Research Of Geology,geochemistry And Zircon Geochronology Of The Granite-greenstone Belt In Dengfeng Area,Henan Province

Predecessors have done a lot of research in North China Craton.But most of them are based on the north and central parts of North China Craton.And there is less research on the south.Dengfeng is an important research area on the southern edge of the North China Craton.Dengfeng granite-greenstone belt is one of the important crustal tectonic units in North China Craton,located in Songshan area,central Henan province.The granite-greenstone belt comprised of Dengfeng group and the surrounding tonalite,trondhjemite,granodiorite gneiss and granitoids.Taking Dengfeng granite-greenstone belt as research content,using zircon U-Pb geochronology,major and trace element geochemistry and Hf isotope methods.We try to explore further the formation time,material source,genetic mechanism using zircon U-Pb geochronology,major and trace element geochemistry and Hf isotope methods.The structural background and evolution of the early Precambrian in Dengfeng area are also discussed.(1)We can get the age of Dengfeng group's Laoyanggou is 2522±13 Ma through the U-Pb dating of zircon.Combining with previous data,we limit the formation time of the Dengfeng group to 2.55?2.50Ga.It belongs to the late Neoarchean.The?_Hf(t)values of Dengfeng group from-2.51 to+9.56.The average is+5.2.The vast majority are positive,indicating magma from the mantle and minor crustal material.The geochemical characteristics of amphibolites indicate that they are originated from depleted lithospheric mantle.According to the characteristics of the mid ocean ridge basalt and the characteristics of island arc basalt,it is presumed to be produced in the back arc basin.Because it also possesses the properties of the mid-ocean ridge basalt and island arc basalt,we assume that it comes from the back-arc basin.(2)The formation time of granitoids is late Archean.Combining zircon U-Pb dating with previous data,we believe the formation time is 2.60?2.50 Ga.They are from continental margin arc or island arc environment.The characteristics of the ratios of trace elements are similar to those mafic rocks affected by subducted plate hydrothermal melts.They are quasi-aluminum,weak peraluminous and calc-alkaline,which conform to I type granite characteristics.They basically rich Na and low K.The?_Hf(t)values of granites+0.97 to+7.55.The average is+4.87.It is formed by partial melting of primary basaltic oceanic crust.(3)The comprehensive studies gave the conclusion that the structural background is the oceanic crust subduction of convergence plate margin.There is a transition from compression to post-collision in Dengfeng the late Archean.