Zircon U-Pb Age,Sedimentary Characteristics And Tectono-Paleogeographic Implications Of Bajie Group In Central Yunnan

Central Yunnan where is considered one of complete continuous strata of the Late Precambrian in China is a key to the object on the precambrian geological process of the Yangtze Paleocontinent and the relation between the Rodinia supercontinent and the Yangtze Paleocontinent.The Bajie Group and its underlying Kunyang Group,the typical stratigraphic units of the Mesozoic and Neoproterozoic in the southwestern Yangtze Paleocontinent,are mainly a set of low-grad metamorphic terrigenous clastic rock and carbonate rocks with pyroclastic rocks.Using methods of Tectonic Stratigraphy,Sedimentary Geology and Isotope Geochronology,this paper focus on their chronostratigraphic and sedimentological features of the Bajie and Kunyang Group.This paper has a discussion on chronostratigraphic framework based on these characteristics of volcanic clastic rocks,and tectonic paleogeographic significances based on provenance.The main progresses are as follows:1.The zircon dating age(945 ± 17 Ma)of tuff siltstone indicates that Meidang Formation formed in Neoproterozoic.2.Two zircon dating ages(878.5 ± 4.7Ma and 878± 4 Ma)of the tuff bed from the Lubiao Formation indicate a formation in Middle Neoproterozoic.These ages prove that the Bajie Group belong to the Qingbaikou System.3.The conglomerates at the top of the Meidang Formation are interlayer conglomerates,which probably represents a regional uplift.Paleocurrent direction of the Lubiao Formation,which points north-north-east,indicates that sediments were derived from the present-day Southwestern.The face of gravel at the bottom of the Huajiaqing Formation indicates that the sediments originated from the present-day North.4.The detrital zircon age histograms from upper part of Lubiao Formation shows that the sediments come from the plate formed by the combination of the Central Yunnan Paleocontinent and the Cathaysia Paleocontinent.5.A strong structural transformation that occurred inside the sedimentary basin caused conglomerates at the bottom of Huajiaqing Formation,and thus Paleocurrent direction changed from North to South.This transformation probably represents the initiation of collisional orogeny between the Central Yunnan Paleocontinent and the Yangtze Paleocontinent.