U–Pb zircon age of the base of the Ediacaran System at the southern margin of the Qinling Orogen

Lecai Xing?Taiyi Luo?Zhilong Huang?Zhikuan Qian?Mingzhong Zhou?Hongtao He

1 Introduction

There are at least two episodes of global glaciations during the Cryogenian Period,namely Sturtian,ending at about 662 Ma(Rooney et al.2014),and Marinoan,ending at about 635 Ma(Hoffmann et al.2004;Condon et al.2005).Particularly,the Marinoan glacial diamictites have been observed on almost all the present-day continents,for example,the Ghaub Formation in Namibia(Hoffman et al.1998,2004),Elatina Formation in Australia(Shields2005),Nantuo Formation in South China(Zhou et al.2004;Condon et al.2005),and Icebrook Formation in Canada(James et al.2001).Thereafter,a meter-scale carbonate deposit hasbeen found to directly cap the Marinoan glacial diamictites.These cap carbonates are characterized by enigmatic sedimentary structures and distinctive carbon and sulphur isotope compositions(e.g.Shields 2005;Jiang et al.2006).The subsequent Ediacaran deposits bear the earliest records of multicellular metazoan on Earth(Xiao et al.2002;Yuan et al.2005).

Geochronological data of the cap carbonates and diamictites,although limited,have played an important role in correlating depositsboth regionally and globally.In Africa,a 15 cm thick ash bed 30 m below the top of the Ghaub tillite in Namibia yielded a concordia TIMSU–Pb age of 635.5±1.2 Ma(Hoffmann et al.2004)and refined age of 635.21±0.59 Ma(Prave et al.2016).In Australia,chemical ablasion thermal ionization mass spectrometry(CA-TIMS)U–Pb dating of zircons from a 0.7 m thick dolomitic sandstone that grades up into a cap carbonate sequence in Tasmania showed a weighted mean age of 636.41±0.45 Ma(Calver et al.2013).In South China,a 1–1.5 cm thick ash bed 2 m above the top of Nantuo diamictite in the Jiuqunao section at the Yangtze Gorges area gave a sensitive-high-resolution-ion-microprobe(SHRIMP)age of 628.3±5.8 Ma(Yin et al.2005).A similar ash bed in the Wuhe-Gaojiaxi Section,also at the Yangtze Gorge area, yielded a TIMS age of 635.23±0.84 Ma(Condon et al.2005).These ages,acquired both from and below the cap carbonates,are consistent within analytical uncertainty,limiting the time for the Marinoan glaciation termination and the deposition of cap carbonates at around 635 Ma.

Researches on the cap carbonates in South China have focused on shallow-water platforms(e.g.the Yangtze Gorge area)or the slope facies(e.g.,Yuanjia and Wenghui sections in eastern Guizhou and middle Hunan area)at the southeastern part of the Yangtze Block.Although regional stratigraphic correlation suggests the occurrence of similar sedimentary package,few equivalents of the Marinoan diamictite-cap carbonate sections have been reported in the Qinling Orogen.Here we report a diamictite-cap dolostone sequence in Heyu,Chengkou County in Chongqing,which belongs to the southern part of the Qinling Orogen but is adjacent to the northern margin of the Yangtze Block.A K-bentonite bed,containing abundant magmatic euhedral zircons without any inherited core,has been discovered at the lower part of the cap dolostones in the Heyu section.This study focuses on the zircon U–Pb ages from this K-bentonite.

2 Geological setting

The Neoproterozoic glacial and post-glacial sedimentary successions are widely distributed across the Yangtze Block in southern China,where sedimentary tectonic setting transformed from the early rift to late passive margin with a NE–SW strike along the southeastern side of the Yangtze Block.The glacial sequences thickened from shallow-water platform to deep-water basin,corresponding to the reduced thickness of post-glacial marine sequences(Jiang et al.2006;Zhang et al.2008).The Neoproterozoic glacial and post-glacial sequences on the Yangtze Block comprise of,in ascending order,the Cryogenian Chang’an/Gucheng/Tiesi’ao diamictite(equivalent to the Sturtian glaciation),the Datangpo Mn-rich siltstone and shale(interglaciation),the Nantuo diamictite(equivalent to the Marinoan glaciation), the post-glacial Ediacaran Doushantuo carbonate and shale or siltstone,the Dengying carbonate(equivalent to Liuchapo or Laobao chert in deepwater basin).

