Late Oligocene crustal anatexis and melt/fluid migration in the Ailao Shan tectonic belt: Evidences from zircon U-Pb ages and trace element compositions
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 and 231 +/- 4Ma and similar metamorphic ages of 27. 8 similar to 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 similar to 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.
