1. Introduction

End-Permian time is one of the most important critical transitions in Earth's history,and the largest mass extinction occurred at the Permian—Triassic transition.According to detailed studies mainly in the Meishan Global Stratotype Section and Point(GSSP)section,Zhejiang Province,southeastern China,the main mass extinction pulse happened just before the defined Permian—Triassic boundary(PTB)(Shen et al.,2011;Yin and Song,2013;Yin et al.,2001,2012),the main episode of the end-Permian mass extinction and PTB age were constrained as 251.941±0.037 Ma and 251.902±0.024 Ma(Burgess et al.,2014),respectively.The peak mass extinction occurred just before 252.28±0.08 Ma at Meishan section,and lasted no more than 200±100 thousand years(kyr)(Shen et al.,2011;Yin et al.,2012).The PTB age obtained from different sections varies to some degree due to the precision of dating methods and possibly-incomplete stratigraphic record in some sections.So far,most of the PTB ages are from clay beds(bentonite)in marine sections,and few reliable PTB ages from terrestrial sections,particularly from the Permian coal beds that are widespread in South China.

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The eastern Yunnan Province of southwestern China has well-preserved terrestrial Permian strata including coal beds that may cross the time of the PTB.Coal-bearing strata in eastern Yunnan belong to the Upper Member of the Upper Permian Xuanwei Formation.The uppermost Xuanwei Formation contains the C1coal seam which is overlain by the Kayitou Formation of the Lower Triassic.Because coal beds may record important information about the palaeoclimate and palaeoecological changes across the PTB in terrestrial environments,the age of the C1coal seam would be a critical anchor for establishing the time framework of events surrounding the end-Permian mass extinction.

In this paper,we report the first U—Pb SHRIMP ages of the C1coal seam in the Yantang Mine in Xuanwei County,eastern Yunnan Province.Zircons are collected from the tonsteins in the C1coal seam,which have formed from contemporaneous volcanic eruptions according to the mineralogical and geochemical evidences.With the new age data,we discuss the end-Permian mass extinction and PTB in coal-bearing terrestrial sections,as well as their correlation with the marine records(Shen et al.,2011;Yin and Song,2013;Yin et al.,2012).

（2）淋水冲洗：将筛子叠放于尼龙绢袋上，尼龙绢袋下口用夹子夹住。取受检者粪便30 g置于筛子中，用水冲淋，压舌板或竹签调浆，使粪液滤入尼龙绢袋中。移去筛子，继续用水冲洗袋内粪渣，用压舌板或竹签轻轻振荡尼龙绢袋使过滤加速，直至滤出液变清为止。将尼龙绢袋下口移入烧瓶口内，取下夹子，将袋内的滤渣用孵化用水淋洗入烧瓶，继续加孵化用水满至离瓶口1 cm处。

(3)域之间的映射。它是以人们对组群中的物体(objects in groups)的数量顿识和数数的经验为基础的。[2]55

2. Geological setting

During the Late Permian,the study area,Xuanwei County,eastern Yunnan Province,was located in the western Yangtze Craton of the South China Block(Fig.1;Wang and Jin,2000).Siliciclastic sediments in this area were supplied predominantly from the Khangdian Oldland(Fig.1B)in the west.Controlled by transgression from the east throughout the Late Permian,and the sedimentary environments in eastern Yunnan and western Guizhou during Late Permian vary from marine,transitional to terrestrial facies from east to west.The Xuanwei Formation of Late Permian in Xuanwei County is mainly terrestrial fluvial facies(Shao et al.,1998;Wang et al.,2011).The C1coal seam comprises low—medium volatile bituminous coal with a low to medium-high ash yield and a very low sulfur content(Shao et al.,2015).

Samples from the C1coal seam were collected from the Yantang Mine,located in Laibin Town,northeastern Xuanwei County(Fig.1A).The C1coal seam consists of the B1,B2and B3sub-seams in descending order in this mine(Fig.2).No further coal seams occur above B1 until the Middle Triassic,marking the onset of the global“coal gap” during the Early Triassic(Retallack et al.,1996;Shao et al.,2000).The three sub-seams(B1,B2and B3)are the only minable seams in the Yantang Mine,and their thicknesses at the sampling sites are 0.75 m,0.70 m and 0.75 m,respectively.The spacing between sub-seams B1and B2,B2and B3are 2.73m and 3.39m,respectively.In addition,within the sub-seams B1and B3,there are two layers of volcanic ash(tonstein)(Zhou et al.,2000)with a thickness of about 5 cm each.

