Chronology,Petrogeochemistry and Geological Significance of Garnet Bearing Granodiorite in Jianfengshan Area,Northern Qaidam Margin
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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