Time of the ocean-continent transition in West Junggar: Constraints from zircon U-Pb dating and Lu-Hf isotopic composition

The West Junggar area of the western Central Asian Orogenic Belt is one of the most typical regions related to transition from the ocean to the continent in the late Paleozoic. It is still controversial about the specific time of ocean-continent transition and the continental growth of the West Junggar area at present. This paper reports zircon U-Pb age and zircon Lu-Hf isotope analytical results from the Miaoergou batholith and the newly discovered columnar jointed rhyolites on the northwest side of the Miaoergou batholith, West Junggar area. The zircon U-Pb ages indicate that the Miaoergou granitoids were formed at 302.8~308.8 Ma and the columnar jointed rhyolites were formed at 303.6~294.5 Ma, which indicates that the specific time of ocean-mainland transformation of the West Junggar area is limited to the Late Carboniferous-Early Permian. The zircon Lu-Hf isotope analytical results show that the Miaoergou granitoids and rhyolites have some similar characteristics in ¹⁷⁶Hf/¹⁷⁷Hf ratios and ε_Hf(t) values. High ¹⁷⁶Hf/¹⁷⁷Hf ratios and ε_Hf(t) values, similar to features of depleted mantle in nature, suggest that they resulted from partial melting of juvenile crust. Integrating regional geological data and previous research, the authors have reached the conclusion that the basement of West Junggar area was composed of juvenile oceanic crust and island arc which grew from late Neoproterozoic to late Paleozoic, with no old crystalline basement.