Ediacaran Glaciation In The Southwestern Margin Of The North China Craton And Northern Margin Of The Qaidam Block

Neoproterozoic is characterized by the final assembly and succeeding breakup of the supercontinent Rodinia,global glaciations,and the Ediacaran biological radiations,representing one of the most crucial periods of the Earth history.Unlike the global distribution of the Cryogenian glacial deposition,the Ediacaran glacial deposits have been reported in limited areas.The Luoquan Formation diamictite from the southwestern margin of the North China Craton(NCC),and the Hongtiegou Formation diamictite from the northern margin of the Qaidam block represent the Neoproterozic glacial deposits in these two blocks.Detailed sedimentological and geochronological studies of the Luoquan Formation as well as the relevant strata can help discuss the developing process of the Luoquan glaciation,and paleogeographic relationship between the North China and Qaidam blocks during the late Neoproterozoic to Cambrian transitional period.A systematic sedimentological study of 7 sections that include the Luoquan Formation is carried out in this study,and 8 lithofaicies are classified.Based on facies associations developed in different sections,three sedimentary facies are recognized,they are the proximal glaciomarine,distal glaciomarine and non-glacial marine sedimentary facies.The sedimentary facies changes from the lower part massive diamictite to the middle part stratified massive diamictite,and upper dropstone bearing siltstone are recognized in the Luoquan Formation at 6 sections,indicating a gradual glacierretreat during the Luoquan glaciation.The great ice sheet originated from the NCC interior,and expanded southward to relatively deep water ocean.The U-Pb ages of detrital zircons from the Luoquan,Dongpo and Xinji formations show similar distribution patterns,they are mainly composed of Paleoproterozoic and even older zircons,with only a small number of Mesoproterozoic to Neoproterozoic ones.Two distinctive age peaks,?1800Ma and?2500Ma,can be recognized in all samples.These age distribution patterns are consistent with the major magmatic events occurred in NCC,indicating that NCC is likely the provenance of detrital zircons of these formations.This inference is consistent with the developing pattern of the Luoquan glaciers suggested by sedimentological analysis.The U-Pb ages of detrital zircons from the upper Quanji Group show an apparent different distribution pattern.The detrital zircon age distribution of the Shiyingliang Formation shows a great similarity to that of the basement of the Olongbuluke terrane(OLT),indicating that OLT is probably the provenance of the Shiyingliang Formation siliciclastics.There is a small amount of Neoproterozoic zircons in the Heitupo Formation,the age distribution pattern is quite similar to that from the Tonian strata in NCC,indicating the probable same source rocks.Zircons older than 1500 Ma are dominant in the Zhoujieshan Formation,which can be well compared with the Luoquan and Dongpo formations,suggesting a close paleogeographic relationship between the OLT and the NCC during late Ediacaran.The occurrence of late Ediacaran index fossil Shaanxilithes from the strata immediately overlying the Luoquan and Hongtiegou diamictites suggests that the Luoquan and Hongtiegou diamictites may represent deposition of a late Ediacaran,probably a post-Gaskiers glaciation.The widespread Luoquan glacial deposits in the southern and western margins of NCC and northern margin of the Qaidam block may suggest that both blocks located in the middle to high latitudes during the late Ediacaran,rather than in the equatorial region as suggested before.