Chengkou County is located in the junction of Sichuan,Shaanxi and Chongqing Provinces/City.According to Regional Geology of Sichuan Province (1991),the Chengkou region spans the Yangtze Block and the Qinling Orogen(Fig.1a).The Chengkou-Fangxian Fault divides the two first-order tectonic and stratigraphic units into southern Dabashan arcuate zone(SDAZ)of the Yangtze Block and northern Dabashan orogenic zone(NDOZ)of the Qinling Orogen(Fig.1b).The Neoproterozoic glacial and post-glacial sedimentary sequences in SDAZ are similar to those at the Three Gorges area,Yangtze Block.There are four Neoproterozoic sedimentary formations in NDOZ,from the lowermost Dai’anhe,then Muzuo,Wugongkou,and finally,to the uppermost Shuijing Formation.The Dai’anhe Formation primarily consists of 2000–2500 m thick volcanic-clastic rocks interbedded with fine-detrital slates in Chengkou region.The Muzuo Formation is mainly comprised of massive glacial diamicties,～ 500–1500 m thick in the Chengkou region,where the Sturtian and Marinoan deglacial depositions have not yet been clearly distinguished.The Wugongkou Formation is mainly composed of 45–75 m thick carbonaceous slates interbedded with silicious slates.The Shuijing Formation consists of thick and massive cherts with a thickness ranging from 20 to 50 m in the Chengkou region.The Wugongkou and Shuijing Formations probably deposited in Ediacaran Period and are equivalent to the Doushantuo and Liuchapo(or Laobao)Formationsin central Hunan and eastern Guizhou respectively,which represent deep-water basin facies in the southern part of the Yangtze Block.

The Heyu section(GPS:N31°51′43.4′′,E108°56′54.3′′)is situated near Heyu town in Chengkou County(the red star in Fig.1a,b,c).The Neoproterozoic strata of the Heyu section include,in ascending order,the Dai’anhe,Muzuo,Wugongkou and Shuijing Formations.The Muzuo Formation mainly consists of about 1200 m thick diamictites.A 10–20 m thick Mn-rich sandstone probably equivalent to the Datangpo Formation has been observed within the Muzuo diamictites.Interbedded between the Muzuo diamictites and the overlying cap dolostones,thereisa 2 m thick boundary mudstones enriched in pyrite nodules.The overlying Wugongkou Formation begins with a 10 m thick cap dolostonethen is followed by～60 m of carbonaceous thinly bedded shales and cherts.A 20–30 m massive chert cliff,the Shuijing Formation,overlies upon the Wugongkou Formation.Specifically,the massive cap dolostone at the base of the Wugongkou Formation is typical with the abundant and fine carbonaceous laminae(Fig.1e),which is characterized by obviously negativeδ13C values(Fig.1f,unpublished data).

Fig.1 Simplified palaeogeographic map of South China or the Yangtze Block during the Ediacaran(a)(modified after Zhou et al.2010;Wang et al.2014),tectonic map of the Chengkou region(b),and regional geological map of the Heyu section(c);field photograph of the K-bentonite at the bottom of the Wugongkou Formation in the Heyu section in Chengkou,Chongqing.NT/DST represents the boundary between the cap dolostones and the underlying mudstone(d);carbonaceous laminae in the cap dolostones of the Wugongkou Formation(e);litho-stratigraphic columns withδ13C data in the cap carbonate sequences of the Heyu section in Chengkou,Chongqing(f)

The Ediacaran sequence in the Heyu section begins with a cap dolostone overlain by thin-bedded carbonaceous shales and cherts and ends with thick cherts of the Shuijing Formation,which indicates a typical deep-water basinal sedimentary setting similar to those of the Laobao or Liuchapo Formation in eastern Guizhou and northern Guangxi(Zhu et al.2003).Sedimentary structures widely observed in cap carbonate of shallow-water facies in the Yangtze Block or other places in the world,such as tepeelike structures,sheet cracks,stromatactis-like cavities,cemented breccia(e.g.Allen and Hoffman 2005;Jiang et al.2006;Zhou et al.2010)have not been found in the Heyu section.Vanished shallow-water sedimentary structures,abundant finely carbonaceous laminae,overlain by carbonaceous thin-bedded cherts and thick cherts of the Shuijing Formation,jointly deduce that the 10-m cap dolostone in Heyu deposited within a deep-water basin.

