哀牢山构造带晚渐新世地壳深熔与熔/流体迁移:锆石U-Pb年龄与微量元素证据

Late Oligocene crustal anatexis and melt/fluid migration in the Ailao Shan tectonic belt: Evidences from zircon U-Pb ages and trace element compositions

哀牢山构造带是青藏高原东南缘重要的边界构造带,其内出露的深变质岩系一直被认为是古老的变质基底岩石。利用LA-ICP-MS原位微区分析技术对哀牢山深变质岩系锆石进行U-Pb年龄、微量元素分析。结果表明深变质岩系的原岩有728±8Ma、727±3Ma、231±4Ma的花岗质岩石和其它年龄的碎屑岩,变质时代为27.8~23.7Ma。综合野外地质特征和分析结果,我们认为哀牢山深构造带在晚渐新世27.8~23.7Ma发生了大规模的地壳深熔作用,现今所见深变质岩系是由不同时代、不同岩性的原岩在晚渐新世(27.8~23.7Ma)变质形成,不全是古老的变质基底岩石。深熔过程中熔/流体发生了明显的迁移。哀牢山变质带具有混合岩化特征的岩石很可能是峰期变质作用后减压熔融的产物。晚渐新世地壳深熔作用与左行走滑剪切是哀牢山深变质带折返过程中近似同时发生的两种不同变质表现形式,两者相互影响、相互制约。

The Ailao Shan tectonic belt is an important tectonic boundary in southeast of the Qinghai-Tibetan Plateau and consists mainly of high-grade metamorphic rocks previously thought to be Paleoproterozoic basement rocks. In-situ LA-ICP-MS U-Pb age and trace element of zircons were determined for the Ailao Shan high-grade metamorphic rocks. Zircon U-Pb geochronological data indicate that the high-grade metamorphic rocks have various parent rocks with different intrusive/depositional ages of 728 ± 8Ma,727 ± 3Ma and231 ± 4Ma and similar metamorphic ages of 27. 8 - 23. 7Ma. Our new results demonstrate that the high-grade metamorphic rocks from the Ailao Shan belt zone are a mixture of metamorphic both igneous and sedimentary rocks of different times,instead of solely Paleoproterozoic basement rocks. Migmatization and metamorphic crystallization of these rocks took place at 27. 8 - 23. 7Ma,indicating a significant period of crustal anatexis in the Ailao Shan tectonic belt. The syntectonic felsic dykes hosted by gneiss were derived from a mixture of locally-derived and allochthonous melts. The migmatized rocks from the Ailao Shan deformation zone were most probably the results of decompression melting following peak metamorphism. The crustal anatexis and left-lateral shearing were two synchronous metamorphic patterns resulted from a shared geological event. They interact and influence each other during exhumation of the Ailao Shan metamorphic zone in Late Oligocene.