SHRIMP U-Pb Ages Of Zircons From The Banded Iron Formation In Miyun Of Beijing And Its Geological Significance

The Precambrian Banded Iron Formations(BIF)is an important type of iron ore resource in the world,which is widely distributed in the world.BIF,as a kind of Early Precambrian special geochemical sediments,which experienced complex metamorphic deformation in the process of formation,has an important role in the early tectonic inversion of Earth(mantle plume and the early tectonic evolution).Located in the northern margin of the North China Craton,Miyun District,Beijing,has experienced a complex tectonic metamorphic event in the Archean to early Proterozoic and have been formed a large number of large and medium-sized sedimentary metamorphic iron deposits in this process.This paper reports zircon SHRIMP U-Pb ages for four samples,which collected from Fengjiayu iron deposit,Shachang iron deposit,and the host rock near the Fangmayu iron ore.The age data obtained are as follows:(1)the metamorphic zircon U-Pb207Pb/206 Pb ages of the magnetite quartzite from Feng Jiayu iron ore are 1777±9 Ma and 2456±8 Ma,respectively;(2)the metamorphic zircon U-Pb ages of the magnetite quartzite from Shachang iron ore are 1806±13 Ma and 2469±21 Ma,respectively.And the207Pb/206 Pb age of detrital zircons was 2517+12 Ma.Then,The two groups zircon U-Pb207Pb/206 Pb ages of amphibolite vein that invades into the iron ore are 1798+15 Ma and 2531+7Ma,which represent the ages of metamorphic zircon and Captured zircon,respectively.Therefore,it is concluded that the sedimentary age of the Banded Iron Formations is possible 2469±21 Ma to 2517+12 Ma;(3)The metamorphic zircon U-Pb ages of the magnetite-garnet-plagioclase gneiss near the Fangmayu iron ore are1817±13 Ma and 2456±6 Ma.And the207Pb/206 Pb age of detrital zircons was 2529+8Ma.It also indirectly constrains that the banded iron formation was formed between2456±6 to 2529±8 Ma,most likely at the end of Neoarchean by combining previous research data.Miyun BIF type iron ore indicates an important BIF mineralization events in the region through the late Neoarchean in the North China Craton.Based on the cathodoluminescence images of zircons,together with the petrography of rocks,the metamorphic zircon ages representing two stages ofmetamorphism of the Banded Iron Formations from the four samples,which corresponding to similar ages of two large-scale metamorphic events(namely ca.2.44 Ga and ca.1.82Ga)reported from the eastern block of North China Craton,shows that the metamorphism of Banded Iron Formations are closely related with two metamorphic events of the North China Craton in Proterozoic.So we speculate that the metamorphic mineralization in the area of BIF has a close genetic relationship with two major tectonic thermal events occurred throughout the eastern North China Craton landmass in the early Proterozoic,and the two is a continuous process in time.Based on the tectonic evolution history of North China Craton,we suggest that the BIF formation in the Miyun area may be formed in the island arc tectonic setting.