1. College of Earth Sciences, Jilin University, Changchun 130061, China;
2. Geological Team 7 of Qinghai Provincial Nonferrous Metal and Geological Exploration Bureau, Xining 810000, China

The Zhalaxiageyong lead-zinc-copper polymetallic deposit is a typical porphyry deposit of the Tuotuohe area. Whole-rock geochemical analyses, Zircon U-Pb dating and Hf isotope analysis are undertaken for the ore host trachydacite with the aim of constraining its petrogenesis, magma source and regional tectonic setting. LA-ICP-MS zircon U-Pb dating indicates that the trachydacite was formed in 32.68±0.50 Ma (MSWD=1.6), i.e., Oligocene. The trachydacite is rich in potassium and poor in Mg^#(5.10-9.70), belonging to the peraluminous shoshonite series. The rocks are enriched in LILE (large ion lithophile elements) Rb, Ba, K and LREE, depleted in HFSE (high field strength elements) Nb, Ta, P, Ti, with high Sr and low Y and Yb, having the characteristics of the C type adakite. It is calculated that the initial εHf(t) of the zircons range from -0.92 to 2.07 and their two-stage Hf model ages T_DM2 range from 978 Ma to 1 169 Ma. The magma source should be mainly the partially melt mafic rocks of the thickened Middle Neoproterozoic lower crust of the Northern Qiangtang massif with the addition of ancient aluminosilica material in the melting process. The rocks formed in the tectonic setting of delamination of lithosphere and extension of the thickened crust. During the period of 40-32 Ma, large-scale potassium rich alkaline magmatism occurred in this area. The porphyry metallogenesis is related to the magmatic activities in this period.