Zircon LA-ICP MS U-Pb Age,Sr-Nd-Pb Isotopic Compositions and Geochemistry of the Triassic Post-collisional Wulong Adakitic Granodiorite in the South Qinling,Central China,and Its Petrogenesis

The Indosinian post-collisional Wulong pluton intruded into the Mesoproterozoic Fuping Group,South Qinling,central China.In the southern part of the pluton,some mafic enclaves have sharp or gradational contact relationships with the host biotite granodiorite.Geochemistry,zircon LA-ICP MS(laser ablation inductively-coupled plasma mass spectrometry)U-Pb chronology and Sr- Nd-Pb isotope geochemistry of the pluton are reported in this paper.The biotite granodiorite shows close compositional similarities to high-silica adakite.Its chondrite-normalized REE patterns are characterized by strong HREE depletion(Yb=0.33-0.96 10and Y=4.77-11.19×10),enrichment of Ba(775-1386×10)and Sr(643-1115×10)and high Sr/Y(57.83-159.99)and Y/Yb(10.99-14.32) ratios,as well as insignificant Eu anomalies(δEu=0.70-0.83),suggesting a feldspar-poor, garnet±amphibole-rich residual mineral assemblage.The marie enclaves have higher MgO(4.15- 8.13%),Cr(14.79-371.31×10),Ni(20.00-224.24×10)and Nb/Ta(15.42-21.91)than the host granodiorite,implying that they are mantle-derived and might represent underplated mafic magma. Zircon LA-ICP MS dating of the granodiorite yields aPb/U weighted mean age of 208±2 Ma (MSWD=0.50,1σ),which is the age of emplacement of the host biotite granodiorite.This age indicates that the Wulong pluton formed during the late-orogenic or post-collisional stage(≤242±21 Ma)of the South Qinling belt.The host biotite granodiorite displaysSr/Sr=0.7059-0.7062,I=0.7044- 0.7050,Nd/Nd=0.51236-0.51238,ε(t)=-2.26 to -2.66,Pb/pb=18.099-18.209,Pb/pb= 15.873-15.979 andPb/Pb=38.973-39.430.Those ratios are similar to those of the Mesoproterozoic Yaolinghe Group in the South Qinling.Furthermore,its Nd isotopic model age (～1.02 Ga)is consistent with the age(～1.1 Ga)of the Yaolinghe Group.Based on the integrated geological and geochemical studies,coupled with previous studies,the authors suggest that the Wulong adakitic biotite granodiorite was probably generated by dehydration melting of the Yaolinghe Group-like thickened mafic crust,triggered by underplating of mafic magma at the boundary of the thickened mafic crust and hot lithospheric mantle,and that the Wulong adakitic biotite granodiorite may have resulted from thinning and delamination of the lower crust or breakoff of the subducting slab of the Mianlue ocean during the Indosinian post-collisional orogenic stage of the Qinling orogenic belt.