Detrital Zircon Age Of Neoproterozoic Sedimentary Rocks In The Southeastern Margin Of The Yangtze Platform And Its Implication

Sedimentary facies analyses and detrital zircon U-Pb dating were performed on some critical intervals of the Neoproterozoic strata in the west part of the southeastern margin of the Yangtze platform,with the aim to improve regional strata correlation and our understanding of basin evolution.First,facies analyses suggest that the Wuqiangxi Formation in the Siduping section of western Hunan Province,the top unit of the Banxi Group,was deposited in predominately littoral to offshore shelf environment.Two large blocks of sandstone were collected from the middle and upper part of the Wuqiangxi Formation for detrital zircon LA-ICPMS U-Pb dating.The results show similar age distribution ranging from Paleoproterozoic to Neoproterozoic.Two major peaks at ca.820 Ma and 780 Ma suggest that the Wuqiangxi sandstone was mainly sourced from Neoproterozoic rocks,corresponding to two phases of magmatic events associated with the rifting of the Nanhua basin.The youngest zircon group from the topmost Wuqiangxi Formation yield a weighted mean age of 714.6 ± 5.2Ma,which is interpreted to be close to the depositional age of the uppermost Banxi Group.This age,along with the ages reported from other sections,constrains that the Banxi Group was deposited between ca.820 Ma and ca.715 Ma.The age of 714.6 ± 5.2 Ma from the top of the Wuqiangxi Formation is indistinguishable with the SIMS U-Pb age of 715.9 ± 2.8 Ma from the upper Gongdong Formation in the Sibao village section of northern Guangxi,South China.It is also,within uncertainties,overlapped with two TIMS U-Pb ages from pre-Sturtian strata in Oman and Canada.These ages indicate that the Jiangkou(Sturtian)glaciation in South China started at ca.715 Ma instead of ca.780 Ma and supports a globally synchronous initiation of the Sturtian glaciation at ca.715 Ma.Additionally,one sample from the Nantuo Formation has yielded a similar detrital age distribution with the of the Wuqiang sample,indicating similar source mainly from Neoproterozoic rocks.Secondly,we conducted SHRIMP U-Pb dating for a tuffaceous claystone layer from the lowermost Liuchapo Formation in the Wuhe section in Guizhou Province.The youngest zircon group has a weighted mean age of 556.4 ± 8.1 Ma,which was interpreted to represent the lower boundary age of the Liuchapo Formation.This age,along with the published zircon U-Pb ages from the top boundary of the Liuchapo Formation,constrains the Liuchapo Formation being deposited in ca.556-523 Ma and indicates that the Ediacaran-Cambrian boundary(E-C)in deep water area is located within the Liuchapo Formation.The Dengying Formation(ca.551-542 Ma)in the shallow-water platform is equivalent to the lower part of the Liuchapo Formation.The new age also provides age constraint on the Wenghui Biota,corroborating its correlation with the Miaohe biota(ca.551 Ma).Moreover,it testifies that the termination of the DOUNCE ?13C negative shift in deep water facies is indeed synchronous with that in shallow platform.The oldest detrital zircon aged at ca.3.4 Ga from these samples indicates the existence of unexposed Archaean basement in the Yangtze craton.Thirdly,a systematic compilation of the detrital zircon age spetra of the Neoproterozoic strata in the west part of the southeastern margin of the Yangtze platform reveals no obvious tectonic environment changes across the Wuling unconformity and the Xuefeng unconformity.Instead,two other tectonic transform surfaces were recognized: the Wuqiangxi/Madiyi boundary(ca.800 Ma)and the Changan/Datangpo boundary(ca.660 Ma).The former marks a tectonic change from foreland basin to rift basin,while the latter represents a shift from rift basin to passive continental margin.We propose four evolution phases of the west part of the southeastern margin of the Yangtze platform: 1)plate-plate collision phase(ca.1000-880 Ma),2)orogen erosion and the activity of the mantle plume(ca.880-800 Ma),3)rift basin phase(ca.800-660 Ma)and 4)the passive margin phase(ca.660-542 Ma).