Geoscience Frontiers 8 (2017) 1161e1173

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Research paper

New U-Pb age constraints on the upper Banxi Group and synchrony ofthe Sturtian glaciation in South China

Gaoyuan Song a, Xinqiang Wang a,b,*, Xiaoying Shi a,b, Ganqing Jiang c

a School of Earth Science and Resources, China University of Geosciences (Beijing), Beijing 100083, Chinab State Key Laboratory of Biogeology and Environmental Geology, Beijing 100083, ChinacDepartment of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA

a r t i c l e i n f o

Article history:Received 8 October 2016Received in revised form24 October 2016Accepted 24 November 2016Available online 19 December 2016Handling Editor: X. Wan

Keywords:Nanhua basinWuqiangxi FormationBanxi GroupU-Pb zircon agesSturtian glaciationSouth China

* Corresponding author. School of Earth Science and RGeosciences (Beijing), Beijing 100083, China.E-mail addresses: wxqiang@cugb.edu.cn, wxqiang307@Peer-review under responsibility of China University

http://dx.doi.org/10.1016/j.gsf.2016.11.0121674-9871/� 2017, China University of Geosciences (BND license (http://creativecommons.org/licenses/by-n

a b s t r a c t

The Nanhua basin in South China hosts well-preserved middleelate Neoproterozoic sedimentary andvolcanic rocks that are critical for studying the basin evolution, the breakup of the supercontinentRodinia, the nature and dynamics of the “snowball” Earth and diversification of metazoans. Establishing astratigraphic framework is crucial for better understanding the interactions between tectonic, paleocli-matic and biotic events recorded in the Nanhua basin, but existing stratigraphic correlations remaindebated, particularly for pre-Ediacaran strata. Here we report new Laser Ablation Inductively CoupledPlasma Mass Spectrometry (LA-ICPMS) U-Pb zircon ages from the middle and topmost Wuqiangxi For-mation (the upper stratigraphic unit of the Banxi Group) in Siduping, Hunan Province, South China. Twosamples show similar age distribution, with two major peaks at ca. 820 Ma and 780 Ma and one minorpeak at ca. 910 Ma, suggesting that the Wuqiangxi sandstone was mainly sourced from Neoproterozoicrocks. Two major age peaks correspond to two phases of magmatic events associated with the rifting ofthe Nanhua basin, and the minor peak at ca. 910 Ma may correspond to the Shuangxiwu volcanic arcmagmatism, which represents pre-collision/amalgamation subduction on the southeastern margin of theYangtze Block. The youngest zircon group from the topmost Wuqiangxi Formation has a weighted meanage of 714.6 � 5.2 Ma, which is likely close to the depositional age of the uppermost Banxi Group. Thisage, along with the ages reported from other sections, constrains that the Banxi Group was depositedbetween ca. 820 Ma and ca. 715 Ma. The age of 714.6 � 5.2 Ma from the top of the Wuqiangxi Formationis indistinguishable with the SIMS U-Pb age of 715.9 � 2.8 Ma from the upper Gongdong Formation in theSibao village section of northern Guangxi, South China. It is also, within uncertainties, overlapped withtwo TIMS U-Pb ages from pre-Sturtian strata in Oman and Canada. These ages indicate that the Jiangkou(Sturtian) glaciation in South China started at ca. 715 Ma instead of ca. 780 Ma and support a globallysynchronous initiation of the Sturtian glaciation at ca. 715 Ma.

� 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting byElsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

The middleelate Neoproterozoic sedimentary and volcanicstrata (ca. 820e541 Ma) are well preserved in the Nanhua basin,South China (Wang and Li, 2003; Li et al., 2008a,b; Jiang et al., 2011;Zhao et al., 2011). These records are crucial for understanding theevolution of the Neoproterozoic Nanhua basin (e.g., Wang and Li,

esources, China University of

126.com (X. Wang).of Geosciences (Beijing).

eijing) and Peking University. Produc-nd/4.0/).

2003; Li et al., 2008b, 2009; Zhao et al., 2011; Wang et al., 2012b,2015; Zhang et al., 2013, 2015a,b) and provide a unique archive tostudy the breakup of the supercontinent Rodinia (e.g., Li et al., 1999,2008b, 2013; Cawood et al., 2013), the nature and dynamics ofNeoproterozoic “snowball” Earth (Evans et al., 2000; Jiang et al.,2003a; Zhou et al., 2004; Bao et al., 2008; Zhang et al., 2008c;Huang et al., 2014; Lan et al., 2014, 2015a,b) and the diversifica-tion of multicellular eukaryotes (e.g., Yuan et al., 2011; Chen et al.,2014; Xiao et al., 2014a). An unresolved issue related to this sedi-mentary succession is the regional stratigraphic correlation of pre-Sturtian strata across the Nanhua basin or more specifically, thestratigraphic relationship between the Banxi Group in HunanProvince and the Liantuo Formation in Yangtze Gorges area. Some

ction and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-

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researchers suggested that the Liantuo Formation is equivalent orpartially equivalent to the Banxi Group (e.g., Jiang et al., 2003b,2011; Wang and Li, 2003; Zhang et al., 2008a,b; Du et al., 2013;Lan et al., 2015a), while others argued that the deposition of theBanxi Group was earlier than the Liantuo Formation (e.g., Xue et al.,2001; Wang et al., 2009b; Gao et al., 2013a,b; Lin et al., 2013; Yinand Gao, 2013; Liu et al., 2015a).

