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Discovery of coesite from Indus Suture Zone (ISZ), Ladakh, India

¹ Wadia Institute of Himalayan Geology, 33, G.M.S. Road, Dehra Dun - 248 001, India
² Department of Earth Sciences, Waseda University, Tokyo 169–050, Japan
³ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152–8551, Japan

^* Corresponding author's e-mail: himanshusachan{at}rediffmail.com

Coesite, the high-pressure polymorph of quartz has been identified for the first time in the Tso-Morari Crystalline Complex, Ladakh (India) in the Himalayan belt. The preservation of coesite grains as inclusions in garnet within eclogite boudins indicates the existence of UHP metamorphism in this continental collision setting.

The coesite was identified optically and its presence confirmed by its characteristic Raman bands. Both coesite and polycrys-talline quartz inclusions exhibit prominent radial fractures in their host garnet. The silica inclusions (monomineralic coesite, monomineralic quartz and bimineralic quartz+coesite) are associated with various textural features and well-developed chemical zonation within the garnet, which show the prograde nature of the UHP metamorphism.

Preliminary P-T estimates suggest that the coesite growth took place at pressure > 28 kbar (> 90 km depth) and temperatures > 640°C. Significantly, the coesite inclusions are interpreted as having suffered decompression during exhumation without changing to quartz, most likely due to the rapid uplift. This finding also indicates deep subduction of the Indian plate beneath the Asian continent. The occurrence of coesite and subduction of the Indian plate was essentially governed by a low geothermal gradient which occurred prior to rapid exhumation. This was vital for generating the coherent picture of metamorphism and exposure of UHP rocks.

Key-words: coesite, garnet, UHP eclogite, Raman spectroscopy, Tso-Morari, Western Himalaya.

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