3 Sampling and analytical methods

An about 5 kg sample(12HY-1)wascollected from a5 cm thick K-bentonite bed occurring at 0.7 m above the base of the cap carbonates,in the Heyu section (Fig.1d).Approximately 500 zircon grains were extracted from this K-bentonite sample by first immersing and dispersing the sample in water,then concentrating heavy minerals by panning,finally separating and concentrating zircons using magnetic techniques and standard heavy liquids.About 200 representative zircons were selected under a binocular microscope,mounted in an epoxy mount,and then investigated under microscope with transmission and reflect light along with cathodoluminescence(CL)images.The CL images were taken by a HITACHI S-3000 N scanning electron microscope(SEM)and ChromaCL at Beijing SHRIMP center.The standard zircon Nancy 91500 was used for calibration of the U–Th–Pb isotopic ratios and elemental fractionations.

Twenty zircon grains were selected from the 200 zircons of sample 12HY-1 for SIMSU–Pb geochronology analysis at the Institute of Geology and Geophysics,Chinese Academy of Sciences in Beijing.The SIMS analysis was conducted by spot analysis with ellipsoidal spot of 20× 30μm.Analytical procedures and data processing are the same as those described by Li et al.(2009).The measured Pb isotopic compositions were corrected for common Pb using non-radiogenic204Pb.Because the measured206Pb/204Pb values exceeded 10,000,corrections were sufficiently small to be insensitive to the choice of common Pb composition.An average of present-day crustal composition(Stacey and Kramers 1975)was used for the common Pb correction with the assumption that the common Pb was largely surface contamination introduced during sample preparation.All the obtained data were processed using the Isoplot/Ex ver.3.27 program(Ludwig 2005).The uncertainty of isotope ratios and ages are reported as 1σ,and the weighted mean ages are quoted with 95%confidence interval.The analytical results are presented in Table 1.

4 Geochronological results

Zircons extracted from the K-bentonite in the Heyu section are mostly euhedral and prismatic,up to 90-180μin length and length to width ratios in the range of 1.4:1 to 2:1.No infective inclusions and inherited core but oscillatory zoning with hourglass structure has been observed in CL images(Fig.2).The Th/U ratios of all data range from 0.70 to 1.65,with an average of 1.01,which is characteristic of zircons of typical magmatic origin(Zhou et al.2008,2013).In addition,data processing using ISOPLOT reveals that there are two modes for calculating the MSWD and Probability:one is the MSWD(and Probability)of concordanceonly and the other is MSWD(and Probability)of concordance and equivalence(MSWDCEand ProbabilityCE).We chose the MSWDCEand ProbabilityCEin data processing following Condon et al.(2005).

U–Pb dating results by SIMS are presented in Table 1 and Fig.3.The data of spot 7 are most probably attributed to the fact that spot 6 and 7 were laser-ablated sequentially in the same zircon,and the heated zircon was not cool enough before the analysis of spot 7.Operation issues like this in zircon U–Pb dating were encountered before at Beijing SHRIMP center.Therefore,data from spot 7 is not included in the age calculation and the later discussion.The206Pb/238U ages of the twenty analytical spots range from 623.1±9.2 to 643.7±9.2 Ma,with a weighted mean206Pb/238U age of 634.3±4.0 Ma (MSWD=0.34,Probability=0.997)and a weighted mean207Pb/206Pb age of 633.4±8.8 Ma(MSWD=0.30,Probability=0.999).Meanwhile,a confident concordia age of 634.1±1.9 Ma(1σ, MSWDCE=0.31, ProbabilityCE=1.000) was obtained.The three ages mentioned above are concordant,in which the concordia age of 634.1±1.9 Ma is considered as the best result of SIMSdating.

