The Structure, Composition And Tectonic Evolution Of The Xuelongshan Metamorphic Complex, Western Yunnan

Xuelongshan metamorphic complex is a northwest elongation of the Ailaoshan-Red River shear zone. It locates in the Sanjiang area, where the Jinshajiang and Lancangjiang tectonic zones converged. After the distinct deformation during Cenozoic, the tectonic characteristics of the complex became more complicated. This paper, in an attempt to ascertain the structure and composition of the complex combining with other previous studies, will explore the features of the structure deformation, and discuss the tectonic evolution of the Xuelongshan metamorphic complex. The results show that the Xuelongshan metamorphic complex consists of various deformed schist, granite, and some metamorphic enclaves. Due to the Cenozoic shear deformation, many rocks were mylonitized to form tectonites with different characteristics. The western granitic mylonites are typical L-tectonites, whereas the eastern deformed schist and phyllonites are S(or S-L)-tectonites. On the basis of the field investigation and the observation from microscope, the metamorphic rocks, as a whole, have NNW striking subhorizontal stretching lineations and NE or SW striking steeper dipping foliations, suggesting a NS striking shear. Additionally, the rocks in the east have super mylonitization, whereas the rocks in the west are protomylonites, indicating a decresing degree of deformation from east to west. The EBSD analysis on fabric and the metamorphic mineral assemblages show an increasing grade of metamorphism from east to west, and indicate a high greenschist-lower amphibolitic facies with a metamorphic temperature at ca.400-600℃. Based on the zircon U-Pb isotopes dating, a magmatic crystallization age of ca.240 Ma was obtained from the western granitic mylonites of Xuelongshan, correlating the Dabaoshan-Bailongliwuba granitic rocks to the Triassic intrusions, attributing the granitic mylonites to be products of the Paleo-Tethys’closure during Indosinian, situations that is similar to the synorogenic extrusions in Weixi area. The ages of the granitic mylonites also imply that Xuelongshan metamorphic complex is not a completed Pre-cambrian crystalline basement.