The settlement of current debate requires further age constrainton both units. Recent geochronological studies indicate that theLiantuo Formation was deposited between ca. 780 Ma and 714 Ma(Du et al., 2013; Lan et al., 2015a). The deposition of the Banxi Groupwas considered to begin at ca. 820 Ma (Wang and Li, 2003; Zhanget al., 2008b, 2012b; Wang et al., 2009a), but the top boundary ageof this unit remains debated, either at ca. 720 Ma (Zhang et al.,2008a; Wang et al., 2012a; Bai et al., 2015) or at 780 Ma (Liu et al.,2015a). The upper age of the Banxi Group also provides constrainton the onset of the Sturtian glaciation because the Banxi Group inthe Nanhua basin is overlain by the Jiangkou diamictites correlativewith the Sturtian glaciation (e.g., Lan et al., 2014). The Sturtianglaciationwas thought to have initiated synchronously at ca. 715Main at least three continents including South China, Oman and Canada(Bowring et al., 2007; Macdonald et al., 2010; Lan et al., 2014), whichis consistent with the prediction of the “snowball” Earth hypothesis(Hoffman and Schrag, 2002). However, two SHRIMP U-Pb ages of778.4 � 5.2 Ma and 785 � 11 Ma were recently reported from thetuff bed/breccia at the base of the Chang’an/Jiangkou diamictites inthe Nanhua basin (Gao et al., 2013a,b; Liu et al., 2015a), challengingthe synchroneity of the Sturtian glaciation.

In this paper, we present the result of LA-ICPMS U-Pb datinganalyses of detrital zircons from the middle and topmostWuqiangxi Formation of the Banxi Group at Siduping, westernHunan Province (Fig. 1). The new ages, in combination with avail-able zircon U-Pb ages in literature, provide age constraints on theBanxi Group and the synchroneity of the Sturtian glaciation. Wealso discuss the provenance of Wuqiangxi sandstone based on theage distribution and regional stratigraphic correlation of pre-Sturtian strata in the Nanhua basin.

2. Geological background and sampling

The Nanhua basin was formed as a result of intracontinentalrifting along the Jiangnan Orogen that marks the amalgamation ofthe Yangtze and Cathaysia blocks (Fig. 1A; Li et al., 1999, 2008a,b;

Figure 1. (A) The Neoproterozoic tectonic framework of South China. The Nanhua basin sepJiang et al., 2003b). (B) Simplified geological map of the study area (red star marks the Sid

Wang and Li, 2003; Li et al., 2010a,b; Zhao and Cawood, 2012).The rift basin evolved into a passive continental margin during thelate Neoproterozoic (e.g., Jiang et al., 2003b, 2011) or a failed rift(e.g., Wang and Li, 2003; Yao et al., 2015). Alternatively, the Nanhuabasin has been interpreted as a back-arc basin (Zhou et al., 2002a;Zhao et al., 2011; Zhang et al., 2012a).

Unconformably overlying the metamorphic rocks of earlyNeoproterozoic Sibao/Fanjingshan/Lengjiaxi groups, the middleelate Neoproterozoic strata were well outcropped in HubeieHunaneGuizhoueGuangxi provinces of the Nanhua Basin and areconventionally divided into three parts (Fig. 2A; Jiang et al., 2011;Xiao et al., 2014b): pre-glacial deposits, glacial and interglacialdeposits and post-glacial deposits. The pre-glacial deposits arecharacterized by siliciclastic rocks intercalated with volcanic rocks,including the Liantuo Formation in Hubei Province, the BanxiGroup in Hunan Province, the Xiajiang Group in Guizhou Provinceand the Danzhou Group in Guangxi Province (Fig. 2A). The rela-tionship of these units, particularly the Liantuo Formation and theBanxi Group, remains controversial (e.g., Du et al., 2013; Gao et al.,2013a,b; Lan et al., 2015a; Liu et al., 2015a) and will be the focus ofthis paper. Two intervals of glacial diamictites were documentedin the Nanhua basin: the older (Sturtian) Jiangkou Group (or theirequivalents) and the younger (Marinoan) Nantuo Formation (e.g.,Zhang et al., 2008b; Jiang et al., 2011; Lan et al., 2014). The Jiangkouglaciation was further divided to the earlier Chang’an glaciationand the later Gucheng glaciation (Zhang et al., 2011; Lan et al.,2015b), although it is quite likely that these two “glaciations” arediachronous stratigraphic record of the same glaciation (Jianget al., 1996, 2003b, 2011). Interglacial manganese-bearing shale/siltstone of the Datangpo/Xiangmeng Formation dated at ca.663e654 Ma (Zhou et al., 2004; Yin et al., 2006; Zhang et al.,2008c; Liu et al., 2015b) is sandwiched between the JiangkouGroup and the Nantuo Formation. In proximal shelf sections, theJiangkou diamictites are largely missing probably due to erosion(Fig. 2A). Post-glacial mixed carbonates and shales of the Doush-antuo Formation are dated as from 635 Ma to �551 Ma (Condonet al., 2005; Zhang et al., 2005; An et al., 2015) and carbonates/cherts of the Dengying/Liuchapo formations extend to Cambrian.These Ediacaran strata have been intensively investigated due topreservation of abundant fossils, including acanthomorphic acri-tarchs, macroscopic algae, possible animal embryos and meta-zoans (e.g., Yin et al., 2007; Yuan et al., 2011; Chen et al., 2014; Liuet al., 2014; Xiao et al., 2014a).

