Garnet bearing granodiorite in the Jianfengshan area on the northern margin of Qaidam basin is vein distributed in Dakendaban rock group.The rocks are mainly composed of quartz(30%-35%),plagioclase(45%-50%),potassium feldspar(15%-20%),biotite(5%)and a small amount of muscovite(<5%),garnet(<5%).Zircon U-Pb dating shows that the formation age of garnet bearing granodiorite is(441.1±1.4)Ma,belonging to Early Silurian.The rocks have high SiO2(66.27%-74.42%),Na2O(3.47%-6.75%),Al2O3(14.71%-20.43%),low MgO(0.07%-0.56%),Mg#(19.8-43.6)contents and strong peraluminite(A/CNK ranges from 0.99～1.34).The minerals are enriched in U,K,Ba and depleted in Nb,Ta,Sr,P,Eu,Ti and other elements.The REE distribution curves show a right-leaning pattern,showing a weak negative Eu anomaly(δEu=0.31-0.81).The rocks have high ISr values(0.707 123-0.708 081)and negative εNd(t)values(-1.91 to-2.37),and the two-stage model age TDM2 is 1 660-1 756 Ma.After correction,(206Pb/204Pb)i value is 18.354 7-18.582 2,(207Pb/204Pb)i value is 15.365 4-15.641 2,(208Pb/204Pb)i value is 38.254 7-38.654 1.Zircon Hf εHf(t)ratios range from 2.5 to-9.1,and the corresponding Hf isotopic crust model ages TDM2 range in 1 263-2 012 Ma.The study of rock geochemistry and isotopes shows that the garnet bearing granodiorite in the Jianfengshan is an S-type granite,which was formed in the collision orogenic stage between Qaidam block and Qilian block.It is the early subducted continental and oceanic crust that experienced ultra-high pressure metamorphism(eclogite facies conditions)and the rise of partially molten fluid,resulting in the partial melting of the continental crust.
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