Structural deformation and geochronology of the ductile shear zone along the southern margin of the Foping dome, South Qinling

A typical granulite–migmatite–gneiss dome developed in the Foping area of the central Qinling orogenic belt. This area is key to studying the metamorphic deformation of continental crust and the Mesozoic tectonic evolution of Qinling. The Yangtianba–Shimudi ductile shear zone along the dome's southern margin records information on middle–deep structural deformation during the late Triassic compressional–extensional transition, offering crucial constraints on the exhumation mechanism of the Foping dome.

A detailed investigation of representative metamorphic and deformed rock samples from the shear zone was conducted using structural analysis, mineral geochemistry, crystallographic preferred orientation (CPO), and geochronology. Field observations and kinematic vorticity analysis show that this shear zone developed under right-lateral ductile shear deformation controlled by pure shear.

In the felsic mylonite, quartz primarily shows prism <a> and prism <c> slip systems, suggesting deformation occurred under amphibolite facies conditions at approximately 550–650 °C. The characteristics of the metamorphic mineral assemblages and the results of garnet–biotite–plagioclase thermobarometry indicate a clockwise P–T path, with peak metamorphic conditions of 568–611 °C/5.2–5.3 kbar and 630–654 °C/7.1–7.9 kbar. The isothermal decompression stage M2 recorded conditions of 590–616 °C/3.5–4.5 kbar. Zircon U–Pb dating of the leucosomes in the migmatites within the shear zone yielded an age of 180.8 ± 3.8 Ma, representing the lower limit of the ductile shear deformation.

Integrated with regional geological data, the metamorphic and deformational evolution of the study area can be reconstructed as follows: Prior to ~210 Ma, the central segment of the South Qinling tectonic belt was dominated by collisional orogenesis, leading to crustal thickening and the development of progressive metamorphism (M1) in the Foping area. During 210–200 Ma, the Foping region transitioned into post-collisional extension. This transitional phase was characterized by a bidirectional stress regime combining horizontal shortening and vertical collapse, which triggered ductile shear deformation (D1) in the Yangtianba-Shimudi area and initiated the isothermal decompression metamorphic event (M2). The region entered a phase of post-collisional extension at about 180 million years. Continued extension resulted in the formation of partial melts in the northern part of the study area. During the subsequent exhumation of the ductile shear zone, the mylonitic foliation was reformed by late fold deformation. [Significance] The findings provide a reference for discussing the detailed process of metamorphic deformation response in the process of Late Triassic–Early Jurassic tectonic transformation in the south of Foping dome.