arated the Yangtze Block in the northwest from the Cathaysia Block in southeast (afteruping section in Fig. 1A)

Figure 2. (A) Generalized shallow-water to deep-water transect of the middle to late Neoproterozoic strata in South China (modified after Xiao et al., 2014b). (B) Middle Neo-proterozoic stratigraphic section in Siduping, western Hunan Province, South China. The Banxi Group is subdivided into the Madiyi Formation in the lower part and the WuqiangxiFormation in the upper part. Stratigraphic position of samples is marked in the stratal column.

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The study section at Siduping is located in the Zhangjiajie area,western Hunan Province, South China (Fig. 1). The Banxi Group inthis locality is divided into the Madiyi and Wuqiangxi formations(Fig. 2B). The Wuqiangxi Formation is more than 1000 m thick andis composed of purple and gray sandstone, siltstone and pebblysandstone. Abundant wavy (Fig. 3A) and cross beddings (Fig. 3B)have been observed in this unit, indicative of predominately littoralenvironment. The Wuqiangxi Formation is overlain by a 1-m-thicksandy diamictite with parallel beds (Fig. 3C). This unit, locallytermed as the Dongshanfeng Formation, was considered to becorrelative with the Chang’an Formation (Jiang et al., 1996), whichis unconformably overlain by massive diamictites of the NantuoFormation (Fig. 3D). The missing of the interglacial deposits in thissectionmay have been resulted from the erosion during the Nantuoglaciation. Two samples were collected from the Wuqiangxi For-mation for LA-ICPMS dating. Sample WQX13-1 is dark-purple,medium-grained sandstone from the middle part of theWuqiangxi Formation, about 425 m below the top the WuqiangxiFormation. SampleWQX13-2 is dark-green, fine-grained sandstoneimmediately below the Chang’an/Dongshanfeng diamictite.

3. Analytical methods

Rock samples (w4 kg in weight) were crushed in a steel jawcrusher, and standard density and magnetic separation techniqueswere used to separate heavy minerals. Zircons were hand-pickedunder binocular microscope. Representative grains, together withstandard zircon 91500, were mounted in an epoxy resin and thenpolished to expose the longitudinal section of crystals. Prior toanalysis, transmitted and reflected light photomicrographs as wellas cathodoluminescence (CL) images were used to reveal the zircongrains’ internal structures and to help selecting appropriateanalytical spots.

U-Pb dating of zircon grains was conducted using LA-ICPMS atthe State Key Laboratory of Geological Processes and Mineral

Resources, China University of Geosciences (Beijing). Laser sam-pling was performed using a Coherent’s GeoLasPro-193nm system.A Thermo Fisher’s X-Series 2 ICP-MS instrument was used to ac-quire ion-signal intensities. Helium was applied as a carrier gas.Argonwas used as the make-up gas and mixed with the carrier gas.All data were acquired on zircon in single spot ablation mode at aspot size of 32 mm with 6 Hz frequency. SRM610 was used tooptimize the LA-ICPMS instrument, and as an external standard fordetermination of trace elements. Zircon 91500 was used as anexternal standard for U-Th-Pb isotope ratios (Wiedenbeck et al.,2004). Meanwhile zircon Mud Tank was used as a monitoringstandard for each analysis. Time-dependent drifts of U-Th-Pbisotope ratios were corrected using a linear interpolation for everyfive analyses according to the variations of 91500. Each analysisincorporated a background acquisition of approximately 20 s (gasblank) followed by 50 s data acquisition from the sample. Off-lineselection and integration of background and analytical signals,time-drift correction and quantitative calibration of trace elements,and U-Pb dates were performed by ICPMSDataCal (Liu et al., 2008a)and plotted using Isoplot 3.0 (Ludwig, 2003). Common Pb correc-tion was carried out using the Andersen method (Andersen, 2002).

