Characteristics of Cr-spinel in mantle peridotite-chromitite from
Manipur Ophiolite Complex, India: Implication for magmatic processes in
mantle and discrimination of tectonic settings.
Abstract
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.