1． College of Earth Sciences，Jilin University，Changchun 130061，China; 2． MLＲ Key Laboratory of Metallogeny and Mineral Ｒesources Assessment，Institute of Mineral Ｒesources，Chinese Academy of Geological Sciences，Beijing 100037，China; 3． School of Earth and Space Sciences，Perking University，Beijing 100871，China; 4． Qinghai Nuclear Industrial Geology Bureau，Xining 810008，China

Zircon U- Pb dating，whole- rock geochemical analyses and Hf isotope are undertaken for the monzogranite in Niubiziliang area with the aim of constraining its formation time，petrogenesis and the regional tectonic setting． The zircons from monzogranite are euhedral- subhedral in shape，and display rhythm growth zoning，indicating a magmatic origin． LA- ICP- MS zircon U- Pb dating indicates the monzogranite formed most probably in the Late Devonian (359． 0 ±2． 6 Ma)． The monzogranite has SiO 2 = 74． 69%- - 76． 11%，Al 2 O 3 = 12． 07%- - 12． 81%，Na 2 O + K 2 O =8． 24%- - 8． 70%，Na 2 O/K 2 O =0． 60- - 0． 68，A/CNK ＞1，which shows that it belongs to high- Si and high- K weakly peraluminous calc- alkaline series． The monzogranite is enriched in K，Ｒb，Th (LILEs) and La，Ce，Sm，Nd(LＲEEs); and depleted in Ba，U(HＲEEs)and Ta，Nb(HFSEs)． Their zircon εHf(t) values range from 1． 21 to 3． 46，in response to their two- stage Hf model ages (T DM2 ) ran- ging from 1 034 Ma to 1 159 Ma，respectively，indicating that the primary magma was derived from the young crust in Meso- Neoproterozoic． Combined with the regional geological evolution background，it is considered that the Niubiziliang monzogranite formed the closure of North Qaidam ocean，which was the stretching stage product after the collision between Qaidam block and Qilian block．