4. Results

One hundred and fifty-eight zircon grains were dated, and theresults are listed in Supplementary Table A1. Uncertainties arepresented with an error of 1s. Representative zircon CL images areshown in Fig. 4 and the results are presented in concordia (Fig. 5)and probability plots (Fig. 6). REE plots are shown in Fig. 7. We onlyuse ages that are less than �10% discordant. 207Pb/206Pb ages areused for zircons older than 1000Ma, while 206Pb/238U ages are usedfor zircons younger than 1000 Ma.

Zircons from the WQX-13-1 are dominated by euhedral to sub-euhedral morphology, with a size range of 80e200 mm in lengthand aspect ratios of 1.5e2 (Fig. 4), implying a short transport

Figure 3. Field photos of the upper Wuqiangxi Formation at the Siduping section. (A) Wavy bedding in purple sandstone of the Wuqiangxi Formation; (B) planar cross bedding inpebbly sandstone of the Wuqiangxi Formation; (C) the sandy diamictite above the top of theWuqiangxi Formation that is possibly remains of the Sturtian glacial diamictites; (D) theboundary between the sandy diamictite and the Nantuo Formationdiamictite. The interglacial Datangpo/Xiangmeng Formation is entirely missing in this section. Coins as scale in(A) and (B) are 2 cm in diameter. Hammer and pencil as scales in (C) and (D) are 33 cm and 15 cm in length, respectively.

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distance from the source. Under CL images, most zircons displaytypical oscillatory growth zonation (Fig. 4) and have Th/U ratios>0.4. The REE distribution pattern shows enrichment of HREEs,negative Eu anomalies and positive Ce anomalies (Fig. 7A). All thesefeatures indicate a magmatic origin of zircons (Koschek, 1993;Hoskin and Ireland, 2000; Belousova et al., 2002; Rubatto, 2002;Hoskin and Schaltegger, 2003). A total of 84 analytical spots weretaken on 80 grains, and 67 concordant analyses yield three agepeaks at ca. 780 Ma, ca. 820 Ma and ca. 920 Ma, respectively(Fig. 6B). The youngest detrital zircon age is 745.1 � 8.6 Ma, andthere is one zircon showing an age of ca. 1975 Ma (Fig. 6A).

Zircons from sample WQX-13-2 show similar features with thatof sample WQX-13-1. Most zircons are euhedral to sub-euhedral,with a size range of 60e200 mm and aspect ratios of 1e2 (Fig. 4).In CL images, most zircons show clear oscillatory growth zonation(Fig. 4) and have Th/U ratios of >0.4, indicating a magmatic origin(Koschek, 1993; Hoskin and Schaltegger, 2003). This inference isalso supported by the REE distribution pattern which displaysnegative Eu anomalies, positive Ce anomalies, and HREE enrich-ments (Hoskin and Ireland, 2000; Belousova et al., 2002; Rubatto,2002; Hoskin and Schaltegger, 2003) (Fig. 7B). A total of 84analytical spots were conducted on 78 grains, and 64 out of 84concordant analyses fall in the range between ca. 740 and ca.930Ma, giving three peaks at ca. 780Ma, ca. 820Ma and ca. 910Ma,respectively (Fig. 6D). The youngest detrital zircon age group yiel-ded a weighted mean of 714.6 � 5.2 Ma (2s, n ¼ 5, MSWD ¼ 0.17)(Fig. 6D). There are four zircons giving pre-Neoproterozoic ages atca. 1579 Ma, ca. 1766 Ma, ca. 1853 Ma and ca. 2177 Ma, respectively(Fig. 6C).

5. Discussion

5.1. Depositional age of the Banxi Group and stratigraphiccorrelation

The new ages provide further constraints on the depositionalage of the Banxi Group. In the past few years, a number of radio-metric ages have been reported from the Banxi Group (Wang et al.,2003; Yin et al., 2003; Zhang et al., 2008a,b; Wang et al., 2010c,2012a; Gao et al., 2012, 2014; Bai et al., 2015; Liu et al., 2015a;Zhang et al., 2015b). It is widely accepted that the deposition ofthe Banxi Group initiated at ca. 820 Ma (Wang and Li, 2003; Zhanget al., 2008b, 2012b; Wang et al., 2009a). However, current datashow large difference on the termination of the Banxi Group (Zhanget al., 2008a; Wang et al., 2012a; Bai et al., 2015; Liu et al., 2015a;Zhang et al., 2015b). For example, a tuffaceous siltstone from theupper Banxi Group in Zhijiang section, western Hunan Province,yields a SHRIMP U-Pb age of 725 � 10 Ma (Fig. 8; Zhang et al.,2008a), which was considered as the youngest age of the BanxiGroup. However, a more recent study reported a SHRIMP U-Pb ageof 786 � 11 Ma from a tuffaceous bed at the top of the Banxi Groupin the Xiushan section, western Chongqing (Liu et al., 2015a), whichhas also been argued to represent the depositional age of the upperBanxi Group. The youngest zircon group from the topmostWuqiangxi Formation yields a weight mean age of 714 � 5.2 Ma(MSWD ¼ 0.17) (Fig. 6D). Several lines of evidence argue againstmetamorphic influence on crystallization age: (1) clear oscillatorygrowth zonation (Fig. 4); (2) relatively high Th/U ratios (>0.4Table A1), and (3) REE distribution pattern showing negative Eu

Figure 4. Representative CL images of zircon grains from sample WQX-13-1 (upper panel) and WQX-13-2 (lower panel); most of them show clear oscillatory growth zonation.

