滇西点苍山—哀牢山变质岩系锆石SHRIMP定年及其地质意义
Zircon SHRIMP dating and its geological implications of the metamorphic rocks in Ailao Shan-Diancang Mountain Ranges, west Yunnan
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摘要: 滇西点苍山和哀牢山主体分别由下元古界苍山群和哀牢山群深变质岩系组成,被认为是前寒武纪结晶基底。选取点苍山的花岗闪长质糜棱岩(DCS-1)和哀牢山元江段黑云母花岗闪长质片麻岩(SM07-1)之中的锆石进行U-Pb SHRIMP定年。样品DCS-1和SM07-1测年结果分别为233±2.6Ma和239.8±2.8Ma。结合前人对滇西地区花岗质岩类所做的研究,揭示滇西地区在三叠纪经历一期在空间上广泛分布的岩浆事件,点苍山花岗质糜棱岩和哀牢山花岗质片麻岩的原岩为三叠纪侵位的花岗岩,其构造环境属于大陆裂谷,反映了滇西地区在三叠纪地壳物质的重新调整,而该变质岩系不是前寒武纪的结晶基底,其变质作用与山体出露地表过程和新生代构造作用有关。Abstract: Ailao Shan-Diancang Mountain Ranges, which are situated in the west part of Yunnan Province, are famous for their distinctive topography, geomorphology and geological structures. The principal part of Ailao Shan-Diancang Mountain Range is composed of high metamorphic rock series, which are considered as the Precambrian metamorphic basement of the Yangtze Plate. In this paper, U-Pb ages of zircons from granodioritic mylonite of Diancangshan Mountain (DCS-1) and biotite granodioritic gneiss of Ailaoshan Mountain (SM07-1) have been determined by sensitive high resolution ion microprobe method (SHRIMP). The zircons from the samples yield a concordant U-Pb ages of 233±2.6Ma (DCS-1, MSWD=1.47) and 239.8±2.8Ma (SM07-1, MSWD=1.9) respectively. According to the former researches on the granitoid rocks in west Yunnan Province and geochronological studies, it is concluded that the west Yunnan areas have been experienced an extensively Triassic magmatism. The protolith was the result of the Triassic magmatic event, which related with the continental rift tectonic environment. The Triassic magmatism brought a great deal of intrusions to the crustal, which have characteristics of “ bimodal”. The metamorphic rocks in the study area are unlikely to be the metamorphic basement of Yangtze Plate, its metamorphism is due to the process of the rocks uplifting and Cenozoic tectonic evolution.
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Key words:
- Zircon U-Pb dating /
- Metamorphism /
- Continental rift /
- Ailao Shan-Diancang Mountain Range
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