Origin and serpentinization of ultramafic rocks of Manipur Ophiolite Complex in the Indo-Myanmar subduction zone, Northeast India
Highlights
► Ultramafic rocks are serpentinized in low P-T conditions. ► The protoliths were abyssal peridotite, derived from less depleted mantle melt. ► They are originated at an MORB setting after low degree (10–15%) partial melting. ► Serpentinite may have created at slow-spreading ridges, rather than a SSZ setting. ► We develop a model of origin and tectonic evolution of the Indo-Myanmar Ranges.
Introduction
The closure of the Tethyan oceanic realm during the Late Cretaceous and Early Tertiary preserved many ophiolite complexes within the Indo-Myanmar-Australia suture zones (Mitchell, 1981), including the MNOB in the Indo-Myanmar suture zone (Acharyya, 2007, Singh, 2008). The ophiolites, including serpentinite of the belt were derived from tectonic activities developed in the Tethyan oceanic lithosphere due to subduction of Indian plate below the Eurasian plate (Nandy, 1981, Oldham, 1883, Brunnschweiler, 1966).
The MOC is confined within the eastern sector of the IMRS (Vidyadharan et al., 1989, Singh, 2008). The complex is a dismembered ophiolite sequence with development of a mélange zone and an olistostromal plate margin (Evans, 1964, Vidyadharan et al., 1989). Well-preserved mantle sequences occur in the ophiolite belt (Acharyya, 1986, Ghose et al., 1986, Vidyadharan et al., 1989). However, the occurrence of serpentinite is observed as the most voluminous litho-tectonic unit in the complex. Pelagic sediments, pillow basalts, volcanic rocks, mafic dyke rocks and podiform chromitites are also exposed in minor in the complex. Rare occurrences of gabbros are observed in the study area.
Several contrasting views were put forward concerning the origin of the MNOB. One school of thought believed that the belt originated from multiple subduction processes of the Indian plate beneath the Eurasian Plate (Mitchell, 1993, Acharyya, 2007). Another school believed that the MNOB is rootless sub-horizontal bodies, which are westward-propagated nappes from the Eastern Belt Ophiolite of Myanmar (Sengupta et al., 1990, Acharyya et al., 1990). Another theory also believed that the belt originated in an Island Arc developed in the Tethyn Ocean (Bhattacharjee, 1991, Nandy, 2001). Furthermore, due to remote and inaccessible nature of the terrain, very less research work has been done in the area. An understanding of the origin of the MOC serpentinite is significant in evaluating the tectonic evolution of the subduction zone. A combination of petrological and geochemical studies of serpentinite can assist in determining the origin and geodynamic settings of serpentinization (Auzende et al., 2002, Hattori and Guillot, 2007, Saumur et al., 2010, Monsef et al., 2010). Raman spectroscopy can be used to study serpentine and the serpentinization processes (Kloprogge et al., 1999, Rinaudo et al., 2003, Auzende et al., 2004, Groppo et al., 2006). This paper details the origin and serpentinization of ultramafic rocks of the MOC through a combined effort of petrology and geochemical analysis. Origin and evolution of the MOC serpentinites, and in consequence, model of subduction of the Indian Plate and Myanmar Plate is discussed.
Section snippets
Geology and field setting
The MOC is located in the MNOB, which is confined within the eastern sector of the IMRs (Fig. 1). The belt constitutes a part of the Arakan-Yoma Fold Belt separating Northeastern India from Myanmar (Nandy, 1981). The belt forms a NNE–SSW trending linear tract with a length of about 200 km and has an average width of about 15 km. The IMR are mainly composed of the Late Cretaceous–Paleogene marine sedimentary rocks, unconformably overlying the Upper Triassic flysch-type sedimentary rocks and
Sampling and analytical methods
Serpentinite samples with various degrees of serpentinization were collected from different locations of the MOC (Fig. 1B). Detailed petrographic studies of the selected thin section slides were carried out at the Department of Geology, Centre for Advanced Studies, University of Delhi. Leica Workstation DMRX with the image analysis system was used to analyze the types of serpentine minerals present in ultramafic rocks.
Raman spectroscopy of samples was done at the Ecole Normale Superieure of
Partially serpentinized peridotite
Most of the peridotites of MOC are highly serpentinized, but few are partially serpentinized. Relicts of harzburgite and lehrzolite frequently occur in the less serpentinized samples of the complex (Fig. 4A1). Wehrlite, dunite and pyroxenite are relatively infrequent as relict in the samples while websterite is absent. The presence of two generations of pyroxenes (orthopyroxene and clinopyroxene), with inclusion of olivine in both pyroxene grains are observed in most of the partially
Source and origin of serpentinite
AFM and ACF ternary plots of the serpentinite samples of MOC inferred that the samples belong to a combined field of metamorphic peridotite and ultramafic cumulates. Higher average value of Al2O3 (2.66 wt.%) in the samples reflects a higher proportion of clinopyroxene phase than that of normal ophiolitic harzburgite and dunite, so the source of the samples is more fertile (Coleman, 1971). The nearly flat pattern of chondrite normalized REE, also indicates that the source of rocks is from a
Conclusions
There is much confusion regarding the origin of the MNOB. This study focusing on petrology and geochemistry of the serpentinite component of the MOC helps clarify its tectonic setting. Our research work concluded that, even though the MOC serpentinites are exposed in the Indo-Myanmar subduction zone, they were originated at a spreading regime (MORB setting) due to the low degree partial melting (10–15°) of a less depleted, fertile mantle melt. Subsequently, the ultramafic rocks were
Acknowledgements
Ningthoujam would like to thank UGC-CAS for providing fellowship for Ph. D. and R&D-DU for providing funds for this research work. We are indebted to the Prof. An Yin and Prof. Kéiko H. Hattori, for reviewing and giving constructive comments that greatly improved the manuscript. We are also thankful to Dr. Sandeep for providing facilities for ICP-MS. Thanks are due to Prof. Soibam Ibotombi, Manipur University for guiding us during our field work.
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