3. Sampling and analysis

3.1. Sampling

The sedimentary sequence surrounding the Permian—Triassicboundary(PTB)in the eastern Yunnan Province consists of a series of intercalated siltstone and mudstone overlain by coal seams of Late Permian age(Peng et al.,2001,2005;Shao et al.,2012),and thin-medium layers of fine sandstone,siltstone,pelitic siltstone,silty mudstone of earliest Triassic age.The Upper Permian in Xuanwei County comprises the Xuanwei Formation,which unconformably overlies the Emeishan Basalt,and is in turn conformably overlainby the Kayitou Formation of the Lower Triassic(Fig.2).The Xuanwei Formation,consists mainly of gray finegrained sandstone,siltstone and mudstone,some volcanic ash beds,and numerous coal seams including the C1coal seam at the top(Dai et al.,2008).The Yantang Mine sequence sampled in this study is locatednear the Xiangyangcun syncline with small faults developed on both sides of the mine area(Fig.1A).

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The characteristics of the three sub-seams in the Yantang Mine are as follows:

Two tonstein samples B1-3 and B3-4 were collected from the underground coal face in the Yantang Mine.The coordinates of the sample site are N 26°18′18′′and E 104°08′35′′.Each sample collected is ≥2 kg and all the samples were immediately stored in plastic bags to ensure as little contamination as possible.

2)The B2coal lies 2.73 m below the B1coal.The thickness of the B2coal is 0.70 m,and no parting is presented inside.The roof and floor are also carbonaceous mudstone.

The Meishan section located at Meishan town,Changxing County,Zhejiang Province,and described as a well-preserved,continuous marine succession across the Late Permian and Early Triassic,and is the most intensively studied PTB section(Bowring et al.,1998;Burgess et al.,2014;Mundil et al.,2004;Shen et al.,2011;Yin and Song,2013;Yin et al.,2001,2012).Here we compare the ages of the Meishan section with those of Yantang Mine,which represent marine and terrestrial deposits,respectively.In the Meishan section it is generally accepted that the PTB lies at the base of Bed 27c,and Beds 25 to 28 are considered as the main extinction beds,essentially Bed 25,which contains the main episode of the end-Permian mass extinction(Fig.8;Burgess et al.,2014;Shen et al.,2011;Yin et al.,2014).The latest U—Pb age of Bed 25 is datedas 251.941±0.037 Ma(Burgesset al.,2014).The age of upper sub-seam B1from Yantang Mine is 252.0±2.3 Ma,slightly older than 251.941±0.037 Ma.In other words,the time of deposition of the C1coal seam of Xuanwei Formation is a little earlier than Bed 25 of the marine Meishan section,which means the coal-forming plant of C1coal seam occurred just before end-Permian mass extinction.Maybe the C1coal seam,especially the sub-seam B1,is the witness of the terrestrial end-Permian mass extinction.

1)The B1coal lies at the top of the Xuanwei Formation,with a thickness of 0.75 m.This sub-seam contains a kaolinite claystone parting(tonstein)with a thickness of 5 cm.Its roof and floor are both carbonaceous mudstone.

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3.2. Methods

3.2.1.Mineral analysis

The mineralogical composition of the tonstein samples(B1-3 and B3-4)was examined by scanning electron microscopy coupled with an energy dispersive X-ray analyzer(SEM-EDX)and powder X-ray diffraction(XRD).Samples were broken into small pieces first;then the fresh and smooth surfaces of samples were selected and were coated with a very thin layer of gold film.The micro-morphology of minerals was observed and determined by SEM(JSM-6390LV)equipped with EDX(INCA-ENERAGY 250).

