Early-Paleozoic Structural Features And Tectonic Evolution Of The Eastern Part Of The Northern Boundary Of The Proto-Tethys Ocean

The North Qinling Terrane (NQT), including the Kuanping, Erlangping and Qinling groups, is located in the northern part of the Qinling Orogen, and is a key area to evaluate the tectonic evolution of the northern boundary of the Proto-Tethys Ocean. Many researches have studied the petrological, geochronological, geochemical features of the NQT. Most of these studies focused on the different rock units within the NQT and the Shangdan Suture Zone. However, only very few studies about the Luonan-Luanchuang Fault have been carried out. Based on (1) detailed structural analyses of different units within the NQT and the southern margin of the North China Block, (2) metamorphic age data of the NQT and (3) detrital zircon U-Pb age analytical results from the NQT and surrounding regions, this study aims to reveal the subduction-collision-exhumation process of the Kuanping Ocean.Three episodes of deformation (D1, D2 and D3) are recognized in the study area. The D1 deformation is characterized by NNE-SSW-dipping penetrative foliations (S1), The D2 deformation is marked by tight asymmetric folds with NNE-SSW-dipping axial plane foliations (S2). The D1 and D2 deformations occurred during ～440-400 Ma, coinciding with the collision and exhumation of the NQT. These two deformations are overprinted by the D3 deformation characterized by ENE-trending upright open folds and widely distributed kind bands. The D3 deformation might occur during the Late Jurassic-Early Cretaceous. The main faults in the study area, such as the Luonan-Luanchuan, Guanpo-Qiaoduna, Zhuyangguan-Xiaguan and Shangdan faults, show as ductile shear zones in the early stage and thrust faults in the late stage. In addition, these faults have different inclinations of fault planes or different shearing movements, which might be generated by local adjustment under regional stress.The Kuanping Ocean is a big ocean existing during the Mesoproterozoic-EarlyPaleozoic. The ancient suture zone representing the closure of the Kuanping Ocean is the Luonan-Luanchuan Fault. Huge differences existing on either side of this fault are more significant than those for the Shangdan Suture Zone, which suggests that the Luannan-Luanchuan Fault marks the northern boundary of the Proto-Tethys Ocean in the study area.The existence of the passive margin depositional system in the southern North China Block indicates the southward subduction of the Kuanping Ocean. The north-vergent major faults and axial plane foliations, and the high-pressure eclogites and granulites located in the northern and southern part of the NQT, respectively, also suggest that the Kuanping Ocean subducted southward beneath the NQT.Detrital zircon U-Pb age data show that the age spectra for the Kuanping Group with the youngest zircon at ～640 Ma are quite different from the ages recorded for the southern margin of the North China Block, while the age spectra with the youngest zircon at ～510 Ma is similar to both the NQT and the southern margin of the North China Block. This suggests that the Kuanping Ocean subducted southward continuously during the Neoproterozoic to Early Paleozoic interval, with final closure in the early Paleozoic.