Stability of zircon during mylonitization: a case study of granitic mylonite in Gaoligong and Ximeng, Southwest China

The stability of zircon under natural conditions is of great significance for reasonable interpretation of zircon age and further understanding of zircon isotopic chronology. In this paper, the microstructure and zircon U-Pb chronology of granitic mylonites in Gaoligongshan and Ximeng areas of the Sanjiang orogenic belt in the southwestern China have been studied. Finerals feldspar, which are the main rock-forming quartz and mica in granitic mylonite from the Gaoligong and Ximeng areas were significantly deformed and recrystallized, while zircon preserved the original structure in the form of independent crystal or residual core. The zircon U-Pb ages of the Gaoligongshan and the Ximeng mylonites are 513 ±8 Ma and 459 ±2 Ma, respectively, which represent the age of granitic protolith. Influenced by the Cenozoic tectonic metamorphism, some zircon grains developed spongy structure and dissolution structure at the rim, indicating the metamorphic recrystallization under the condition of a small amount of fluid. The metamorphic mechanism of quartz is grain boundary migration recrystallization, which indicates that the deformation temperature is 500-700℃. Quartz c-axis fabrics revealed by the electron backscatter diffraction (EBSD) technique reflect two stages of metamorphic temperature in the mylonites:the early stage of 550-650℃ and the late stage of 400-550℃. The results of rock microstructure, zircon structure and chronology show that the crystal structure of zircon is destroyed and metamorphic recrystallization occurred at 550-650℃ in the process of mylonization with a small amount of fluid involved. These conditions provide important constraints for the interpretation of zircon ages under complex natural conditions.