摘要
部分学者根据西藏1:10O万改则幅区调资料和戈木日、果干加年山浅变质岩中获得的14个锆石Pb-Pb年龄(3204～509 Ma),建立了太古宙—元古宙岩石地层系统,提出了“结晶基底”与“软基底”的双层结构模式。资料证实样品取自沉积岩,锆石为碎屑锆石,无法确切反映成岩或变质年龄。作者等在原划为古元古界都古尔片麻岩中获得了384 Ma的锆石年龄、日湾茶卡东山下石炭统之下火山岩(原划AnD)获得346 Ma的Rb-Sr年龄、玛依岗日组玄武岩夹层中获得318 Ma的Rb-Sr年龄,野外工作中将“基底岩系”与上覆石炭三叠系角度不整合关系厘定为整合关系。基于野外地质产状和同位素年龄资料分析,对羌塘地区基底和双层结构模式提出质疑,认为羌塘中部的浅变质岩系主体时代为晚石炭世。太古宙—元古宙岩石地层、变质期次、双层基底结构及其演化的认识显然是不正确使用同位素年代数据得出的结论。
According to the regional geological surveying data from the Gerze area, Xizang( Tibet) (1 : 1000000) and 14 zircon-single Pb-Pb ages (3204-509 Ma) from the low-metamorphic rocks in Gomo Ri and Guoganjianian Mountains, some geologists constructed a Pt-Ar lithostratigraphic system and proposed a two-layer structure model with a hard crystallized basement and a soft basement. But some geological data have made certain that those zircons came from debris sediments, and that those ages could not represent the diagenetic and metamorphic time. The author has obtained 384 Ma Zircon-single isochron age from the Douguer gneiss (which was thought to be Paleoproterozoic before) , 346 Ma Rb-Sr isotopic age from the volcanics deposited under the early Carboniferous layer to the east of Riwanchaka (which belonged to AnD before) and 318 Ma Rb-Sr isotopic age from the basalts interbedded within the Mayer Kangri Formation. During the field study, the author also corrected the angle unconformity between the basement and the Carboniferous-Permian cap rocks to the conformity. On the basis of the analyses for the field geological occurrence and the isotopic ages, a question related to the basement and its two-layer structure of the Qiangtang micro-plate was put forward. At last, the author thinks that most low metamorphic rocks are thought to be Carboniferous and that the previous conclusion about the Pt-Ar lithostratigraphics, metamorphic times, two-layer basal structure and the evolution is not true because it resulted from the misuse of isotopic ages.
出处
《地质论评》
CAS
CSCD
北大核心
2003年第1期4-9,共6页
Geological Review
基金
原地质矿产部"九五"重大基础项目(编号9501101-05)和中国石油天然气集团公司"九五"科技工程(编号QZ-97-204102)联合资助的部分成果。