Sample B1-3 was crushed and ground to less than 200 mesh and directly analyzed by powder XRD.Sample B3-4 was crushed and ground to less than 200 mesh,then further subjected to low-temperature ashing(LTA)in an EMITECH K1050X Plasma Asher.After that,the LTA ashes were analyzed by XRD(Bruker D8 Advance).The XRD patterns were collected in the 2θ rangeof5°—90°using CuKα radiation.The SEM-EDX and XRD analyses of tonstein were carried out at the Key Laboratory of Biogenic Traces and Sedimentary Minerals of Henan Province,Henan Polytechnic University.

The analytical results are summarized in Table 1 and concordia diagrams are shown in Fig.6.The Th/U ratios of zircons are generally taken as indicators for their conditions of formation.In general,the Th/U ratio of magmatic zircons is greater than 0.4,while that of metamorphic zircons is less than 0.1(Li et al.,2014).In this study,most of the Th/U ratios of analyzed zircons are greater than 0.4,mostly in the range 0.6—1.2,except for B1-3-7.1 with 0.39.The high Th/U ratios and distinct oscillatory growth zoning observed in CL images(Fig.5)indicate that all these zircon grains are of igneous origin.

The two tonstein samples,each about 1 kg,were crushed and sieved to 60 mesh.The zircon grains were separated using standard heavy liquid from light minerals(such as feldspar and quartz),then using magnetic techniques from magnetic minerals(magnetite,etc.).Zircon separation was carried out at the Hebei Institute of Geological Minerals Investigation.Approximately 100 zircon grains were hand-picked from each sample under a binocular microscope,and then mounted on an adhesive tape,enclosed in epoxy resin together with several grains of standard zircon TEMORA.The mount was polished to expose the grain interiors.In order to examine internal textures of zircons,cathodoluminescence(CL)images of individual zircon grains were obtained using Hitachi S-3000N scanning electron microscope(SEM)operating at an accelerating voltage～10 KV and ion beam current～10 μA.The zircons were dated in the Sensitive High Resolution Ion Microprobe II(SHRIMP II)at the Beijing SHRIMP Centre,Chinese Academy of Geological Sciences.Detailed analytical procedures and data reduction followed those described by Song et al.(2002).

Spots that have the following features are avoided for analysis:textural complexity,rounded grains,and grains containing bubbles,fracture and inclusions.Those zircon grains with uniform CL color,long columnar structure and oscillatory zoning according to CL images were selected for dating(Liu et al.,2011).

Zircons were analyzed for U,Th and Pb using the SHRIMP II.Five scans through the mass stations were carried out for each age determination.The standard zircon was analyzed first and after every three unknown analyses.The TEMORA zircon,with an age of 417 Ma(Black and Jagodzinski,2003),was used as a standard.The raw ICP-MS data was processed using the ICPMSDataCal procedures,and the ages of zircons were the weighted means calculated using the Isoplot 3.7 programs(Ludwig,2003),and measured204Pb was applied for the common lead correction.The uncertainties for individual analysis are quoted at the 1σ confidence level,whereas errors for the weighted mean ages are quoted at the 95%confidence level.

4. Results

4.1. Mineralogical evidence for tonstein

The mineral composition in the two partings is similar.The XRD results show that kaolinite is the dominant mineral in the partings,followed by quartz,calcite,and minor anatase and ankerite(Fig.3).Kaolinite occurs in the form of “accordion” and“vermicular” aggregates in B1-3 and B3-4,respectively(Fig.4A and B).No zircons are identified in the partings using SEM-EDX and XRD.However,numerous zircon grains were hand-picked under a binocular microscope from these partings.The CL images(Fig.4C and D)for zircon grains of the two samples have high length/width ratios and clear oscillatory zoning indicative of volcanic origin(Hoskin and Schaltegger,2003;Wu and Zheng,2004).The vermicular texture of kaolinite aggregates are often interpreted as volcanogenic origin(Zhou et al.,2000).In addition,magmatic zircons in volcanogenic tonstein partings are different from the zircons in sediments of terrigenous origin(Zhou and Ren,1994).Hence,combining the mineral composition and micro-morphological characteristics,the two partings are identified as tonsteins—the products of alteration of volcanic ash.

