1. Xi'an Center of Geological Survey, China Geological Survey, Xi'an 710054;
2. Ministerial Key Laboratory for the Study of Forcused Magmatism and Giant Ore Deposits, Xi'an 710054;
3. Comprehensive Exploration Institute of Geology and Mineral Resources of Qinghai Qaidam, Geermu, Qinghai 816000
The gabbro and hornblende gabbro are widely occur at the nickel-copper mining area of Yanchangbeishan in Lenghu,which located in the west segment of the north margin of Qaidam. The Rittmann index of the rocks show they belong to sub-alkaline series. The chondrit and primitive mantle-normalized curve show two group characteristics,but have similar partition pattern,which indicate they came from the same magmatic source. The REE standard curve show weak right dip type and no Eu anomalies nearly,that may reflect weak fractional crystallization of plagioclase. The abundance and ratio of Rb,Nb,Zr,Cr show the evolution trend of partial melting. Combined with the fairly high Mg# and SI,the primary magma have a feature of primary basaltic magma. It is believed that the magma originated by partial melting,based on the subduction slab fluid metasomatized mantle wedge,and the abundance of Nb,Zr,Y imply the rocks derived from transitional mantle source. Moreover,these samples are low HFSE (Nb,Ta,Zr,Hf,Ti),high LILE (Ba,Th,U,Pb),reflecting the typical characteristic of subduction zone and island-arc magma. LA-ICPMS zircon U-Pb dating shows that the formation age of the gabbro is 254±3 Ma. Based on the regional geological background,we consider that the north margin of Qaidam was in volcanic arc or active continental margin tectonic environment during Permian epoch.