intrusive - extrusive rocks and oceanic pelagic sediments. The Cr-spinel in peridotites are characterised by high Al2O3 (45.59-50.85), Mg# (89.25-90.48) and low Cr# (19.66 - 23.56) collectively suggest a typical abyssal peridotite derived from fertile mantle at mid-oceanic ridges (MOR) tectonic setting. Whereas, chromitites are low Al2O3 (14.35 to 19.68) and melt compositions (12.01 – 13.69 wt.%) with high Cr# (65.5 to 71.83) suggests that they were crystallized from boninitic magma series in the fore-arc subducting environment. Our petrological and geochemical evidences reveal that two stages of magmatism evolved during the formation of MOC. In the initial stage, magma was generated at the mid - oceanic spreading centre (MOR) by a small degree of partial melting (8–10 %) resulting high Al-rich and low Cr-rich chromian spinel bearing peridotites essentially of lherzolitic composition. At the second stage, high Cr-rich and low Al-rich magma evolved by high degree of partial melting intruded the earlier formed lherzolitic mantle exhibit harzburgite with high Cr# chromitite that formed at the fore-arc related setting above supra-subduction zone. It has been concluded from the present study that the magmatism in the mid-oceanic ridges environment followed by subduction tectonic process were responsible for the evolution of the Nagaland-Manipur Ophiolites that were emplaced along the Eastern plate margin of Indian subcontinent.