Abstract:

Located between the North Qaidam Block and the Qilian Orogen, the Quanji Massif (i.e. Oulongbuluke microcontinent) is characterized by a cratonic continental remnant composed of a medium to highgrade metamorphosed basement and an unmetamorphosed cover, representing the typical doublecrust units. The Quanji Group, which unconformably overlies the Palaeoproterozoic Dakendaban Group and underlies the Cambrian Oulongbuluke Group, is the first sedimentary cover of the Quanji Massif. At present, with imprecise constraints, most geologists believe that the Quanji Group is the late Neoproterozoic in age. In this study, as the first time, we report the LAMCICPMS zircon UPb ages of the tuff from the Hongzaoshan Formation of the Quanji Group, which have been divided into Mahuanggou, Kubaimu, Shiyingliang, Hongzaoshan, Heitupo, Hongtiegou, and Zhoujieshan formations, in ascending order. Two LAMCICPMS zircon UPb ages, 1640±15 Ma and 1646±20 Ma, were obtained from the tuff beds in lower part of the carbonatedominated Hongzaoshan Formation at the Quanjishan section, where the Hongzaoshan Formation was defined as a formal lithostratigraphic unit in 1980s. These new ages suggest that the lower Quanji Group, including the Mahuanggou, Kubaimu, Shiyingliang, and Hongzaoshan formations, should be revised into late Paleoproterozoic (Statherian Period) time. Combined with the palaeoweathering crust and disconformity on the top of the Hongzaoshan Formation, and the previous studies on Ediacaran fauna in the Zhoujieshan and glacial diamictite in the Hongzaoshan Formation, it may infer that there should be a huge time gap between the Hongzaoshan Formation and the overlying formations, and the Quanji Group, as a lithostratigraphic unit, should be redefined. Moreover, now it may suggest that the “Quanji Movement”, represented by the unconformity between Dakendaban Group and Quanji Group and conventionally corresponding to the ~850 Ma Jinning Movement in Yangtze Craton, should correlate to the ~1.8 Ga Lüliang Movement in North China Craton (NCC). It seems that, during the Mesoproterozoic time, the Quanji Massif should have more closely tectonic affinity with the NCC than that with the Yangtze Craton.

Key words: Quanji Group, Hongzaoshan Formation, tuff, zircon UPb age, Mesoproterozoic, Neoproterozoic, Qaidam Basin