Syn-tectonic emplacement duringsinistral transpression: The Late Triassic Gaoqiao pluton in the SouthQinling Belt, central China

Internal structures of the pluton may provide key information on magma emplacement mechanism and the relationship with regional tectonics. We present a case study on the Late Triassic Gaoqiao pluton from South Qinling Belt, which was emplaced under a syn-collisional setting. Multi-approaches including zircon U-Pb geochronology, field and microstructural observations, anisotropy of magnetic susceptibility analysis was conducted to reconstruct the growth process of the pluton and to investigate the internal structures. Zircon U-Pb geochronology reveals that the pluton is a composite intrusion which was constructed by two successive units with distinct ages (similar to 220 and similar to 210 Ma). Field observations show that the Gaoqiao pluton and its country rocks recorded abundant evidences of syn-plutonic deformations, indicating a syn-tectonic emplacement. Microstructural observations show that the internal fabrics were mainly acquired during the stage from magma flow to high-T solid-state deformation. The fabric distributions display 'onion-skin' patterns on the pluton scale, which are characterized by zoned foliations and lineations. Combined with the syn-collisional convergence setting, we propose a tectonic-controlled incremental growth model for the pluton, in which the pluton was successively constructed by the Fujiahe unit and the Hetaoping unit, and the emplacement process of the pluton was controlled by the regional deformation with sinistral transpressional kinematics. Furthermore, the internal fabric patterns of the Gaoqiao pluton were probably formed by tectonic-controlled helical magma flow, which appears to be common in fabric formation of many other plutons with similar features.