Formation Time,Geochemical Feature And Petrogensis Of The Dongjiangkou Pluton In The South Qinling Block

The South Qinling block is an important part of the Qinling orogenic belt,with complex formation process and extensive Early Mesozoic granitoid magmatism.The Dongjiangkou pluton is a significant Early Mesozoic granitoid pluton exposed in the eastern South Qinling tectonic-magmatic belt.Based on detailed field investigation,petrology,zircon U-Pb dating,lithological and isotopic geochemistry,this study focuses on geological and geochemical features of this pluton.Field investigation indicates that the predominant rock type of the Dongjiangkou pluton is granodiorite,with minor monzonitic granite and abundant mafic enclaves.Zircon U-Pb dating indicates that the granodiorite and monzonitic granite from the Dongjiangkou pluton were emplaced in 224?215 Ma,and mafic enclaves crystallized in 221?214 Ma,which belong to the products of magmatic activities in the same period.Whole rock geochemical data show that granitoid rocks have characteristics of metaluminous to weak peraluminous,high Mg#values(57?63)and adakitic signature,belonging to the high-K calc-alkaline series;mafic enclaves have characteristics of low SiO2(52.92?61.45 wt.%),metaluminous,and high Mg#values(59?70),belonging to the high-K calc-alkaline to shoshonite series.Evidence from Sr-Nd-Hf isotopic composition suggests that the granitoid rocks might form from crust-derived granitic magmas,while the mafic enclaves came from mantle-derived mafic magmas,which have exchanged chemically and isotopically with the granitic magmas during the magma mixing process.Mineralogical evidence indicates that the probable emplacement depths of the pluton are in the range of 5.50?10.50 km,with an average value of 8.17 km,and the crystallization temperatures of the rocks vary from 605? to 665?.The injection of the mafic magmas into the felsic magmas led to the rise of the temperature of the magmatic system in the early stage,and the mafic magmas quenched to form the mafic enclaves in the late stage.The characteristics of granitoid rocks and mafic enclaves point to a complex process of magma mixing with mingling and chemical mixing.In combination of ages of the inherited zircons and isotopic composition of the Dongjiangkou granitoid rocks,it can be suggested that the magmas of the Dongjiangkou granitoid mainly originated from the Neoproterozoic basement of South Qinling and/or the northern margin of the Yangtze block,and Early Paleozoic crustal rocks might also contribute material to the magma source of this pluton.