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).

意象指客观形象与主观心灵融合而成的带有某种意蕴与情调的东西[6]。它往往是主客观融为一体的形象，融合了表达者的主观体验、感受和情感，赋予客观事物以某种象征意义。意象为人们发挥想象力提供了广阔的思维空间,它不仅指示某个事理,而且代表着具有特殊意义的生活意象[2]。意象作为表征实践性知识的一种形式，可以反映出教师对自身、教学、学生和教育情境的认知。

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).

ⒸScience Press,Institute of Geochemistry,CAS and Springer-Verlag GmbH Germany,part of Springer Nature 2017

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.

选择任务 在多样化的集体课堂中，为了让不同学生从事不同层次和富有个性的数学创造，希斯赞特米哈伊的建议是，任务或活动必须能适合每位学生的能力，或者至少是可以改变的．

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

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).

1.4 质量控制 本次研究对象的选取标准由北京友谊医院营养科营养医师共同讨论确定，并对本研究的调查方法及用表进行审核完善，确保调查方法科学可行、真实有效。现场调查前进行培训及预试验，由专业调查员填写调查表，保证资料获取的准确性。数据处理均严格按照卫生统计分析的要求进行，保证调查结果的分析准确。

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-radiogenic 204Pb.Because the measured 206Pb/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.

采用L-B技术、RIE和湿法刻蚀技术等，制备出具有优良SERS特性的PS@Ag NPs和Si@Ag基底，并提出了基于PS@Ag免疫探针、AFP和PS@Ag免疫基底组成的“三明治”结构SERS特性的免疫检测方案.结果表明，该检测方案对AFP具有高的检测灵敏度、宽的检测动态范围和好的可靠性，具有潜在的临床应用价值.

4 Geochronological results

Compliance with ethical standards

根据教育部修订的最新的《中等职业学校专业目录(2010年)》和《普通高等学校高等职业教育(专科)专业目录(2015年)》显示，中职设321个专业，高职设761个专业，远多于中职[3]，这必然使得一部分中职毕业生难以到高职院校的对应或相应专业继续深造，制约了中高职衔接的连贯性。

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.The 206Pb/238U ages of the twenty analytical spots range from 623.1±9.2 to 643.7±9.2 Ma,with a weighted mean 206Pb/238U age of 634.3±4.0 Ma (MSWD=0.34,Probability=0.997)and a weighted mean 207Pb/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

The value of 206Pb/204Pbm is the measured value;f206 represents the percentage of common 206Pb in total 206Pb;the data listed in italics have been rejected for the all the age calculation

