Journal of Asian Earth Sciences

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1 Journal of Asian Earth Sciences 48 (2012) 1 8 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: New U Pb age from the basal Niutitang Formation in South China: Implications for diachronous development and condensation of stratigraphic units across the Yangtze platform at the Ediacaran Cambrian transition Xinqiang Wang a,, Xiaoying Shi a,b, Ganqing Jiang c, Wenhao Zhang a a School of Earth Science and Resources, China University of Geosciences, Beijing , China b State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing , China c Department of Geoscience, University of Nevada, Las Vegas, NV , USA article info abstract Article history: Received 6 October 2011 Received in revised form 22 December 2011 Accepted 27 December 2011 Available online 13 January 2012 Keywords: Ediacaran Cambrian boundary U Pb age Niutitang Formation Metal-rich ore deposits South China The black shale dominated Niutitang Formation covers a large portion of the early Cambrian Yangtze platform in South China. The base of this unit has been traditionally taken as the Ediacaran Cambrian boundary (ca. 542 Ma). Recent radiometric ages from the basal Niutitang Formation and its correlative units, however, are significantly younger than 542 Ma and raise questions about the diachrony of stratigraphic units across the Yangtze platform at the Ediacaran Cambrian transition. Here we report a new U Pb sensitive high-resolution ion microprobe (SHRIMP) age of ± 4.9 Ma from a tuffaceous bed of the basal Niutitang Formation in a deep-water section in Taoying, Guizhou Province of South China. This age, in combination with existing ages and biostratigraphic data, suggests that the base of the Niutitang Formation is of Tommotian or upper Meishucunian age and the Ediacaran Cambrian boundary is most likely within the Liuchapo Formation. The depositional age of the unusual metal-enriched (Ni Mo PGE) sulfide deposits, which are approximately 1 m above the ca. 523 Ma tuffaceous bed, should be early Tommotian (upper Meishucunian) rather than early Nemakit-Daldynian (lower Meishucunian). Regional correlation confirms the diachronous development and condensation of stratigraphic units across the early Cambrian Yangtze platform and provides important information for understanding the paleogeography and paleoceanographic events at the Ediacaran Cambrian transition in South China. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The Ediacaran Cambrian (E C) transition witnessed the great biological innovation characterized by the replacement of Ediacaran soft-body animals by Cambrian-type shelly fauna (e.g., Narbonne, 2005, 2010) and dramatic ocean geochemical changes including large carbonate carbon isotope (d 13 C carb ) excursions (e.g., Maloof et al., 2010), sulfur isotope variations (e.g., Schröder et al., 2004; Goldberg et al., 2005, 2007), trace elemental enrichment and molybdenum isotope change (e.g., Guo et al., 2007; Lehmann et al., 2007; Wille et al., 2008; Zhou and Jiang, 2009; Wen et al., 2011). The casual link between these events, however, remains uncertain and requires further investigation, particularly the time and synchrony of biological and geochemical events. Owing to the relatively good preservation, the Ediacaran-early Cambrian succession in the Yangtze platform of South China plays important roles in exploring the relationship between biological evolution and paleoenvironmental changes. In the last decade, Corresponding author. Tel.: address: wxqiang@cugb.edu.cn (X. Wang). enormous paleontological (e.g., Qian, 1999; Shu et al., 1999, 2001, 2003a, 2003b, 2006; Zhang et al., 2002, 2003, 2004; Steiner et al., 2007; Dong et al., 2009; Liu et al., 2011) and geochemical (e.g., Shen and Schidlowski, 2000; Goldberg et al., 2007; Ishikawa et al., 2008; Wille et al., 2008; Wen et al., 2011) data have been obtained from the early Cambrian strata in this region, but stratigraphic correlation among shallow- to deep-water facies has been difficult due to facies-dependent fossil preservation and the lack of radiometric age data in general. Existing radiometric ages from the lower Cambrian strata were mostly obtained from shallow-water facies and show significant discrepancy. U Pb age dating from the basal Cambrian units in eastern Yunnan Province yielded ages of ± 1.5 Ma (Jenkins et al., 2002), ± 2.9 Ma (Compston et al., 2008), and ± 3.9 Ma (Zhu et al., 2009), confirming their proximity with the E C boundary. However, much younger ages of ± 0.7 Ma (Jiang et al., 2009), 518 ± 5 Ma (Zhou et al., 2008), and 521 ± 5 Ma (Xu et al., 2011) were obtained from the basal Niutitang Formation in Guizhou Province; the base of this unit has been traditionally considered as the E C boundary. These age differences raise questions about the diachrony of stratigraphic units across the early Cambrian Yangtze platform and compel for /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi: /j.jseaes