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anomalies, positive Ce anomalies, and HREE enrichments (Fig. 7).Thus, the age of 714 � 5.2 Ma provides a maximum age constrainton the upper boundary of the Banxi Group. This age is slightlyyounger than the 725�10 Ma age from the Zhijiang section (Zhanget al., 2008a). Considering that the tuffaceous siltstone producingthe ca. 725 Ma age was located about 300 m below the top of theBanxi Group, these two ages are consistent.

However, the ca. 715 Ma age from the Wuqiangxi Formation ismuch younger (about 70 Myr) than the ca. 786 Ma age claimed tobe the top boundary age of the Banxi Group in Chongqing (Liu et al.,2015a). A reasonable explanation for this difference is erosion andstratigraphic truncation at the top of the Banxi Group. The thick-ness of the Wuqiangxi Formation in the Nanhua basin show sig-nificant variations, ranging from<100 m to>1000 m. A SHRIMP U-Pb age of 809.3 � 8.4 Ma was reported from a tuffaceous bed about60 m below the top of the Wuqiangxi Formation in the Guzhangsection, western Hunan Province (Fig. 8; 60 km south to our sec-tion) (Zhang et al., 2008b). TheWuqiangxi Formation in Guzhang isonly 152 m and may have been significantly eroded. Therefore, theca. 809 Ma age in that section may record the depositional age ofthe lower rather than upper part of the Wuqiangxi Formation(Zhang et al., 2008b). TheWuqiangxi section in the Xiushan section,western Chongqing, may be equally subject to erosion. Hence, theage of ca. 786 Ma reported by Liu et al. (2015a) may represent theloweremiddle part of the Wuqiangxi Formation.

For comparison, sample WQX13-1 was collected from themiddle Wuqiangxi Formation (about 425 m below the topboundary) in the Siduping section, and the youngest zircon groupfrom this sample gives an age of ca. 745 Ma (Fig. 6B), which may bethe closest to the depositional age. TheWuqiangxi Formation in theSiduping section is about 1200 m thick, and a rough estimation/

extrapolation would put the lower part of the Wuqiangxi Forma-tion at ca. 800Ma, roughly consistent with the ca. 809Ma age of thelower Wuqiangxi Formation at Guzhang (Zhang et al., 2008b) andXiushan (Liu et al., 2015a).

The new ages present here, coupled with existing ages withinthe Banxi Group (Wang et al., 2003; Yin et al., 2003; Zhang et al.,2008a,b; Wang et al., 2010c, 2012a; Gao et al., 2012, 2014; Baiet al., 2015), constrain the deposition age of the Banxi Group be-tween ca. 820 Ma and ca. 715 Ma and provide insights into regionalstratigraphic correlation in the Nanhua basin (Fig. 8). The relationbetween the Banxi Group and the Liantuo Formation in YangtzeGorges area has long been contentious. Some suggested that theLiantuo Formation is equivalent or at least partially equivalent tothe Banxi Group (e.g., Jiang et al., 2003b; Wang and Li, 2003; Zhanget al., 2008a,b; Du et al., 2013; Lan et al., 2015a); while othersargued that the deposition of the Banxi Group predated the LiantuoFormation. In this later view, the Liantuo Formation is eithercorrelated with the Chang’an Formation (mainly based on a lessreliable age of ca. 780 Ma from the basal part of the Chang’anFormation; Gao et al., 2013a,b, 2015; Yin and Gao, 2013; Liu et al.,2015a) or with the middleeupper Fulu Formation (e.g., Xue et al.,2001; Peng et al., 2004; Lin et al., 2013). The correlation betweenthe Liantuo Formation and the Fulu Formation is mainly based onlithological and paleoclimate interpretations and is devoid ofgeochronological evidence. Recent geochronological study indicatethat the Liantuo Formation was deposited between ca. 780 Ma and<730Ma or ca. 714Ma (Fig. 8; Gao and Zhang, 2009; Du et al., 2013;Lan et al., 2015a). These ages support that the Liantuo Formation isequivalent to the middleeupper Banxi Group, or more specificallyto middleeupperWuqiangxi Formation (ca. 810e715Ma). It shouldbe noted that the Liantuo/Nantuo boundary in the Yangtze Gorge

Figure 5. U-Pb concordia plots of zircons from sample WQX-13-1 (A, B) and WQX-13-2 (C, D). The ellipses show 1s errors.

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area is characterized by an erosion surface (Fig. 2; Lan et al., 2015a),implying that the entire Sturtian glacial record and uppermost partof the Liantuo Formation may have been eroded.

