Oct 31, 2007 - Ophiolite nappes were thrusted southwards along this zone over the Indian ... late influx of volatiles,. Amphibolite. "This rock is essentially.
JOURNAL GEOLOGICAL SOCIETY OF INDIA Vo1.70, October 2007, pp.595-604
Mayodia Ophiolites of Arunachal Pradesh, Northeastern Himalaya BISWAJIT
GHOSH!$
JOHN
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and
JYOTISANKAR
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'Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019 20epartment of Geology and Geophysics, University of Hawaii, SOEST, 1680 East-West Road, Honolulu, Hl96822, USA. sPresent address: Geological Survey of India, Op.: WB-SK-AN, ER, Kolkata - 462 016 Email: bghosh geoltshotmail.com Abstract: The ophiolite assemblage around Mayodia, Dibang Valley, Arunachal Pradesh falls in the eastern extension of the Indus suture belt and is represented by peridotite tectonite, hornblendite (dyke) and amphibolite. The entire succession is overlain by metabasalt carapace interlayered with metapelitic pelagic sedimentary rocks. The basal peridotite may be classified as wehrlite. The hornblendite typically occurs as intrusive within the peridotite tectonite and is represented by cumulus primary amphibole. The amphibolite is characterized by well developed gneissose banding. The pillow lava is represented by actinolite-chlorite-albite-epidote schist. The evolutionary trend of the ophiolite suite has been assessed based on major, trace and rare earth element data which favours partial melting of a depleted mantle source. Tectonic discrimination diagrams for the amphibolite and metabasalt clearly indicate their MORB affinity. Such ophiolite assemblage has developed as a result of collision of India and Asia that started with the closing of Tethyan ocean during Mesozoic and Early Tertiary. Keywords:
Mayodia ophiolite, Petrology, Geochemistry, Tectonic setting, Northeastern Himalaya, Arunachal Pradesh.
INTRODUCTION The Himalayan mountain system is part of the world's largest concentration of mountain ranges in the central part of Eurasia. A series of continuous thrusts divide the long Himalayan arc into four tectonically distinct and stratigraphically contrasted zones with multifaceted histories of sedimentation, magmatism, metamorphism and tectonism that extends continuously from east to west (Gansser, 1964; Sinha, 1992). From north to south these zones are: (1) Trans-Himalaya including the Indus-Tsangpo suture zone (ITSZ), (2) Higher Himalaya or Great Himalaya including the Central Himalayan crystalline and Tethyan Himalaya, (3) Lesser Himalaya or Southern Himalaya and (4) Sub-Himalaya or Outer Himalaya. The Triassic-Pliocene rocks of Indus-Tsangpo Suture Zone show evidences of Late Cretaceous convergence (subduction) and Cenozoic collision (- 45Ma) of the Indian and the Eurasian plates (Sinha and Upadhyay, 1997). This suture zone has a complex accretionary history and is marked by association of turbidites and ophiolite melanges with volcanics, calc-alkaline magmatism and molasse type sediments. Ophiolite nappes were thrusted southwards along this zone over the Indian continental 00\6-762212007-70-4-595/$
margin in the Paleogene during the final stages of closure of Tethys and these were recorded from a number of occurrences from northeastern and northwestern part of Trans-Himalaya ranges (Prasad et al. 1980; Radhakrishna et al. 1984; Sharma et al. 1991). We describe the first account of the petrology and geochemistry of ophiolitic rocks around Mayodia, Dibang valley district, Arunachal Pradesh (Lohit Himalaya).
GEOLOGICAL
SETTING
In Lohit Himalaya, a sequence of metasediments· bounded by NW-SE trending Mishmi and Lohit boundary thrusts were studied in the southern part along Tezu Hayuliang section (Dhoundial et al. 1976; Thakur and Jain, 1975; Sharma et al. 1991 and Kumar, 1997) (Fig. 1). The Tidding serpentinite (a mantle derived rock) and limestone occur as a narrow band within this section (Chattopadhyay and Chakraborty, 1984). Another section presenting similar geological setup between Roing and Hunli (Fig. 1), a stretch of90 km forms the subject of present investigation around Mayodia (Fig.2). This belt extends further northeastward up to 1.00
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are substantially elongated, highly stretched and serpentinised. It shows features similar to L-S tectonite. Hornblendite dyke occurs as intrusive into peridotite tectonite and is found to be restricted within it. Amphibolite of the investigated area occurs above the peridotite tectonite and is marked by prominent mesoscopic zebra banding, and possibly represents a part of the dyke swarm that acted as channel to the overlying lava flow. The widths of dark bands are comparatively high in comparison to lighter ones. The foliations in amphibolite at places are marked by prominent inch scale banding. The metabasalt is represented by actinolite-chloritealbite schist and shows a prominent schistosity plane. The schistosity plane is folded at places possibly due to later deformation. The sedimentary unit (associated with the metabasalts) towards the top of the ophiolite association is now represented mainly by metapelite. Local metacarbonate bands are also present. The metapelite unit reveals predominant schistosity plane marked by elongated and flaky constituent minerals. The schistosity plane is locally crumpled and puckered with consequent development of crenulation cleavage. In some cases, locally developed close mesoscopic folds are noticed within metapelite.
PETROGRAPHY 95 8' Fig.I. Generalised Geological map of Lohit Himalaya, Arunachal Pradesh (after Dhoundial et al. 1976). J-Alluvium: 2 - Low grade metasediments; 3 - Serpentinites; 4 - Crystalline limestone;5 - Quartzite; 6 - Garnetiferous amphibole schist; 7 - High grade schists and para-gneisses; 8 - Low grade schists and quartzite; 9 - Diorite-granodiorite complex and 10 - Amphibole schist, gametiferous staurolite-kyanite schist. Etolin. Thakur and Jain, (1975) carried out reconnaissance survey for 30 km starting from Roing along this section and have noted reverse metamorphism. Sarvothaman (1990) worked in the Hunli - Etolin section and noted an assemblage of carbonate rock, chlorite-actinolite-epidote schist and feldspathic quartzite. He suggested that the epimarble band around Hunli could possibly mark the northwesterly extension of Tidding Limestone. The ophiolite association of Roing - Hunk section comprises basal peridotite tectonite, hornblendite (~¥ke) and amphibolite with overlying metabasaltic carapace The latter unit is interlayered with metasediment slivers (Fig. 2). The peridotite tectonite preserves well developed foliation and is deformed. The constituent olivine grains
The detailed petrographic characters of each lithounits of the study area and their implications are addressed elsewhere (Ghosh and Ray, 2003a) and hence a brief description of petrography of each lithounits is presented here. Peridotite Tectonite The rock consists of olivine (Fo_~ + clinopyroxene (endiopside) + serpentine + opaque mineral (spinel) and plots in the field of wehrlite (Fig.3). Hornblendite The most dominant mineral of the hornblendite is cumulus primary amphibole (mostly pargasitic) with some intercumulus oligoclase plagioclase (An_2o). The rock is very coarse grained (pegmatitic), possibly influenced by late influx of volatiles, Amphibolite "This rock is essentially composed of amphibole (magnesio-hornblende to tschermakitic-hornblende), plagioclase (