Spot U Th Pb Th/U 206Pb f206 207Pb ±1σ 207Pb ±1σ ppm ppm ppm 204Pbm % 206Pb % 235U %12HY-1@1 265 228 36 0.86 1.87E+4 {0.10} 0.06133 0.86 0.85275 1.77 12HY-1@02 335 365 50 1.09 2.83E+4 {0.07} 0.06113 0.83 0.88516 1.71 12HY-1@03 200 171 28 0.85 2.17E+4 {0.09} 0.06027 1.20 0.85614 1.93 12HY-1@04 260 208 36 0.80 1.47E+4 {0.13} 0.06083 0.99 0.87246 1.80 12HY-1@05 318 223 43 0.70 4.05E+4 {0.05} 0.06158 1.17 0.87599 1.92 12HY-1@06 233 167 31 0.72 1.88E+4 {0.10} 0.06106 0.97 0.85942 1.79 12HY-1@07 313 264 47 0.84 2.72E+4 {0.07} 0.06182 0.89 0.93940 1.79 12HY-1@08 309 268 43 0.87 5.59E+4 {0.03} 0.06022 0.87 0.85429 1.78 12HY-1@09 343 433 53 1.26 8.05E+4 {0.02} 0.06055 1.35 0.86810 2.02 12HY-1@10 351 366 51 1.04 2.18E+4 {0.09} 0.06108 0.82 0.87895 1.73 12HY-1@11 450 623 71 1.38 4.10E+4 {0.05} 0.06081 0.92 0.87049 1.76 12HY-1@12 356 375 52 1.05 2.18E+4 {0.09} 0.06089 0.82 0.87877 1.72 12HY-1@13 273 200 37 0.73 1.42E+4 {0.13} 0.06192 1.36 0.87249 2.09 12HY-1@14 327 318 47 0.97 1.10E+5 {0.02} 0.06071 0.86 0.87249 1.73 12HY-1@15 454 747 75 1.65 4.59E+4 {0.04} 0.06109 0.73 0.86789 1.67 12HY-1@16 452 637 72 1.41 5.32E+4 {0.04} 0.06145 0.73 0.87275 1.68 12HY-1@17 455 676 73 1.49 5.43E+4 {0.03} 0.06122 0.73 0.86792 1.68 12HY-1@18 268 195 36 0.73 1.97E+4 {0.09} 0.06031 0.95 0.86021 1.77 12HY-1@19 399 445 58 1.11 4.90E+4 {0.04} 0.06120 0.90 0.86204 1.79 12HY-1@20 338 347 47 1.03 1.28E+4 {0.15} 0.06183 1.23 0.84937 2.03 12HY-1@21 286 252 39 0.88 1.65E+4 {0.11} 0.06026 1.53 0.85416 2.15 Spot 206Pb ±1σ corr. 207Pb/206Pb ±1σ 207Pb/235U ±1σ 206Pb/238U ±1σ 238U % coef. Age(Ma) Age(Ma) Age(Ma)12HY-1@1 0.1021 1.51 0.85036 623.2 20.0 626.2 8.3 627.0 9.0 12HY-1@02 0.1050 1.50 0.87608 643.9 17.6 643.8 8.2 643.7 9.2 12HY-1@03 0.1030 1.50 0.78032 613.2 25.8 628.0 9.1 632.1 9.1 12HY-1@04 0.1040 1.50 0.83435 633.2 21.2 636.9 8.5 637.9 9.1 12HY-1@05 0.1038 1.50 0.78035 646.9 25.6 638.8 9.2 636.5 9.1 12HY-1@06 0.1021 1.51 0.84055 641.4 20.7 629.8 8.4 626.6 9.0 12HY-1@07 0.1112 1.52 0.85176 649.4 20.0 672.6 8.8 679.5 9.8 12HY-1@08 0.1029 1.55 0.87117 611.6 18.8 627.0 8.4 631.3 9.3 12HY-1@09 0.1040 1.51 0.74599 623.4 28.8 634.5 9.6 637.7 9.2 12HY-1@10 0.1044 1.52 0.88055 642.0 17.5 640.4 8.2 640.0 9.3 12HY-1@11 0.1038 1.50 0.85295 632.4 19.7 635.8 8.3 636.8 9.1 12HY-1@12 0.1047 1.51 0.87860 635.3 17.5 640.3 8.2 641.8 9.2 12HY-1@13 0.1039 1.50 0.71881 635.6 30.9 636.9 9.9 637.3 9.1 12HY-1@14 0.1042 1.50 0.86658 629.1 18.5 636.9 8.2 639.1 9.1 12HY-1@15 0.1030 1.50 0.89870 642.4 15.7 634.4 7.9 632.2 9.1 12HY-1@16 0.1035 1.50 0.89377 645.4 16.1 637.1 8.0 634.7 9.1 12HY-1@17 0.1033 1.51 0.89609 637.6 16.0 634.4 8.0 633.5 9.1 12HY-1@18 0.1035 1.50 0.84570 614.6 20.3 630.2 8.4 634.6 9.1 12HY-1@19 0.1026 1.53 0.85156 636.0 20.1 631.2 8.5 629.9 9.2 12HY-1@20 0.1015 1.54 0.75985 628.6 28.2 624.3 9.5 623.1 9.2 12HY-1@21 0.1028 1.50 0.69900 612.9 32.8 626.9 10.1 630.8 9.0

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.

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.

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

aHoffmann et al.(2004);bPrave et al.(2016);cCondon et al.(2005);dYin et al.(2005);eZhang et al.(2005);f Zhang et al.(2008);gCalver et al.(2013)

Location Palaeo-Geography Dating result Location and thickness Central Namibia Basin Within the Ghaub Formation;15 cm The dating bed in the profile Measurement techniques ID-TIMS Concordia age=635.5± 1.2 Ma(2σ,n=6,MSWD=0.38)a An aliquot of sample dated by Hoffmann et al.(2004) CA-TIMS Weighted mean 206/238U=635.21± 0.59 Ma(2σ,n=5,MSWD=3.4)b Yangtze Gorge,South China ID-TIMS Concordia age=635.23± 0.57 Ma(2σ,n=4,MSWDCE=0.55)c Yangtze Gorge,South China Platform Within the cap carbonate;1 cm Platform 5 m above the cap carbonate ID-TIMS Concordia age=632.5± 0.48 Ma(2σ,n=3,MSWDCE=0.38)c Yangtze Gorge,South China SHRIMP Weighted mean 206/238U=628.3±5.8 Ma(n=15,MSWD=0.86)d Yangtze Gorge,South China Platform Within the cap carbonate;1–1.5 cm SHRIMP Weighted mean 206/238U=621± 7 Ma(1σ,n=13,MSWD=1.13)e Jishou,Hunan,South China Platform ～2.5 m above the cap carbonate;4–6 cm SHRIMP Weighted mean 206/238U=636.3± 4.9 Ma(1σ,n=12,MSWD=1.06)f Tasmania,Australia Basin Within the basal Nantuo Formation;0–5 cm CA-TIMS Weighted mean 206/238U=636.41± 0.45 Ma(2σ,n=8,MSWD=1.5)g Chongqing,South China Above storm wave-base From the uppermost Cottons Breccia;70 cm SIMS Concordia age=634.1± 1.9 Ma(1σ,n=20,MSWDCE=0.31)(this work)Basin Within the cap carbonate;0–5 cm

6 Conclusions

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.

Acknowledgements This 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.

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(MSWDCE and ProbabilityCE).We chose the MSWDCE and ProbabilityCE in data processing following Condon et al.(2005).

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

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