Discovery of ～4.0 Ga detrital zircons in the Changdu Block,North Qiangtang,Tibetan Plateau

在 situ 锆石由 LA-ICP-MS 和 MC-ICP-MS 标明日期使用， 3981 的由岩屑形成的锆石 ??? 鮼铧?? ? 裦蒚铩 ????? 偃 ?? 鮼 ?? 郥趽 ????腘 吗？？

Using in situ zircon dating by LA-ICP-MS and MC-ICP-MS, detrital zircon of 3981±9 Ma age was found in metamorphic rocks of the Ningdno Rock Group, Changdu Block of Northern Qiangtang. This is the oldest age record that has been found in the Qiangtang area. This finding also constitutes the third zircon locality in China with an age older than 3.9 Ga. Thus, the discovery provides new information for the study of Hadean crust. In addition, we found 3.51-3.13 Ga, ~2440 Ma, ~ 1532 Ma, ~982 Ma and ~618 Ma age peaks from 100 test spots. The younger ages of ~982 Ma and ~618 Ma correspond to the formation of the Rodinian super-continent and the Pan-African event, respectively. These findings suggest a close relationship between these zircons and the Gondwanan super-continent. The age of ~618 Ma defines the lower limit on the deposit time of the protolith for the garnet-micaquartz schist in the Ningduo Rock Group. Zircons with an age of ~982 Ma generally display a negative ε_Hf（t） and a two-stage Hf model with concentrated ages around 1933-2553 Ma. This pattern indicates that the source area of the Ningduo Rock Group underwent a significant separation of depleted mantle into the crust during the Paleoproterozoic Era. However, zircons with ages of 2854-3505 Ma also show a negative ε_Hf（t） and a two-stage Hf model with a concentration of ages around 3784-4316 Ma. These results demonstrate that the source area of the Ningduo Rock Group contains a residual amount of ancient （Hadean） crustal materials. This paper provides new information on the relationship between the basement of the Qiangtang area and the Paleoproterozoic basements of the Gangdese and Himalayan regions, which constrains the northern boundary of Gondwana.