UHPM Workshop 2001 at Waseda University 4A04 (Page 124-128) DISCOVERY OF COESITE FROM INDIAN HIMALAYA: CONSEQUENCES ON HIMALAYAN TECTONICS. Himanshu K. Sachan 1 , Barun K. Mukherjee 1 , Y. Ogasawara 2 , S. Maruyama 3 , Anand K. Pandey 1 , A.Muko 2 , N. Yoshioka 2 and H. Ishida 2 1 Wadia Institute of Himalayan Geology, 33, G.M.S.Road, Dehra Dun –248 001,(India). 2 Department of Earth Sciences, Waseda University, Tokyo 169-8050, Japan 3 Department of Earth and planetary Sciences, Tokyo Institute of Technology, Tokyo 152- 8551, Japan Coesite inclusions have been identified for the first time from eclogite in the Tso-Morari crystalline complex of Ladakh (India) by petrography, Ramanspectroscopy and microprobe analysis. Here, we present detail account of new evidences for ultra-high pressure metamorphism in Indian Himalaya. The Tso-Morari crystalline complex occurs as dome and is separated from the Indus suture zone to the north by Zildat detachment fault and Zanskar sedimentary unit in the south. The crystalline complex is divided into three principal tectono stratigraphic units refers as Puga Formation, Taglang La Formation and Polokong La and Rusphu granitoids (Fig.1). The Puga formation is composed of Cambro-Ordovician medium to high-grade gneisses of sillimanite-kyanite grade. The eclogite occurs in the form of lenses and irregular bodies in the sillimanite –kyanite grade rocks of Puga Formations. These eclogite bodies (1-40 m) wrapped by sillimanite-kyanite grade of gneisses (Thakur, 1983). The eclogites are medium to coarse grained. Garnet contains minute inclusions of omphacite, amphibole and phengite. The amphiboles are sodic in nature in which glaucophane is predominant and mostly associated with omphacites. Phengite occurs mostly in the matrix and also inclusion in garnet. The garnet porphyroblasts have some lensoid inclusion containing subhedral and rounded coesite rimmed by palisade quartz aggregate with radial fractures in the host garnet. The silica inclusions in the garnet are of monomineralic quartz, bimineralic quartz+coesite and monomineralic coesite. The core of the larger garnet contains several inclusions of monomineralic quartz. These quartz grains have sharp termination of boundary and their size range from 30 to 150 µm. The mantle region of garnet contains bimieralic coesite+quartz, which comprise relic coesite and surrounded by palisade quartz. They display intense and concentric fracturing in the host garnet. Monomineralic coesite inclusions are generally present in the outermost part of the garnet mantle. They are rounded, lanticular and occasionally subhedral (Fig.2). Coesite is a high-pressure polymorph of SiO 2 and 30 –80 microns in size characterized by it distinctly higher relief and lower birefringence as compared with quartz. Quartz is euhedral, monocrystalline and range in size from 30 to 100 µm. In some others eclogite samples, polycrystalline quartz is observed which clearly reflect the presence of former coesite. Garnet has almandine rich composition. Core has high Mn and low Mg whereas rim has the highest Mg. The coesite-bearing garnet shows chemically three distinct zonal domains. Large euhedral cores, mantle and thin outer rims (Fig-3a, b,c,d). In the garnet, core regions have low concentration of MgO (i.e. 1.08 –4.2wt.%) whereas the mantle has higher concentration (i.e.508wt%) while the rim has rhe highest MgO content (i.e.8.9-10.64 wt.%) vice versa Mn is showing very low concentration at rim (i.e.0.05- 0.5wt.%) and higher at core (i.e. 1.07-1.86wt.%). The CaO content is higher in the core as well as in the rim part of the garnet (i.e.6.65-7.64 wt.%). The FeO content is higher in the core to mantle region and then it decreases towards rim. (Fig.3d).The Mg and Mn distribution 124