Water-fluxed melting and rheological weakening within the continental-scale Ailaoshan-Red River strike-slip shear zone, Western Yunnan, China

Abstract

While dehydration melting is universally acknowledged as a crucial factor in the evolution of the continental crust, the significance of water-influxed melting frequently receives insufficient attention. The Ailaoshan-Red River shear zone (ASRR-SZ) stands as one of the continental-scale exhumed strike-slip faults that have significantly contributed to the intraplate deformation and the collision between the Indian and Eurasian Plates. In a combined analysis of field-based structures, petrology, whole-rock geochemistry, zircon geochronology, phase equilibrium modeling, and Electron Backscatter Diffraction (EBSD) textures of leucogranites and migmatites within ASRR-SZ, the results reveal typical water-influxed melting characteristics. The leucogranites are characterized by high contents of Sr, Ba, and Ca, elevated ratios of Th/U and Zr/Hf, and low levels of Rb, Nb, Ta, as well as low Rb/Sr and Sr/Y ratios. Despite increasing Sr contents and decreasing Ba contents, the Rb/Sr ratios of leucogranites remain remarkably stable. The melting reactions identified are Qz + Pl + Kfs + H₂O = Melt and Pl + Ms + Qz + H₂O = Melt, with an additional 2.75 wt% of H₂O. Quartz grains from both leucogranites and migmatites display ductile deformation features indicative of grain boundary migration recrystallization, occurring at temperatures ranging from 424 °C to 678 °C. The timing of water-influxed melting within the shear zone was determined to be 32–26 Ma, which is consistent with left-lateral strike-slip shearing (31–20 Ma). The process of anatexis not only significantly affects the thermodynamic and rheological properties of rocks, but also the migration of melts can facilitate crustal differentiation, resulting in the formation of extensive leucogranites. The process plays a crucial role in the origin, modification, and stabilization of the continental crust.