RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 112, NO. 10, 25 MAY 2017 2122 *For correspondence. (e-mail: ayani_ovung@yahoo.com) Mineralogy of the Manipur Ophiolite Belt, North East India: implications for mid-oceanic ridge and supra-subduction zone origin Thungyani N. Ovung 1, *, Jyotisankar Ray 1 , Xueming Teng 2 , Biswajit Ghosh 1 , Madhuparna Paul 1 , Proloy Ganguly 1 , Saradee Sengupta 1 and Supriyo Das 1 1 Department of Geology, Calcutta University, 35 B.C. Road, Kolkata 700 019, India 2 School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China Mineralogical studies on the mantle and crustal sec- tions of the Manipur Ophiolite Belt (MOB) lead to important findings pertaining to its genesis and con- trolling tectonic milieu. The wide compositional gap in the Cr# and Mg# content of spinel in the mantle peri- dotites of MOB implies upper mantle melting in two different tectonic settings. The tectonic discrimination diagrams based on spinel chemistry indicate a mid- oceanic ridge (MOR) origin for the high-Al spinel per- idotites and a supra-subduction zone origin for the high-Cr spinel peridotites. The pyroxenite mantle dyke, ultramafic cumulate and pillow-basalt record temperature in the range of 600 –1030C, 600–800C and 700–1005C respectively. Plotting of clinopyrox- ene composition of pillow-basalt in the TiO 2 –Na 2 O– SiO 2 /100 (wt%) tectonic discrimination diagram, implies a subduction-related origin of the basalts. Ex- perimental studies on the serpentine stability indicate that it was dominantly affected by high temperature- low deformation setting. Keywords: Mineralogical study, ophiolite belt, pyroxe- nite mantle dyke, pyroxene thermometry. THE Manipur Ophiolite Belt (MOB) which is a part of the Naga-Manipur Ophiolite Belt (NMOB) has been studied by several workers 1–12 . It occurs within the NNE–SSW trending Indo-Myanmar Orogenic Belt (IMOB), a rem- nant of the Tethyan Ophiolite in the Alpine-Himalayan orogenic system 13–16 . Initially, it was proposed that sever- al subduction processes were involved in the generation of this ophiolite belt resulting from the convergence of the Indian plate with the Myanmar plate 14,17–21 . However, the tectonic processes involved in this are still controver- sial. Recent studies have suggested that the peridotites of MOB are the residues remaining after low-degree partial melting of spinel–peridotite mantle at mid-oceanic ridge environment 3,5–7 . The mafic volcanics are also suggested to be derived from a heterogeneous mantle source at a spreading ridge zone 12 . Implications on the origin of these rocks can be made through the study of the constituent primary mineral phases. Therefore, in this study, we present field observa- tions of MOB and its mineralogical aspects to provide a brief overview on its petrogenesis. Experimental study was also done on the serpentine grains to understand the serpentine stability in terms of regional tectonic frame. In the Indo-Myanmar Ranges, the ophiolitic rocks occur in two parallel belts (Eastern belt and Western belt) along the eastern margin of the Indian plate. The Eastern belt passes through central Myanmar, Sumatra and Java, whereas the Western belt passes through Nagaland, Manipur, western Myanmar and Andaman islands 1 (Figure 1, inset). MOB which is the southern extension of Naga Hills Ophiolite (NHO) shows an easterly dipping thrust contact with the underlying Disang and Barail flysch sediments (Upper Cretaceous–Upper Eocene), whereas the eastern part is overthrusted by continental metamorphic rocks known as the Naga metamorphics 22 . NHO and its associ- ated rocks are broadly classified into three distinct tectono-stratigraphic units from east to west, viz. (1) the Nimi Formation, (2) NHO and (3) the Disang Formation. A fourth tectono-stratigraphic unit known as the Jopi Formation is a post-orogenic molasse that consists of ophiolite-derived sediments, and occurring as a cover sequence over NHO 9 . The study area is confined within Jessami in the north and Moreh in the extreme south (Figure 1). It consists of mantle peridotite (serpentinite), pyroxenite mantle dyke, ul- tramafic cumulate and mafic intrusives and pillow basalt which are overlain by pelagic sediments. The ophiolitic rocks are absent in the Jessami–Marem and Shangshak– Phungyar sectors. Mantle peridotite is the dominant lithotype which occupies the basal unit. They are mostly highly weath- ered and serpentinized. Few fresh samples of harzburgite, lherzolite, wehrlite and dunite are found in Sirohi, Phangrei, Gamnom, Maku, Kamjong, Lungpha and Khudenthabi. Intrusive dyke solely restricted within the mantle-borne restites has been referred to in the literature as ‘deep dykes’ or mantle dykes 23 . Pyroxenite mantle dyke intruding the mantle peridotite is found at Gamnom in the present study area. It displays a sharp contact with the mantle peridotite, cutting across its foliation planes. The deformed mantle peridotites are overlain by the cu- mulate unit. The lower cumulate sequence is represented by ultramafic cumulates like harzburgite and lherzolite, and the upper cumulate sequence is represented by gab- bro. Ultramafic cumulate rocks are predominantly obser- ved in the Gamnom–Kamjong and Moreh–Khudenthabi sectors. Mafic intrusives like the layered gabbro are scantily distributed in and around Gamnom. They exhibit banding of mafic and felsic minerals. The volcanic rocks