Table 1 SIMSU–Pb isotopic analyses of zircons from the K-bentonites at the bottom of Wugongkou Formation in Heyu section,in Chengkou district

Fig.2 CL images and analytical spots of selected zircons

5 Discussion

Existing zircon U–Pb data from the cap carbonates and their adjacent beds have been summarized in Table 2,and these ages constrain the termination of Marinoan glaciation and the deposition of cap carbonates at approximately 635 Ma.The euhedral zircons in this study are of apparent magmatic origin and almost contain no inherited core or infective inclusions(Fig.2).We interpret the SIMS concordia age of 634.1± 1.9 Ma(1σ,MSWDCE=0.31,ProbabilityCE=1.000,n=20)as the most reliable sedimentary age of this K-bentonite,which is in strong agreement with the existing TIMS U–Pb ages within analytical uncertainties.The dating work further corroborates the global synchroneity of termination of Marinoan glaciation and provides reliable radiometric date for the regional correlation of Neoproterozoic glacial/deglacial sections in the Qinling Orogen.

Fig.3 SIMS U–Pb concordia diagrams of zircons from the K-bentonites at the base of the Wugongkou Formation in the Heyu section,Chengkou

Researches on the Neoproterozoic glaciations mainly focus on the southeastern part of the Yangtze Block.However,according to the regional geology,the sedimentary sequences in South Qinling region contain welldeveloped Neoproterozoic glacial/deglacial successions as exemplified by deposits in the Yangtze Gorge area.In the Qinling region,they are complicated by volcanic deposits(the Yaolinghe Group)but are characterized by deeper basinal sedimentary sequences.Due to the lack of U–Pb dates,however,the Neoproterozoic glacial/deglacial sequencesin the Qinling Orogen and their relationship with the North China,the Yangtze,and other placesin theworld are poorly known.

Sturtian and Marinoan glacial deposits have not been clearly discerned in the Qinling Orogen.In the Yangtze Block,the Datangpo Formation is comprised of Mn-enriched siltstones and shales,which separate the Chang’an/Tiesi’ao and Nantuo diamictites(Zhou et al.2004).Therefore,the occurrence of Mn-enriched sandstone in the Muzuo diamictite in the Heyu section suggests that the thick sequence of the Muzuo Formation could include the equivalents of the Chang’an/Tiesi’ao,Datangpo,and Nantuo formations in the Yangtze Block.Sedimentary and stratigraphic works have correlated the upper part of the Muzuo diamictite to the Nantuo diamictite equivalents in many parts of the Yangtze Block(Regional Geology of Sichuan Province 1991;Tang and Lin 2002;Deng et al.2015).Although thisiswidely accepted,radiometric datais lacking.Our SIMS data offer the first U–Pb age of the Neoproterozoic glacial/deglacial sequences in the Qinling Orogen and confirm that at least the upper part of the Muzuo diamictite is of Marinoan age.

Table 2 Published zircon geochronologic studies on the Marinoan-type cap carbonates deposition

6 Conclusions

A K-bentonite bed,0.7 m above the boundary between a Neoproterozoic diamictite formation and a cap dolostone sequence,has been discovered at the base of the basinfacies Wugongkou Formation in Heyu section,Chengkou County,Chongqing,South China.Magmatic zircons yield SIMS concordia U–Pb ages of 634.1± 1.9 Ma(1σ,MSWDCE=0.31,ProbabilityCE=1.000,n=20).This age is nearly identical to the TIMSU–Pb ages obtained for the deposition of the cap carbonateson top of the Marinoan diamictites in Africa,Australia,and the Yangtze Block.Our data provides geochronological evidence of the Marinoan or Nantuo age of at least the upper part of the Muzuo Formation in this northern margin of the Yangtze Block,supporting aglobal synchronicity on the termination of Marinoan glaciation and the deposition of cap carbonates.

AcknowledgementsThis work was supported by the National Natural Science Foundation of China(Grant Nos.41072054,40672053,41462001).We are grateful to Prof.Li Qiuli and Engineer Tang Guoqiang of the Institute of Geology and Geophysics,Chinese Academy of Sciences in Beijing for their great help in zircon SIMS U–Pb isotopic analysis.

Compliance with ethical standards

Conflict of interestOn behalf of all authors,the corresponding author states that there is no conflict of interest.

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