复杂岩体中锆石的成因判识及其对锆石U-Pb年龄的影响:以辽河坳陷太古宇基底中的锆石为例

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

复杂岩体经历了长期地质演化,内部所含锆石成因十分复杂。综合前人研究成果,根据锆石外部形态、CL发光强度、内部结构以及微量元素特征,本文将锆石成因类型归纳为岩浆作用、变质作用以及热液作用3大类9小类。在锆石U-Pb定年的过程中,单颗粒锆石或锆石上不同位置的成因判识是获取高质量U-Pb年龄数据的前提。以渤海湾辽河坳陷基底复杂岩体为例,对岩体内具有核—边结构的复杂成因锆石进行了系统的研究,分别获取不同成因锆石的U-Pb年龄,进而探讨锆石成因对获取高质量U-Pb年龄的重要意义。阴极发光图像显示,锆石的核部多见岩浆成因环带结构,也可见斑杂状、海绵状及扇形分带等变质成因结构,边部呈现变质重结晶成因的扇形和冷杉叶结构。锆石核部Th/U比值(0.39~1.32)和分散的稀土元素特征,表明核部存在岩浆结晶锆石、变质锆石以及变质残余锆石,边部主要是变质重结晶作用或流体蚀变形成的增生边。基于锆石的成因类型划分,从3个样品锆石核部和边部分别获得了代表原岩形成时间的岩浆年龄和变质作用时间的变质年龄。辽河坳陷基底复杂岩体锆石U-Pb年龄结果表明,基底存在两期新太古代晚期岩浆事件,分别为2586~2583 Ma、2565~2538 Ma,变质事件主要发生在2508~2501 Ma。辽河坳陷基底岩浆事件和变质事件的获得,对于揭示华北克拉通覆盖区前寒武纪基底的形成演化具有重要意义。

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 2586~2583 Ma and 2565~2538 Ma,while metamorphic events mainly occurred at 2508~2501 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.