4.2. SHRIMP U—Pb age data

U—Pb isotope analyses were conducted on 15 and 16 zircon grains,respectively,for B1-3 and B3-4(Fig.5).The CL images show that most of the zircon grains are euhedral with elongated prism or blocky shape,100—400 μm long.The high length/width ratios and clear oscillatory growth zoning of the zircons indicate their volcanic origin(Hoskin and Schaltegger,2003;Wu and Zheng,2004).All the determinations were carried out on the oscillatory zones.

3.2.2.Zircon U—Pb dating

For sample B1-3,the three outliers(spot No.1.1,2.1,and 3.1)were discarded because they are significantly abnormal compared with other 12 spots(Table 1).The analyses 1.1 and 3.1 are outliers due to the distinct higher common Pb(206Pbc)content and yielding younger ages than others.The analysis 2.1 is also an outlier because its age is far less than other data.When these three analyses are discarded,the scatter decreases considerably(MSWD=0.60).The Th and U concentrations of the other 12 zircons change from 56 ppm to 427 ppm and from 101 ppm to 639 ppm,respectively,with Th/U ratios changing from 0.39 to 0.74.When the age of a sample is younger than 1000 Ma,the206Pb/238U ages are preferred because they have smaller uncertainties than the207Pb/206Pb and208Pb/232Th ages(Black and Jagodzinski,2003;Compston et al.,1992).Hence,the206Pb/238U ages are used in present study.The concordia diagrams and the weighted mean calculations were made using the Isoplot 3.7 software(Ludwig,2003).Excluding the three spots(1.1,2.1 and 3.1)which have anomalously young206Pb/238U ages of 247.0 Ma,240.3 Ma and 234.7 Ma,respectively,the remaining 12 spots yielded ages between 248.1 Ma and 258.8 Ma,with a weighted mean206Pb/238U age of 252.0±2.3 Ma(n=12,MSWD=0.60)(Fig.6A and C).

For the sample B3-4,there are 12 valid analyses,while the other 4 analyses(spot No.9.1,10.1,13.1 and 14.1)are obviously discordant(Table 1).The Th and U concentrations of these valid zircons vary from 40 ppm to 516 ppm and from 56 ppm to 346 ppm,respectively.The Th/U ratios range from 0.71 to 1.49.The four abnormal analyses give either too young or too old 206Pb/238U ages,which therefore are excluded from the age calculation.The remaining 12 spots yield ages between 244.8 Ma and 259.8 Ma.They are concordant(Fig.6B and D)and yield a weighted mean206Pb/238U age of 250.3±2.1 Ma(n=12,MSWD=1.09).

有的学生对学习化学积极性不高，是因为他们体会不到学习的乐趣，学案教学可以在课前就把预习目标和任务、课堂学习目标告诉学生，帮助学生带着问题预习新课，对课本进行独立阅读思考，对知识进行分析综合，整理归纳，上课就不会迷迷糊糊，不知所云。

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We interpret the two single-crystal SHRIMP zircon U-Pb ages of samples B1-3 and B3-4 represent the formation ages of the tonsteins in the C1coal seam.In other words,these ages represent the deposition ages of volcanic ash beds.Furthermore,these ages can be taken as a good approximation for age of the C1coal seam because its total thickness is no more than 3 m.

2)本自适应调节系统的动态仿真实验表明，当插秧机行走部振动频率较低时，该插秧机插深自适应平衡系统控制能力良好，工作稳定可靠。而通常情况下水稻插秧前都完成了水田平地作业，水田硬底层相对较为平整，插秧机行走作业过程中颠簸频率不高，因此秧盘插深自适应调节系统是可以满足农田插秧实际工作的要求的。今后还需要开展系统大田实验，在实际工况中对系统的实用性和稳定性进行分析，进一步提升系统的工作性能，为插秧机高速精细化作业提供有力的技术支持。

5. Discussion

5.1. U—Pb ages of typical PTB sections in South China

The Permian—Triassic timescale has undergone major revisions in recent years based on isotope dilution thermal ionization mass spectrometry(IDTIMS)dating of zircons in volcanic ash beds of different locations.Most of these results were obtained from the clay beds adjacent to the PTB sections in South China(Figs.7 and 8),presently the most accurate PTB age is 251.902±0.024 Ma from Meishan section, Zhejiang Province, southeastern China(Burgess et al.,2014).

