Onset Time,Clastic Provenance And Global Correlation Of The Cryogenian Glaciations In The SE Margin Of Yangtze Block

The Cryogenian glaciation,as a remarkable event in the early earth evolution,is a global scientific problem.The Yangtze Block supplies plenty of stuff for the Cryogenian researches for it's one of the blocks on which the Cryogenian strata were most completed.Whereas the precise onset time and global correlations of the Jiangkou and Nantuo glaciations in South China still remain undecided.Given this,detrital zircon LA-ICP-MS U-Pb dating was performed on the glacial diamictites from the basal part of the Chang'an and Nantuo Formations in the southeastern Yangtze Block,in order to accurately constrain the maximum depositional ages of the two glacial units;date the bottom boundary of the Nanhua System;ascertain the onset time and global synchronism of the Jiangkou glaciation and Nantuo glaciation,respectively;analyse the provenance of Chang'an and Nantuo Formations by building their detrital zircon age spectrums,and discuss their paleogeographic significance.Detrital zircon of sample HJC-N1 from the Chang'an Formation give a concentrated U-Pb age range of 958 to 717 Ma,and the youngest 206Pb/238U age population yields a weighted mean of 719.6±6.1 Ma;Detrital zircon of sample FL-N3 from the Chang'an Formation give a concentrated U-Pb age range of 979 to 719 Ma,and seven youngest 206Pb/238U ages yields a weighted mean of 723.0 ± 8.1 Ma.The results together with the CL images suggest that the maximum depositional age of the Jiangkou diamictites is ca.720 Ma and the initiation of Jiangkou glaciation should be later than 720 Ma.Meanwhile,the age proof that the bottom boundary age of the Nanhua System is ca.720 Ma rather than 780 Ma.Taking into account the available age data,it is suggested that the Jiangkou glaciation was most likely initiated at ca.715 Ma,which is synchronous with the Sturtian glaciation in other blocks such as Tarim,Arabia-Nubia and Laurentia.Detrital zircon of sample FL-N1 from the Nantuo Formation give a concentrated U-Pb age range of 987 to 649 Ma,and three youngest 206Pb/238U ages yields a weighted mean of 649.3±6.2 Ma;Detrital zircon of sample SLK-N1 from the Nantuo Formation give a concentrated U-Pb age range of 939 to 650 Ma,and the youngest 206Pb/238U age population yields a weighted mean of 652.1±6.7 Ma.The results together with the CL images suggest that the deposition of the Nantuo diamictites should be later than 650 Ma and the Nantuo glaciation initiated later than 650 Ma,basically synchronous with the Marinoan glaciation in other blocks such as Siberia,Australia and Laurentia.Detrital zircon age spectrums of the Chang'an and Nantuo Formations in this study and their relative strata from north,west and southeast margins of Yangtze Block show that Neoproterozoic zircon ages from all these strata can be completely correlated with each other,which means that Neoproterozoic zircon from all these strata have a main age group ranges from ca.750 to ca.850 Ma,with a dominant peak at ca.800 and several other remarkable peaks at ca.850 Ma,ca.820 Ma and ca.750 Ma,revealing the episodic tectonic-thermal events of South China in Neoproterozoic.The Chang'an and Nantuo Formations in this study have almost the same detrital zircon age distribution patterns,suggesting that the two units have a roughly consistent provenance.When compared with their relative strata,the age spectrums of sample in this study have extremely high similarity with that of preglacial strata in study area.Coupled with the CL images and the petrological features,the result indicates that sediments of the Chang'an and Nantuo Formations in study area were mainly derived from the nearby underlying Neoproterozoic magmatic and sedimentary rocks at the edge of the Nanhua Rifting Basin by exaration.Based on existing data,the continental rifting in the South China maybe continue until the termination of the global Marinoan glaciation when the block evolved independently after breaking away from the Rodinia.