Geochemistry Characteristics Of Early Cretaceous Granitic Rock From Western Dabie Orogen And Its Fractional Mechanism Of Iron Isotope

Granitc rock of Xinxian pluton and Shangcheng pluton from the Western Dabie orogen were selected as the research objects in this study.We studied and analyzed these samples through field investigation and sampling,petrology,mineralogy,metal stable isotope geochemistry and other research methods,with the combination of previous data.The petrogenesis and isotopic fractional mechanism of the two types of granitic rocks were discussed by the characteristics of major elements,trace elements and iron isotopic composition of samples,and further explored their sources.This study mainly obtains the following conclusions:（1）According to the elemental geochemistry,mineralogy and petrogenesis of the samples.It is determined that the rock type of Xinxian pluton is biotite monzogranite,while the genetic type is highly fractionated type I granite.The rock types of each unit from Shangcheng pluton are different.The oldest Sigudun unit is giant porphyritic biotite monzogranite,the Shiguwa unit is porphyritic biotite monzogranite,while the Loufangwan unit is porphyritic biotite monzogranite.The genetic type of Shangcheng is adakitic rock.（2）The petrogenetic physical and chemical conditions of two plutons are quite different.The petrogenetic depth of Shangcheng pluton is deeper than that of Xinxian pluton,and the scale of its petrogenetic depth is large.According to the calculation results of formulas,the petrogenetic temperature of Xinxian pluton is 575℃～760℃,the petrogenetic pressure is 1.57kbar～1.90kbar,and the petrogenetic depth is13.87km～30.97km.The petrogenetic temperature of Shangcheng pluton is 642℃～713℃,the petrogenetic pressure is 3.67kbar～8.19kbar,and the petrogenetic depth is13.87km～30.97km.（3）The fractionation mechanism of iron isotopes in granitic rocks in the western Dabie area is revealed.Theδ⁵⁶Fe value of Xinxian pluton is between 0.19‰～0.36‰,and the Shangcheng pultonδ⁵⁶Fe value is between 0.08‰～0.20‰.The iron isotope fractionation in the two plutons is not affected by fluid exsolution and later alteration.The fractional crystallization of Fe-Ti oxides and the changes of melt and mineral composition are the main mechanisms controlling the iron isotope fractionation of Xinxian pluton.The difference of mineral composition of source rocks and fractional crystallization are the main mechanisms controlling the iron isotope fractionation of Shangcheng pluton.（4）The petrogenetic process of Xinxian pluton is similar to that of Shangcheng pluton.The lower crust material is partially melted to produce initial magma.Fractional crystallization is the control mechanism of the iron isotopic composition of the melt.The source rock,temperature,pressure and partial melting degree have an impact on the composition of the initial magma,making its heterogeneous composition.For the early Cretaceous granitic rocks in the whole western Dabie area,this model may have a certain reference significance.（5）For the early Cretaceous granitic rocks in the western Dabie area,this model may have a certain reference significance.The mechanism of controlling its iron isotope fractionation can give priority to fractional crystallization.The changes of melt and mineral composition,mineral composition of source rock,partial melting and other factors should also be discussed.There are 32 figures,12 tables and 195 references in this thesis.