Many PTB sections were studied in South China,including sedimentary sequences deposited in marine,terrestrial and transitional environments(Burgess et al.,2014;Gao et al.,2013;Shen et al.,2011;Yang et al.,2005;Yin and Song,2013;Yin et al.,2007,2012).Many of these sedimentary sections contain more than one layer of volcanic ash(Bowring et al.,1998),and their dating results can be compared to the ones obtained in this study,as both samples are derived from layer of volcanic ashes(tonsteins),and by the same dating method.

Some previous studies on the radiometric ages of PTB sections in South China are shown in Fig.8.Of them,the Meishan and Chahe sections are very important,which represent marine and terrestrial sequences,respectively.The Yantang Mine is the geographically close to the Chahe section(68 km)and is also a terrestrial PTB sequence(Fig.7).The C1coal seam of Yantang Mine can be compared to the PTB horizons in Meishan and Chahe sections as all of these sections were developed with the similar volcanic ash beds in South China.

3)The B3coal lies 3.39 m below the B2coal.The thickness of the B3coal is 0.75 m,with a 5-cm-thick kaolinite claystone parting(tonstein)inside.The roof of this sub-seam is silty mudstone and the floor is gray mudstone.Evidence for tonstein is discussed in Section 4.1.

在20世纪，民众反对剥削，并寻求将他们在经济中的重要作用转化为政治权力。而现在，民众担心自己被边缘化，他们疯狂地想在为时已晚之前使用剩余的政治权力。2016年，特朗普和英国退欧得到了许多人的支持，他们仍然享有政治权力，但担心自己正在失去经济价值。也许在21世纪，民粹主义的反叛将不是针对剥削人民的经济精英，而是针对不再需要他们的经济精英。

Yang et al.(2005)and Yin et al.(2007)suggested that Bed 67 of the Chahe section is equivalent to Bed27 of the Meishan section which is considered as the P/T boundary bed,and the beds from lower 66f to upper 68(including 68a and 68c)are defined as the boundary interval.The clay beds in 68a and 68c above the PTB at the Chahe section are dated at 247.5±2.8 Ma and 252.6±2.8 Ma,respectively(Yang et al.,2005;Yin et al.,2007).Therefore,the age of Bed 67 should be slightly older than Bed 68a dated as 252.6±2.8 Ma.Shen et al.(2011)also gave asimilarage of 252.30±0.07 Ma for Bed 68 of the Chahe section.However,they proposed that the age of Bed 68 is comparable with that of Bed 25 at the Meishan section,which is the main extinction bed.Thus,the age of the PTB at the Chahe section should be slightly younger than 252.30±0.07 Ma(Shen et al.,2011).This conclusion seems to be different from the view of Yin et al.(2007)and Yang et al.(2005),with the differences focusing on whether the PTB is above or below Bed 68.

The marine—terrestrial transitional Zhongzhai section in Liuzhi,Guizhou Province,also contains ash beds across the PTB.The SHRIMP zircon U-Pb ages of the beds immediately above and below the boundary are 252.00±0.08 Ma and 252.24±0.13 Ma,respectively(Shen et al.,2011).Therefore,the age of the PTB at the Zhongzhai section ranges from 252.00±0.08 Ma to 252.24±0.13 Ma.

Gao et al.(2013)suggested that the SHRIMP zircon U—Pbages from the 10 beds across the Permian—Triassic transition of marine Daxiakou section range from～248.5 Ma to ～253.8 Ma,mostly 250—251 Ma.Lehrmann et al.(2015)reported a new U—Pb zircon age of 251.985±0.097 Ma from the PTB at the Taiping section(Guizhou Province),Nanpanjiang Basin in South China.

The International Chronostratigraphic Chart of 2017 showed the latest revised PTB age at 251.902±0.024 Ma,which was reported by International Commission on Stratigraphy(ICS).

The206Pb/238U age of the tonstein B1-3 with 252.0±2.3 Ma is a little older than that of the extinction peak,while much older than those obtained from the PTB at the marine Meishan section in Zhejiang Province,some 1600 km away,where the PTB age determined by ICS(2017)is 251.902±0.024 Ma.Although the age of the B3-4 is younger than that of the B1-3,the value ranges of two tonstein layers overlap,indicating that these values are still in a reasonable range.As a whole,the age of C1coal seam in the Yantang Mine is very close to the main episode of PTB mass extinction.

