Structural and chronological constraints for Longquan-Badu ductile shear zone: Implication for Triassic intraplate orogeny in South China Block

Abstract

The ductile shear deformation of Precambrian basement rocks in Wuyishan provides a crucial perspective on intraplate orogeny in the South China Block (SCB). This study focuses on the Longquan-Badu ductile shear zone in southeastern Zhejiang, employing field observations, thin section analysis, quartz electron backscatter diffraction (EBSD), zircon U–Pb dating and ⁴⁰Ar/³⁹Ar geochronology. Two distinct phases of deformation, referred to as D1 and D2, have been identified. D1 is primarily characterized by a WNW–ESE striking foliation within a NE-plunging lineation, indicating top-to-SSW shearing. The paragneiss within the Badu complex that experienced D1 deformation has been dated to 247–239 Ma through zircon U–Pb analysis, corresponding to the prevalent high-pressure metamorphic age in the region. This correlation suggests that the D1 deformation event coincided with crustal thickening during the Early Triassic. D2 deformation exhibits folds, foliation, S-C fabrics and mylonitic microstructures and is mainly characterized by striking NNE–SSW with steeply dip, demonstrating a dominant dextral strike–slip component. Quartz c-axis orientations in mylonitic rocks indicate deformation temperatures between 350°C and 550°C with asymmetric girdle patterns suggesting simultaneous basal and prism slip. The plateau ages of muscovite from mylonitic rocks obtained through ⁴⁰Ar/³⁹Ar dating are approximately ~228 Ma implying that the D2 deformation occurred under retrograde amphibolite to greenschist facies metamorphic conditions during Middle Triassic. Collectively these data along with regional geological evidence signify two distinct intracontinental orogenic processes occurring in eastern SCB. Considering Early Mesozoic tectonothermal events in Cathaysia Block, it can be inferred that intraplate orogeny in Wuyishan resulted from plate-margin collisions between SCB and peripheral plates following scissors closure of Palaeo-Tethys from east to west.