Our new data also confirm the correlations of the Banxi Groupwith the Xiajiang Group in eastern Guizhou Province and theDanzhou Group in northern Guangxi Province (Fig. 8). The Xiajiangand Danzhou groups, similar to the Banxi Group, are mainlycomposed of thick siliciclastic and volcanic rocks that unconform-ably overlie early Neoproterozoic metamorphic basement of theSibao/Fanjingshan/Lengjiaxi Group. Intensive geochronologicalstudy have been conducted on these units (e.g., Zhou et al., 2002c;Wang et al., 2006, 2010a, 2013a,b, 2014, 2015; Wang et al., 2012a;Wang and Zhou, 2012; Lan et al., 2014; Qin et al., 2015). Theonset and termination of the Xiajiang Group were well constrainedat ca. 820 Ma (Wang et al., 2006; Gao et al., 2014) and ca. 725 Ma(Wang et al., 2010a; Qin et al., 2015), respectively. The time span ofthe Xiajiang Group (ca. 820e725 Ma) is, therefore, consistent withthat of the Banxi Group (ca. 820e715 Ma). Likewise, the base of theDanzhou Group have been constrained at ca. 820 Ma (Zhou et al.,2002c) or ca. 770 Ma (Wang and Zhou, 2012) and the top of thisGroup is dated at ca. 715Ma (Lan et al., 2014) (but see different viewin Gao et al., 2013a,b, 2015). These ages are comparable with those

from the Banxi Group, but the bottom ages of the Danzhou Grouprequire further confirmation, given the large basal age discrepancy(more than 50 Myr) between the Banxi and Danzhou groups (Zhouet al., 2002c; Wang and Zhou, 2012).

5.2. Age constraint on the Sturtian glaciation

Two severe glaciations (Sturtian and Marinoan) during the lateNeoproterozoic recorded the extreme cold in Earth’s history, the“snowball” Earth (Hoffman and Schrag, 2002). The “snowball”Earth hypothesis predicted an abrupt onset of glacial condition inmiddle to low latitudes in response to runaway ice albedo (Hoffmanand Schrag, 2002). A synchronous initiation of the Sturtian glacia-tion is supported bymany ages (e.g., Macdonald et al., 2010; Rooneyet al., 2014), but additional ages from different continents are stillneeded. The lower age of the Sturtian glaciation has previouslybeen constrained at ca. 725 Ma in Oman and South China (Brasieret al., 2000; Zhang et al., 2008a). The ca. 725 Ma age in SouthChina was obtained from a tuffaceous siltstone about 300 m belowthe base of the Jiangkou diamictite (Zhang et al., 2008a), implyingthat the Sturtian glaciation should be younger than 725Ma. The agereported by Brasier et al. (2000) in Oman has been calibrated to

Figure 6. Relative probabilities of zircons from sample WQX-13-1 (A, B) and WQX-13-2 (C, D) of the upper Wuqiangxi Formation. The youngest age group from sample WQX-13-2gives a weighted mean 206Pb/238U ages of 714.6 � 5.2 Ma. For zircons with ages >1000 Ma, 207Pb/206Pb ages are used. For zircons younger than 1000 Ma, 206Pb/238U ages are used.

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713.7 � 0.5 Ma using the ID-TIMS method (Bowring et al., 2007).Fanning and Link (2004) reported a SHRIMPU-Pb age of 709� 5Mafrom a tuff breccia immediately below glaciogenic diamictite of theScout Mountain Member in Oxford Mountain, Idaho. This age,within uncertainty, is overlapped with ID-TIMS age of

Figure 7. Chondrite normalized REE patterns of zircons from sample WQX-13-1 (A) and Wpositive Ce anomalies, and HREE enrichments, indicating magmatic origin.

713.7 � 0.5 Ma in Oman (Bowring et al., 2007). Recently, a TIMS U-Pb age of 716.5 � 0.2 Ma was obtained from a tuff bed within theSturtian equivalent diamictite of the Upper Mount Harper Group innorthwestern Canada (Macdonald et al., 2010). This age is sup-ported by an indistinguishable SIMS U-Pb age of 715.9 � 2.8 Ma

QX-13-2 (B). The REE patterns show that the zircons display negative Eu anomalies,

Figure 8. Stratigraphic correlation of the Neoproterozoic (pre-Sturtian) strata across the Nanhua basin in South China, with most of the published U-Pb ages shown in each stratigraphic column. The ages were shown in different colorsaccording to the analyzing methods (red: SIMS; blue: SHRIMP; green: LA-ICPMS; brown: TIMS).

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from a tuffaceous siltstone bed in the upper Gongdong Formationconformably overlain by the Chang’an diamictite in South China(Lan et al., 2014). Interestingly, two other SHRIMP U-Pb ages of778.4 � 5.2 Ma and 785 � 11 Ma were recently reported from thetuff beds and vocanic breccia at the base of the Chang’an diamictitein Guangxi and Hunan provinces (Gao et al., 2013a,b; Liu et al.,2015a). These two ages are consistent with each other within un-certainties but are 60Myr older than the SIMS age of 715.9� 2.8Mafrom the upper Gongdong Formation (Lan et al., 2014).

