Identification of zircons of different origin in complex rock mass and its implications for zircon U-Pb age:A case study from zircon of Archean basement in Liaohe Depression
The complex rock mass has undergone an extremely protracted geological evolution,and its genetic types of zircons are very complicated.Synthesizing previous research results for external morphology,CL luminescence intensity,internal structure and trace elements of zircons with different origin types,the genetic types of zircons were summarized into 3 major categories for magmatism,metamorphism and hydrothermal,and they could be further divided into 9 minor categories.During U-Pb dating of zircon,the genetic identification of single-grained zircons or different locations on zircons is a precondition for obtaining high-quality U-Pb age data.Taking a case of the complex rock mass from the Archean basement in the Liaohe Depression,a systematic study was carried out on zircons with complex genesis and a rim-core texture in this rock mass,and U-Pb ages were obtained from zircon domains of different origins to explore the significance of zircon genesis in obtaining high-quality U-Pb ages.The cathodoluminescence images show that the zircon cores are mainly characterized by magmatic oscillatory zoning,and some also show metamorphic structures such as porphyritic,spongy and sector zoning,while the zircon rims mainly show the internal structures of sector or fir-tree sector zoning of metamorphic recrystallization.The characteristics of Th/U ratios(0.39～1.32)and dispersed rare earth elements in zircon cores indicate that they are magmatic crystalline zircon,metamorphic zircon and metamorphic residual zircon.The rims surrounding them are primarily overgrowth edges formed by metamorphic recrystallisation or fluid alteration.Based on the identification of zircons with different genetic types,U-Pb ages were obtained from cores and rims of zircons from three samples,respectively.These ages represent the magmatic age of magmatism and protolith formation,and the metamorphic age of metamorphism.The results of zircon U-Pb dating of the complex rock mass in the Liaohe Depression basement indicated that there were two Late Neoarchean magmatic events of 2 586～2 583 Ma and 2 565～2 538 Ma,while metamorphic events mainly occurred at 2 508～2 501 Ma.The data on magmatic and metamorphic events obtained from the basement of the Liaohe Depression are crucial to reveal the formation and evolution of the Precambrian basement in the North China Craton coverage area.
Internal structure of zirconTrace elements in zirconGenetic types of zirconU-Pb ageLiaohe Depression
万方数据
维普
