Ѿ Zircon U-Pb and Hf Isotopic Constraints from the Linzhou Basin on the Onset Timing and Petrogenesis of the Linzizong Volcanic Successions, Southern Tibet Hao-Yang Lee (李皓揚) Department of Geosciences, National Taiwan University Abstract The Linzizong volcanic successions that crop out in the Lhasa terrane, southern Tibet are interpreted as the products of northward subduction of the Neo-Tethyan oceanic slab and its subsequent breakoff at the early stage of the India-Asia collision (Fig. 1A; Coulon et al., 1986; Pearce et al., 1988; Lee et al., 2007). In this study a dacitic breccia and a rhyolitic ignimbrite were collected from the bottom layers of the Dianzhong and Nianbo Formations, respectively, in the Linzhou basin (Fig. 1B), the type locality of the Linzizong volcanic successions, for zircon U-Pb age and Hf isotope analysis. The results indicate that the former erupted at 62.5±1.1 Ma (Fig. 2) and the latter at 56.4±1.2 Ma (Fig. 3), implying the Linzizong volcanism to have started right after the K/T boundary. Igneous zircons from the Dianzhong and Nianbo samples yield ε Hf (T) values of +9.0~+0.5 and +11.0~+4.8 (Fig. 4), respectively, suggesting involvement of the juvenile mantle source in the magma generation. These ε Hf (T) values are corresponding to those of igneous zircons from the coeval Gangdese batholith (Fig. 4; Chu et al., 2006), thus allowing us to infer a cognate origin related to partial melting of the mantle wedge and associated magma underplating/remelting. We furthermore observe inherited zircons of Carboniferous ages in the Dianzhong sample that show ε Hf (T) values from -0.6 to -3.2 and Mesoproterozoic model ages comparable to those of inherited zircons from the Nyainqentanglha granitoids (Fig. 4; Chu et al., 2006). This implies that the old crust of the Lhasa terrane was also involved in the magma generation. References Coulon, C., Maluski, H., Bollinges, C., and Wang, S., 1986. Mesozoic and Cenozoic volcanic rocks from central and southern Tibet: 39 Ar- 40 Ar dating, petrological characteristics and geodynamical significance: Earth and Planetary Science Letters, 79: 281-302. Pearce, J.A., and Mei, H., 1988. Volcanic rocks of the 1985 Tibet Geotraverse: Lhasa to Golmud: Philosophical Transactions of the Royal Society of London, A327: 169-201.