The new age from the top of the Wuqiangxi Formation providesfurther constraint on the onset of the Sturtian glaciation in SouthChina. The Wuqiangxi Formation is overlain by the Dongshanfengdiamictite in the Siduping section, thus our geochronology studyindicates that the maximum age for the onset of the Sturtianglaciation is 714 � 5.2 Ma, consistent with the SIMS U-Pb age of715.9 � 2.8 Ma from the upper Gongdong Formation (Lan et al.,2014). Within uncertainties, it is also overlapped with the TIMSages of 713.7 � 0.5 Ma and 715.9 � 2.8 Ma in Oman and Canada(Bowring et al., 2007; Macdonald et al., 2010). These ages suggestglobal synchronous initiation of the Sturtian glaciation at ca.715 Ma. The large offset between our result and the two SHRIMPages (both at ca. 780 Ma) from the base of Chang’an diamictite (Gaoet al., 2013a,b; Liu et al., 2015a) is intriguing. A potential explana-tion is that the ca. 780 Ma age suffers from inheritance because thezircon grains are rounded to varying extent or it is a detrital zirconage (e.g., Lan et al., 2015a).

5.3. Provenance of the Wuqiangxi Formation

The two samples from the middle and uppermost WuqiangxiFormation show similar age distributions (Fig. 6). Except for a fewzircons of MesoproterozoicePaleoproterozoic ages from 1579Ma to2177 Ma, most zircons from the samples have Neoproterozoic agesbetween 940 Ma and 750 Ma. This age distribution is consistentwith previous results from the Banxi Group in adjacent areas(Wang et al., 2010b, 2012a) and sections of the broader Nanhuabasin (Fig. 9), including the Danzhou Group in northern GuangxiProvince (Wang et al., 2012a; Wang and Zhou, 2012), the XiajiangGroup in eastern Guizhou Province (Wang et al., 2010a, 2012a), andthe Liantuo Formation in Yangtze Gorges area (Liu et al., 2008b; Cuiet al., 2014), suggesting that sandstones of the upper Banxi Groupand the Liantuo Formation had similar source rocks.

Except for the youngest age group of ca. 715 Ma in sampleWQX13-2, other ages from the Wuqiangxi sandstones include twomajor age groups of ca. 860e800 Ma (peak at 820 Ma) and ca.800e750 Ma (peak at 780 Ma) and a less abundant group at ca.940e870 Ma. The two major age groups correspond well with thetwo phases of magmatic activity during the middle Neoproterozoicin South China (e.g., Li et al., 2003a; Wang and Li, 2003; Wang et al.,2012b; Wang and Zhou, 2012; Zhao, 2015). According to paleo-current and paleogeomorphological reconstructions, Wang andZhou (2012) noted that most Neoproterozoic sediments of theDanzhou Group and its equivalents in the western side of theJiangnan Orogen were mainly derived from the western andnorthwestern parts of the Yangtze Block. Possible candidate sourcerocks of the ca. 860e800 Ma zircons include the ca. 860 MaGuandaoshan and Gezong plutons (Zhou et al., 2002a; Li et al.,2003b; Sun and Zhou, 2008), ca. 824 Ma felsic Gongcai pluton(Zhou et al., 2002a), ca. 821 Ma Bikou basalt (Wang et al., 2008), ca.820 Ma Wangjiangshan complex (Zhou et al., 2002a; Ling et al.,2003), ca. 817 Ma Tiechuanshan basalt and rhyolite (Ling et al.,2003), ca. 814 Ma Beiba gabbro (Zhao and Zhou, 2009a), ca.812 Ma Gaojiacun and 806 Ma Lengshuiqing mafic pluton (Zhouet al., 2006a) and ca. 803 Ma Suxiong bimodal volcanic rocks (Liet al., 2002). The source rocks of the ca. 800e750 Ma zircons may

consist of the ca. 795e750 Ma Kangding complex (Zhou et al.,2002a; Li et al., 2003a), ca. 780 Ma Bijigou mafic intrusion (Zhouet al., 2002b), ca. 765 Ma Miyi complex (Zhou et al., 2002a; Liet al., 2003a), ca. 762 Ma Tianpinghe granites (Zhao and Zhou,2009b), ca. 760 Ma Datian granodiorite (Zhao and Zhou, 2007),ca. 752 Ma Shaba gabbro (Li et al., 2003a) and ca. 750 Ma Xue-longbao adakitic complex (Zhou et al., 2006b). A subordinate agegroup between ca. 940 Ma and ca. 870Ma is consistent with ages ofthe Xixiang volcanic rocks dated as ca. 950e895 Ma (Ling et al.,2003), the Pingtoushan and Guankouya diorite dated as ca.885 Ma (Xiao et al., 2007) and the Liujiping gabbro dated as ca.877 Ma (Xiao et al., 2007).

