Element Distribution Characteristics And Magmatic Evolution Of Coarse-grain Granites In West Qinling Orogenic Belt

As an important part of continental crust,granite records important information of crust-mantle interaction,diagenesis,mineralization and tectonism.It is of great significance to explore the diversity of granite compositions for understanding the evolution of granite source rocks,magma and the formation and evolution of continental crust.The Qinling orogenic belt,as an important part of the central orogenic belt,is a composite orogenic belt formed through multi-stage tectonic evolution.The West Qinling orogenic belt is located in the western extension of the Qinling orogenic belt,where Mesozoic and Cenozoic granitic bodies and Paleozoic granitic bodies are widely developed.The distribution of granites is different in time and space.The study of petrogenesis and magmatic evolution of the granites from Paleozoic to Mesozoic in the West Qinling orogenic belt is of great significance to reveal the tectonic evolution of the Caledonian and Indosinian orogenic belts,and to discuss the formation process and mineralization of the West Qinling orogenic belt.In this paper,the Wujiahe Wenquan granite,Lixian Wuchaba granite,Qingshui granite,and Miba granite in the West Qinling mountains are taken as the research objects.Through the geochemical characteristics of the whole rock and single mineral of the granite,the material source,tectonic background and physical and chemical conditions of rock formation of the granite are comprehensively analyzed.Based on the distribution coefficient characteristics of rocks and rock-forming minerals,this paper reflects the evolution stages and influencing factors of granite magma in the study area of the West Qinling orogenic belt,and thus discusses the relationship between granite magma evolution and tectonic setting in the West Qinling orogenic belt.The main insights gained are as follows:(1)Wujiahe granite and Miba granite are orogenic calc alkaline I-type granite from crust mantle mixed source,Lixian granite is orogenic calc alkaline I-type granite from crust source,and Qingshui granite is peraluminous S-type granite from crust source.(2)Using biotite as geological thermometer and pressure gauge,the physicochemical conditions of granite formation in the study area are as follows: the crystallization temperature range of biotite of Wujiahe granite is 760?786 ?,and the crystallization pressure is about 68 MPa;The crystallization temperature of biotite from Lixian granite is about 766 ?,and the crystallization pressure is about 109 MPa;The crystallization temperature range of biotite of Qingshui granite is 537?722 ?,and the crystallization pressure is about 365 MPa;The crystallization temperature of biotite from Miba granite ranges from 739?782 ?,and the crystallization pressure is about 160 MPa.(3)According to the distribution coefficient of rare earth elements in the whole rock and rock forming minerals,the crystallization degree of magmatic evolution is Qingshui granite,Miba granite,Wujiahe granite and Lixian granite from high to low,which corresponds to the corresponding crystallization pressure and depth.According to the characteristics of trace elements,the crystallization sequence of minerals is biotite-feldspar-quartz.(4)Qingshui granite is related to Caledonian movement and formed in Paleozoic granite magmatism(507?422 Ma)in West Qinling.It was produced by partial melting of subducted plate.Wujiahe granite,Lixian granite and Miba granite are related to Indosinian movement and formed in granite magmatism(225?185 Ma)in the late Early Mesozoic in the West Qinling Mountains.Lixian granite was formed by partial melting of subducted oceanic crust.Wujiahe and Miba granites were produced by the rise of asthenosphere caused by subduction plate fracture and partial melting of lower crust heated by underplating.(5)The degree of magmatic evolution in the study area is mainly affected by the source of magmatic material and the degree of partial melting.According to the composition of whole rock and rock forming mineral,the magmatic evolution degree is Qingshui granite,Lixian granite,Wujiahe granite and Miba granite from high to low,which reveals the difference between the magmatic evolution degree of some S-type granites in the Paleozoic North Qinling area of the West Qinling and the I-type granites in the late Early Mesozoic South Qinling area.