Multi-stage evolution of gneiss from North Dabie: evidence from zircon U-Pb chronology  

作　　者：XIEZhi GAOTianshan CHENJiangfeng 

机构地区：[1]EarthMaterialCyclingLaboratory,SchoolofEarthandSpaceSciences,UniversityofScienceandTechnologyofChina,Hefei230026,China

出　　处：《Chinese Science Bulletin》2004年第18期1963-1969,共7页

摘　　要：TTG gneiss is a common rock to outcrop in the northern part of the Dabie orogen, a few of which are closely associated with eclogites that experienced the Triassic ultra-high pressure metamorphism. Although they were thermally metamorphosed by a large-scale magma activity in this re-gion at the Early Cretaceous, it is unclear whether or not they are also affected by the Triassic metamorphism during continental subduction and exhumation. In order to resolve this issue, SHRIMP zircon U-Pb dating was carried out for the host gneiss of eclogites in North Dabie. The results show that cores from the gneiss have an age of 746?1 Ma, consis-tent with the protolith ages of granitic gneisses in the Dabie orogen. Zircon overgrowing with different U and Th concen-trations give concordant ages of 212?1 and 120?1 Ma, respectively. Th/U ratios of overgrown zircons are both lower than 0.1, suggesting a metamorphic genesis. The present re-sults suggest that the gneiss in North Dabie has the similar protolith ages of Neoproterozoic to those granitic gneisses elsewhere in the Dabie orogen, and experienced not only the Triassic metamorphism but also the thermal metamorphism due to the Early Cretaceous magmatism. This provides an important insight into the geodynamic evolution of gneissic rocks in the Dabie orogen.TTG gneiss is a common rock to outcrop in the northern part of the Dabie orogen, a few of which are closely associated with eclogites that experienced the Triassic ultra-high pressure metamorphism. Although they were thermally metamorphosed by a large-scale magma activity in this re-gion at the Early Cretaceous, it is unclear whether or not they are also affected by the Triassic metamorphism during continental subduction and exhumation. In order to resolve this issue, SHRIMP zircon U-Pb dating was carried out for the host gneiss of eclogites in North Dabie. The results show that cores from the gneiss have an age of 746?1 Ma, consis-tent with the protolith ages of granitic gneisses in the Dabie orogen. Zircon overgrowing with different U and Th concen-trations give concordant ages of 212?1 and 120?1 Ma, respectively. Th/U ratios of overgrown zircons are both lower than 0.1, suggesting a metamorphic genesis. The present re-sults suggest that the gneiss in North Dabie has the similar protolith ages of Neoproterozoic to those granitic gneisses elsewhere in the Dabie orogen, and experienced not only the Triassic metamorphism but also the thermal metamorphism due to the Early Cretaceous magmatism. This provides an important insight into the geodynamic evolution of gneissic rocks in the Dabie orogen.

关 键 词：多级进化 TTG片麻岩 变质作用 U-PB年龄测定 锆 年代学 

分 类 号：P588.345[天文地球—岩石学] P585[天文地球—地质学]