5.2. Correlation of the C1coal seam and PTB study

Several studies focused on dating the PTB strata by conodont zones (as defined by first appearance datum of the conodont Hindeodus parvus)and radiometric ages(Jiang et al.,2007;Kozur,2003;Nicoll et al.,2002;Yin et al.,2001;Zhang et al.,2014).A large number of data were derived from volcanic ash beds or clay beds close to the PTB,including the one in this study.The ultimate goal of this paper is to determine the age of the C1coal seam.In comparison with organic-poor successions,the coal beds obviously contain more information of palaeobotany,palaeoclimate and palaeoenvironment that may help better understand the PTB events.In fact,in addition the dating work of the C1coal seam,the palynological assemblage analysis of the C1coal seam had been undertaken at the same time,and the detailed description and analysis of sporopollen results will be reported in another paper.

Each tonstein layer is a record of volcanic eruption.The only 5-cm-thick tonstein implies a short eruption.He et al.(2007)reported that sediments of the upper Xuanwei Formation surrounding the Khangdian Oldland were derived from maficvolcanic rocks of the Emeishan large igneous province(ELIP).In view of volcanic eruptions close to the PTB,the main episode of Siberian large igneous province(SLIP)was around 252 Ma(Jerram et al.,2016;Ogden and Sleep,2012),while the terminal age of ELIP is 251.0±1.0 Ma(Zhu et al.,2011).It is clear that the ages of the tonsteins in the C1coal seam,determined in this paper,is within the range of both the ELIPand SLIP,and as a result,the origin of the tonsteins in the C1coal seam is hard to determine.However,considering the distribution of large igneous provinces,the ELIP is much closer to Xuanwei County than SLIP.The closest ELIP eruption site with available research data is in Dongchuan,eastern Yunnan,some 109 km to the west-southwest(Xu et al.,2001).According to the spatial and temporal distribution of ELIP,we interpret the tonsteins analyzed in this study to have formed by volcanic eruptions of the ELIP.Furthermore,the minerals and geochemistry characteristics(Shao et al.,2015)of the C1coal also implied the influence of contemporaneous volcanic activities.

As the last coal deposited at the end Permian,the C1coal seam in Yantang Mine contains abundant information,such as carbon and sulfur isotope,minerals,poisonous and harmful trace elements,and organic macerals.These geochemical characteristics of the C1 coal seam may provide critical information about the PTB events,which most importantly,is the largest mass extinction event in the Earth history.

用锹捯7～8次，在捯拌中，依据混拌土的干湿度，适当用喷壶，喷5.5～6.5升水，搅拌均匀，达到手捏成团不滴水，落地混拌土自然散开为宜。

6. Conclusions

Two partings from the C1coal seam in the Yantang Mine,Xuanwei,eastern Yunnan,China,close to the PTB were investigated.Their mineralogical composition indicates that these partings are tonsteins derived from volcanic ash.The coal itself is enriched with silica derived probably from a volcanic source.The SHRIMP zircon U—Pb ages of these tonsteins are 252.0±2.3 Ma and 250.3±2.1 Ma,which are compatible to the U—Pb zircon age of main episode of PTB mass extinction(Bed 25)from the most extensively-studied Meishan section,Zhejiang Province,China.Inparticular,the age of upper tonstein B1-3datedas252.0±2.3 Ma is a little older than Bed 25 at Meishan,which means the coal-forming plant of C1coal seam occurred just before the end-Permian mass extinction.Spatial and temporal analyses suggest that volcanic ashes that formed the tonsteins in the C1 coal seam were likely from the ELIP.These findings encourage further research on the biological,mineralogical and geochemical features of the coal beds and tonstein layers near the PTB in South China.

Acknowledgements

This research is supported by the National Natural Science Foundation of China(Grant No.41572090 and 41602123),the Doctor Foundation of Henan Polytechnic University(Grant No.B2016-72)and Open Fund of Key Laboratory of Biogenic Traces and Sedimentary Minerals of Henan Province(Grant No.OTMP1407).

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