1�� College of Earth Sciences��Jilin University��Changchun 130061��China; 2�� ML�� Key Laboratory of Metallogeny and Mineral ��esources Assessment��Institute of Mineral ��esources��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����b��Th (LILEs) and La��Ce��Sm��Nd(L��EEs); and depleted in Ba��U(H��EEs)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��