The pre-Neoproterozoic zircons in our samples (Fig. 6) partiallypreserve oscillatory zonation and have high Th/U ratios (0.7e1.0)suggesting of magmatic origin. The oldest zircon has an age of2177 Ma, which is significantly younger than the oldest ages ob-tained from the Xiajiang and Danzhou groups (Wang et al., 2010a;Wang and Zhou, 2012) and the Liantuo Formation (Liu et al., 2008b;Cui et al., 2014). The lack of Archean zircons in the Banxi Groupsamples likely implies that the Kongling complex (ca. 2.9e3.3 Ga)may not have been the source rock. The Paleoproterozoic age groupranging from 1766 Ma to 2177 Ma is consistent with the ages ofabundant detrital zircons from the Neoproterozoic sediment inSouth China (e.g., Liu et al., 2008b; Yu et al., 2009, 2012; Cui et al.,2015) and may be associated with coeval magmatic rocks in theYangtze Block and the Cathaysia Block (e.g., Xiong et al., 2009; Yuet al., 2009; Peng et al., 2012; Wu et al., 2012). One zircon fromsample WQX-13-2 yields a Mesoproterozoic age of 1579 Ma. Nocontemporaneous magmatic rocks have been found in either theYangtze or Cathaysia blocks, although similar ages have been re-ported from the Xiajiang Group (Wang et al., 2010a), the DanzhouGroup (Wang and Zhou, 2012), the Liantuo Formation (Cui et al.,2014) and their correlatable units in the eastern side of the Jian-gnan Orogen (Cui et al., 2015). Thus, the possible source of theseMesoproterozoic zircons could be unexposed crystalline basementof the Yangtze Block (Wang et al., 2010a).

5.4. Uncertainties related to age dating methods

Although a significant number of ages have been reported fromthe Liantuo Formation and the Banxi/Xiajiang/Danzhou groups inSouth China (Fig. 8), these ages are obtained using different datingmethods including TIMS, SHRIMP, SIMS, and LA-ICPMS. Thesedating methods have varying precision and accuracy that have notbeen fully calibrated by inter-laboratory comparison. Recent inter-laboratory comparison study indicated that the LA-ICPMS zircon U-Pb ages have approximately 4% (2RSD) uncertainties, while theSIMS ages havemuch less uncertainty (Li et al., 2015). It is uncertainif the SHRIMP ages have similar accuracy problems. Existing agesfrom the top of the Liantuo Formation and Banxi Group were datedusing LA-ICPMS, SHRIMP, and SIMS and some of which are incon-sistent among sections (Fig. 8). Because the accuracy of ages de-termines the amount of erosion inferred at a particularstratigraphic section and the synchrony of geological events,whether the existing age differences among sections (Fig. 8) recorddifferential erosion during the Sturtian glaciation or an artifactraising from the dating methods (and the choice of stratigraphicinterval for dating) requires a systematic investigation by acollaborative research group using the same dating method.

6. Conclusions

We reported new LA-ICPMS detrital zircon U-Pb ages from themiddle and topmost Wuqiangxi Formation of the Banxi Group inSiduping section, Hunan Province, South China. The new age data

Figure 9. Correlation of the U-Pb relative probability plots of detrital zircons from the Banxi Group and its equivalents (the Liantuo Formation, Xiajiang Group and Danzhou Group).Data sources: Banxi Group (Wang et al., 2010b and this study); Liantuo Formation (Cui et al., 2014); Xiajiang Group (Wang et al., 2010a); Danzhou Group (Wang and Zhou, 2012).

G. Song et al. / Geoscience Frontiers 8 (2017) 1161e11731170

G. Song et al. / Geoscience Frontiers 8 (2017) 1161e1173 1171

show two major age peaks at ca. 820 Ma and ca. 780 Ma and oneminor age peak ca. 910 Ma, suggesting that the Wuqiangxi sand-stonewas mainly sourced fromNeoproterozoic rocks. The youngestzircon group from the topmost Wuqiangxi Formation gives aweighted mean age of 714.6 � 5.2 Ma, which was interpreted asclose to the termination of the Banxi Group. The new age, alongwith existing ages in literature, indicates that the Banxi Group wasdeposited between ca. 820 Ma and ca. 715 Ma. The new age of714.6 � 5.2 Ma from the top of the Banxi Group suggests a syn-chronous initiation of the Sturtian glaciation in South China. Recentages of ca. 780 Ma reported from diamictites of the basal Chang’anFormation may suffer from inheritance or is a detrital zircon age.Because of the accuracy differences related to the TIMS, SIMS,SHRIMP, and LA-ICPMS dating methods, a calibration of existingages of the Banxi Group and its time-equivalent units in the samelaboratory using the same dating method seems to be needed toconfirm a reliable stratigraphic framework.

Acknowledgment

This research is supported by the Ministry of Science andTechnology (No. 2011CB808806) and the National Natural ScienceFoundation of China (No. 41402026). We appreciate Ye Qing formakingmounts and Dr. Xiang Peng for LA-ICPMS analyses. Valuablecomments and suggestions from editor and two anonymous re-viewers help improve the quality of this paper.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.gsf.2016.11.012.

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