Petrogenesis of the Silurian Liqiao granites in the Tianshui area,West Qinling:Constraints from geochemistry,zircon U-Pb geochronology and Hf isotope
Granite plays an important role in exploring the growth and evolution of continental crust as well as the geodynamics of orogenic belts.The Liqiao granite in the Tianshui area,West Qinling Orogen,is mainly composed of plagioclase,quartz,K-feldspar,and biotite.Zircon U-Pb dating shows that the crystallization age of the granite is 439±3 Ma.Geochemistry suggests that the granite contains high silicon and alkali,and it belongs to the peralu-minous to high potassium calc-alkaline series.The granite is rich in large ion lithophilic elements(e.g.K and Ba),light rare earth elements(e.g.La and Ce),and depleted in high field strength elements such as Nb,Ta,and Ti,which shows the geochemical characteristics of island arc magmatic rocks.Furthermore,the granite in the Liqiao area shows low Mg# values(25～41),and the La/Yb-La and La/Sm-La diagrams show that the formation of the granite is related to partial melting.In addition,the existence of microgranular enclaves in the host rock mass and the positive εHf(t)values of zircons indicate that the formation of these granites is influenced by mantle-derived materials.Combined with the regional geological background and the results in this study,we believe that the Liqiao granite was formed by partial melting of the juvenile basaltic lower crust before the arc-continental collision.Certainly,it cannot be ruled out that it was the product of partial melting of the thickened of the continental marginal arc.
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