Geochronology of the Paleoproterozoic Helanshan ductile shear zones: Insights into temporal framework of polyphase deformation in the Khondalite Belt, North China Craton

The Khondalite Belt has been regarded as a Paleoproterozoic collisional orogen in the northwestern North China Craton, but the geochronological framework of polyphase deformation in the Khondalite Belt still remains unclear, particularly the timing of orogen-parallel ductile shearing. In this study, we conducted field-based structural analysis and LA-ICP-MS zircon U-Pb geochronology on the Helanshan ductile shear zones (HDSZ) in the Khondalite Belt. The results show that three pre-kinematic leucocratic dykes have been reworked by the shear zones and yielded magmatic zircon ages of 1934 ± 13 Ma, 1924 ± 5 Ma and 1920 ± 24 Ma. Two syn-kinematic leucocratic dykes appeared along the mylonitic foliations and were dated at 1887 ± 21 Ma and 1871 ± 28 Ma. A post-kinematic leucocratic dyke cut the mylonitic foliations and gave a crystallization age of 1816 ± 28 Ma. These emplacement ages of shear zone-related dykes constrain the development of the HDSZ at some time between ∼ 1920 Ma and ∼ 1816 Ma. Meanwhile, zircon overgrowth rims from three intrusions that suffered the shear deformation exhibited metamorphic ages of 1904 ± 11 Ma, 1840 ± 13 Ma and 1826 ± 31 Ma. Similar metamorphic ages of 1901 ± 7 Ma, 1888 ± 8 Ma and 1849 ± 29 Ma were also obtained from high-temperature felsic, pelitic and calcitic mylonites. These results further unravel that the HDSZ most likely developed at ca. 1904–1826 Ma. Additionally, we have summarized available deformation-related geochronological data in the Khondalite Belt, and proposed that this belt underwent three major phases of deformation (D₁-D₃) in the Orosirian. D₁ mainly produced overturned to recumbent isoclinal folds F₁, penetrative transposition foliations S₁ and mineral lineations L₁ at ca. 1.97–1.93 Ga. Subsequently, D₂ dominantly resulted in tight to open upright folds F₂ at ca. 1.93–1.90 Ga. Later, D₃ generated a series of NE- to E-trending orogen-parallel ductile shear zones at ca. 1.90–1.82 Ga. Together with multiple high-grade metamorphism and magmatism, the D₁-D₃ deformation recorded a complex protracted orogeny (>100 Myr) in response to the collision between the Yinshan and Ordos blocks.