Two stages of granulite facies metamorphism in the eastern Himalayan syntaxis, south Tibet: petrology, zircon geochronology and implications for the subduction of Neo-Tethys and the Indian continent beneath Asia Z. M. ZHANG, 1,2 G. C. ZHAO, 3 M. SANTOSH, 4 J. L. WANG, 1 X. DONG 1 AND J. G. LIOU 5 1 Institute of Geology, Chinese Academy of Geological Sciences, No. 26, Baiwanzhuang Road, Beijing 100037, China (zzm2111@sina.com) 2 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China 3 Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China 4 Department of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan 5 Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA ABSTRACT The eastern Himalayan syntaxis in southeastern Tibet consists of the Lhasa terrane, High Himalayan rocks and Indus-Tsangpo suture zone. The Lhasa terrane constitutes the hangingwall of a subduction zone, whereas the High Himalayan rocks represent the subducted Indian continent. Our petrological and geochronological data reveal that the Lhasa terrane has undergone two stages of medium-P metamorphism: an early granulite facies event at c. 90 Ma and a late amphibolite facies event at 36– 33 Ma. However, the High Himalayan rocks experienced only a single high-P granulite facies metamorphic event at 37–32 Ma. It is inferred that the Late Cretaceous (c. 90 Ma) medium-P metamorphism of the southern Lhasa terrane resulted from a northward subduction of the Neo-Tethyan ocean, and that the Oligocene (37–32 Ma) high-P (1.8–1.4 GPa) rocks of the High Himalayan and coeval medium-P (0.8–1.1 GPa) rocks of the Lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the Indian continent beneath Asia. Our results provide robust constraints on the Mesozoic and Cenozoic tectonic evolution of south Tibet. Key words: continent collision; Himalayas; HT and HP metamorphism; Neo-Tethys subduction; paired metamorphic belts. INTRODUCTION Stretching for over 2000 km from Pakistan to Burma, the Himalayan range defines an important segment of the Alpine–Himalayan chain, the worldÕs largest active orogenic belt. Extensive investigations carried out over the last century have led to many achievements in understanding the geology and tectonics of this region. Most of the previous studies were focused on meta- morphic rocks from the central and western Himalaya (e.g. Kaneko et al., 2003; Leech et al., 2005; Goscombe et al., 2006; Parrish et al., 2006; Groppo et al., 2007; Liu et al., 2007; Ahmad et al., 2008; Jessup et al., 2008; Bhattacharrya & Mitra, 2009; Cottle et al., 2009a,b), and magmatic rocks from the central and eastern Himalaya (e.g. Coulon et al., 1986; Booth et al., 2004; Zhang et al., 2004; Chung et al., 2005; Aikman et al., 2008) and the southern Lhasa terrane (Chung et al., 2003, 2005; Ding et al., 2003; Hou et al., 2004, 2009; Mo et al., 2005, 2007a, 2008; He et al., 2007; Wen et al., 2008a,b; Zhu et al., 2008, 2009; Ji et al., 2009; Zhao et al., 2009; Wu et al., 2010), whereas investiga- tions on the metamorphic rocks in the eastern Hima- layan syntaxis are sparse (Liu & Zhong, 1997; Ding et al., 2001; Booth et al., 2009). In particular, the metamorphism and tectonic setting of the south Lhasa terrane during the Mesozoic Andean-type and Ceno- zoic Himalayan-type orogenesis have not been well constrained; this information is essential for a robust understanding of the tectonic history of the whole Himalayan belt. In this paper, we report the petro- logical characteristics and U–Pb ages of zircon from Late Cretaceous and early Oligocene granulites from the eastern Himalayan syntaxis; these data are related to the subduction of the Neo-Tethys and the Indian continent beneath the Asia. The mineral abbreviations used in this paper are after Whitney & Evans (2010). GEOLOGICAL SETTING The Himalayan orogen of south Tibet is composed essentially of the Himalayan and south Lhasa terranes, separated by the Indus-Tsangpo Suture zone, repre- senting the remnants of the Neo-Tethyan ocean J. metamorphic Geol., 2010, 28, 719–733 doi:10.1111/j.1525-1314.2010.00885.x Ó 2010 Blackwell Publishing Ltd 719