Metamorphism-Deformation Phases and Ar-Ar Chronological Constraints of the Lancang Group in the Middle and Southern Sections of the Lancangjiang Tectonic Belt, Western Yunnan

The determination of metamorphic deformation process in Lancang Group can reveal the evolution history of Lancangjiang tectonic belt. Based on the "tectonic-metamorphic" study of the medium-low glacial facies metamorphic rocks and the high-pressure metamorphic rocks exposed in the Shuangjiang-Huimin area, this paper presents the following findings on the metamorphic rocks in the Lancang Group:(1) the Fold cleavage S₂ under the extrusion system is the regional foliation. The facies S₁ represented by the quartz veins in the metamorphic rocks are consistent with the S₀, and S₁ is nearly parallel with S₂ after the second stage. (2) The tectonic pattern and the metamorphic deformation period are basically the same in the resarch area, showing two-stage middle and deep tectonic stratigraphy in Permian-Triassic, which is shallow-surface deformation superposition of the Jurassic-Cenozoic. (3) The high-pressure metamorphic rocks record two periods of deep tectonic deformation, and the tourmaline and garnet have a distinct two-stage growth pattern. The core (M_1aD_1a) may represent the peak eclogite facies metamorphism, The edge represents the blue lithofacies (M_1bD_1b) stage during the exhumation process. The high-pressure metamorphic rocks are consistent with the tectonic level and deformation pattern of the second stage (M₂D₂) of the metamorphic rocks, which represents the process of the compressive stress after the exhumation process. Both shear-type cleavages S₂ indicate that the upper disk shear sliding toward to cleavages. Combined with the Cenozoic Sanjiang area occurred near the 90° block rotation, the recovery of the second phase of the disk against the thrust of the role of E. By the ⁴⁰Ar-³⁹Ar dating of quartz sandstone muscovite in Xingfu area, phengite and blue amphibole in Xiaoheijiang, and reinterpreting the ⁴⁰Ar-³⁹Ar age of the blue amphibole in Xiaoheijiang, it is concluded that Lancang Group has 4 tectonic stages:(1) Metamorphism of blueschist facies in the first stage (D_1b) of high-pressure metamorphic rocks, ~250 Ma. (2) The second phase (M₂D₂) is extrusion in the middle level, 215-214 Ma. (3) The shallow metamorphic rocks N-S to the longitudinal folds belong to the third stage (D_3a):111.9-103.7 Ma with the late (D_3b) thrust, ~82.28 Ma. (4) The fourth period near the E-W to the vertical fold:the new generation (probably later than 59.18 Ma).