Journal of Earth Science, Vol. 29, No. 5, p. 1026–1039, October 2018 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-018-0854-9 Sun, G. M., Li, X.-P., Duan, W. Y., et al., 2018. Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone, Southern Tibet. Journal of Earth Science, 29(5): 1026–1039. https://doi.org/10.1007/s12583-018-0854-9. http://en.earth-science.net Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone, Southern Tibet Guangming Sun 1 , Xu-Ping Li * 1 , Wenyong Duan 1 , Shuang Chen 1 , Zeli Wang 1 , Lingquan Zhao 2 , Qingda Feng 3 1. Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China 2. Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, Bochum 44780, Germany 3. Center of Hydrogeology and Environmental Geology Survey, China Geological Survey, Baoding 071051, China Guangming Sun: https://orcid.org/0000-0001-8964-6315; Xu-Ping Li: https://orcid.org/0000-0002-5408-8963 ABSTRACT: The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone (YZSZ), southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia con- vergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composi- tion indicating that the sodic amphibole has formed during a prograde metamorphic event. The proto- lith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345 C. This con- dition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20–22 km and a thermal gradient of 15–16 C/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experi- enced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage. KEY WORDS: Kadui blueschist, metamorphism, P-T conditions, accretionary complex, southern Tibet. 0 INTRODUCTION Blueschist is formed under HP-LT (high-pressure and low-temperature) conditions and has been recognized as an important marker of subduction zones (Maruyama et al., 1996a; Ernst, 1972; Miyashiro, 1961). Thus the spatial and chronologi- cal study of blueschists is critical for understanding the meta- morphic evolution and tectonic process which occur at conver- gent plate boundaries (Isozaki et al., 2010; Stern, 2005; Platt, 1993). Based on the protolith and subduction nature, Maruyama et al. (1996a) classified blueschist belts into two types, i.e., colli- sional type and Cordilleran type. The former one, also called A-type blueschist (Maruyama et al., 1996a), consists of passive- margin lithologies of limestones, bimodal volcanics, peralumi- nous sediments and granitic gneiss basement rocks, and is *Corresponding author: lixuping@sdust.edu.cn © China University of Geosciences and Springer-Verlag GmbH Germany, Part of Springer Nature 2018 Manuscript received May 20, 2018. Manuscript accepted July 27, 2018. normally formed in continental collision process (Maruyama and Liou, 1998; Maruyama et al., 1996b). The Cordilleran type (B-type) blueschist results from oceanic plate subduction and is characterized by the formation of accretion complexes in an active continental margin, typically containing of bedded chert, MORB basalt, ocean-island basalt and seamount fragments derived from oceanic crust (Maruyama, 1997; Maruyama et al., 1996a). Exam- ples are the Sambagawa metamorphic belt in Japan (Kabir and Takasu, 2016; Aoki et al., 2008) and Franciscan mélange in Cali- fornia, USA (Ukar and Cloos, 2014; Ukar, 2012), which are lo- cated at areas as part rocks encircled the Pacific Ocean. In com- parison with A-type blueschists, B-type blueschists commonly experience lower metamorphic pressure and temperature condi- tions (Maruyama and Liou, 1998). A mineral assemblage of me- tabasaltic rock that experiences blueschist facies metamorphism contains sodic amphibole (glaucophane)+lawsonite (or epidote)+ chlorite+albite+quartz±sodic (jadeitic) clinopyroxene±aragonite (Ota and Kaneko, 2010). Depending on the P-T conditions the metabasalt experienced, different further mineral assemblages are possible, which in general range in the area of subgreenschist to blueschist facies conditions.