Geochemistry And Petrogenesis Of Granites From The Wuduoshan Pluton,the North Qinling Terrane

The Wuduoshan pluton is located in the eastern of the North Qinling terrane with characteristics of complex rock types and multi-period emplacement.In this paper,we have studied the petrological characteristics,zircon age,major elements,trace elements and Sr-Nd-Pb isotopic composition to discuss the petrogenesis,magmatic source and geodynamic significance of the Wuduoshan pluton.The Wuduoshan pluton is mainly composed of biotite granite,granodiorite and monzogranite.Analysis results of zircon U-Pb dating show that the crystallized ages vary from 441Ma to 426Ma and inherited zircons are obtained in some samples with ages varied from Paleoproterozoic to Neoproterozoic.There are also dikes and intrusions with ages about 400 Ma in this pluton.REE discriminant diagram shows that these granites are mainly formed by partial melting.The oxides of most major elements are negatively correlated with the content of SiO2.Granites from the Wuduoshan pluton exhibit characteristics of high silica,low magnesium and alkaline-rich.According to geochemistry and crystallized ages,granitic samples from the Wuduoshan pluton can be divided into three groups:Granites of 441-438Ma are enriched in LREE and depleted in HREE without obvious Eu anomalies(0.73?1.45).They have high Sr/Y ratios ranging from 39.7 to 120,high(La/Yb)N ratios ranging from 18.8 to 56.0,and relatively high initial ?Nd values from-1.9~-4.5.These features indicate that granites of this group were generated at the depth of lower crust without(or a small amount)plagioclase in the residue of source area.Granites with crystallized ages of 433-426Ma have lower Sr/Y ratios ranging from 8.2 to 53.9 and high(La/Yb)N ratios ranging from 14.3 to 174 with obvious Eu anomalies(0.27-0.86,average value is 0.53).These features indicate that granites were formed at a relatively shallow depth of middle-upper crust with a significant plagioclase in the residue of source area.The last group granites of 400Ma show obviously Eu positive anomalies(1.55 and 1.72).These may be the product of feldspar accumulation in the later stage of differentiation for granite in middle-upper crust.In addition,the Pb isotopic composition of the Wuduoshan grantitoid is similar to that of the North Qinling terrane.The source is a mixture of metasedimentary rocks of the North Qinling and juvenile crust materials,and these juvenile crust materials may be intermediate-basic intrusive rocks in the crust in the earlier stage.The North Qinling terrane were under the post-collisional environment from the Late Ordovician to the Early Silurian.The difference in the formation depth for granites indicates that the North Qinling terrane changed from thickened crust to normal crust.In addition,the geochemical characteristics of granites from the Wuduoshan pluton and the North Qinling terrane of Middle and Late Silurian show the petrogenesis of partial melting in middle-upper crust.These features indicate the collapse process of earlier thickened crust in the North Qinling during Early Silurian,then these granites were generated as partial melting of middle-upper crust.