2 2 X. Wang et al. / Journal of Asian Earth Sciences 48 (2012) 1 8 additional age constraints, particularly in deep-water sections where biostratigraphic data are very limited. In this paper, we report a U Pb SHRIMP age of a tuffaceous bed from the basal Niutitang Formation in Taoying, Guizhou Province. Paleogeographically, the Taoying area was located in the slope environment of the early Cambrian Yangtze platform (Fig. 1A and B and Fig. 2). In combination with existing ages and biostratigraphic data, we discuss the possible E C boundary in deep-water sections, diachronous development and condensation of early Cambrian stratigraphic units, and the depositional age of the polymetallic (Ni Mo PGE) ore deposits across the Yangtze platform. 2. Geological setting and sample section It has been inferred that the E C Yangtze platform inherited from a late Neoproterozoic rifted basin that was initiated at the southeastern side of the Yangtze block at ca Ma (Wang and Li, 2003; Zhao et al., 2011). Several lines of evidence including the stratal thickness, spatial distribution of facies and the lack of major tectonic events and igneous activity indicate that the E C succession was deposited in a passive continental margin setting (Jiang et al., 2006a, 2011). During the latest Ediacaran and earliest Cambrian, shallow-water carbonate deposits of the Dengying and Yanjiahe formations (and their correlative units) dominated the northwest part of the Yangtze platform and changed basinward into deep-water mudstone/shale and cherts of the Liuchapo Formation (Fig. 1A; Wang, 1985; Zhu et al., 2003; Steiner et al., 2007). Overlying the Dengying and Yanjiahe formations, the black shale-dominated Niutitang Formation and its correlative units (including Shiyantou Formation in Yunnan, Guojiaba Formation in Sichuan, Shuijingtuo Formation in Hubei, and Xiaoyanxi Formation in Hunan; Fig. 1C) cover the majority of the early Cambrian Yangtze platform, representing a major sea-level transgression. The exact time and number of transgressive regressive cycles across the E C transition are still uncertain, largely due to the difficulties of establishing a precise stratigraphic framework across the Yangtze platform. The basal part of the Niutitang Formation and its correlative units contain an organic-rich polymetallic sufide layer with unusual enrichment of Ni, Mo, and platinum group element (PGE) that have been locally mined as metallic ores (Mao et al., 2002; Jiang et al., 2006b; Lehmann et al., 2007). The extreme enrichment of redox-sensitive elements (Ni, Mo, V, U, Cr, and PGE) and large Mo isotope change across the Ni Mo PGE sulfide layer have been interpreted as recording a major ocean anoxic event at the E C boundary (Lehmann et al., 2007; Wille et al., 2008). More recent U Pb ages of ± 0.7 Ma (Jiang et al., 2009) and ± 5.5 Ma (Chen et al., 2009) from tuffaceous beds below the Ni Mo PGE sulfide layer and Re Os age of 521 ± 5 Ma (Xu et al., 2011) from the metallic sulfide layer itself, however, indicated that the depositional age of Ni Mo PGE enriched sulfides should be much younger than the E C boundary. The study section (Taoying section) is located in northeastern Guizhou Province, South China, about 4 km southeast of the Fig. 1. (A) Paleogeographic reconstruction of the Yangtze platform during the latest Ediacaran to early Cambrian Nammkit-Daldynian stage. (B) Paleogeographic reconstruction of the Yangtze platform during the early Cambrian Tommotian stage. Modified from Wang (1985) and Steiner et al. (2007). Black circles with numbers in A and B indicate location of: 1 Meishucun section; 2 Shatan section; 3 Songlin section; 4 Jijiapo section; 5 Ganziping section. These sections are used for correlation in Fig. 5. Red circle (section 6) is the study section (Taoying). (C) Summary of late Ediacaran early Cambrian stratigraphic units across the Yangtze platform. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3 X. Wang et al. / Journal of Asian Earth Sciences 48 (2012) A B Fig. 2. (A) Generalized shelf-to-basin transect of the late Ediacaran early Cambrian Yangtze platform, showing the relative paleogeographic location of sections 1 6. The Taoying section was located in the slope environment. (B) Late Ediacaran early Cambrian stratigraphic section in Taoying, Guizhou, South China. The Ediacaran Cambrian boundary was traditionally put at the base of the Niutitang Formation in deep-water facies, but the real E C boundary may be within the Liuchapo Formation. A B C D Fig. 3. (A) Field photograph of the polymetallic sulfide layer in the lower Niutitang Formation in Taoying. Trace element analysis indicates that redox-sensitive elements Ni, Mo, V are extremely enriched in this interval (unpublished data); (B) Field photograph of the tuffaceous bed at the base of the Niutitang Formation. Hammer for scale is 30 cm long. (C) Representative zircon grains that yield concordant age show clear oscillatory zones under cathodoluminescence (CL). Circles show the position of the SHRIMP beam spots; (D) Representative zircon grains that have discordant ages. Taoying town. Paleogeographically, this area was within the slope environment of the Yangtze platform (Fig. 2A). Well-exposed outcrop consists of, in ascending stratigraphic order, the Doushantuo Formation, the Liuchapo Formation and the lower Niutitang Formation (Fig. 2B). The upper Doushantuo Formation is characterized by organic-rich black shale containing abundant macroscopic algae of the Wenghui biota (Zhao et al., 2004; Wang et al., 2008; Tang et al., 2008). This formation is conformably overlain by approximately 30-m-thick black cherts of the Liuchapo Formation that host Horodyskia and Palaeopascichnus fossils, which have been also found in the Jiumen section in Guizhou Province (Dong et al., 2008). Overlying the Liuchapo Formation is the pyrite-rich black shale of the Niutitang Formation. The lower part of the Niutitang Formation contains a laterally discontinuous phosphorite layer and an unusually metal-enriched layer (Fig. 3A) that is typical of the polymetallic sulfide layer seen in other sections across the Yangtze platform. Trace elemental analysis (unpublished data) indicates that redox-sensitive elements Ni, Mo, and V are highly enriched in this interval. A 10-cm-thick, greenish grey tuffaceous bed (Fig. 3B) is present just below the polymetallic layer. So far, no biostratigraphic data is available for the Niutitang Formation in this section.

4 4 X. Wang et al. / Journal of Asian Earth Sciences 48 (2012) 1 8 Table 1 SHRIMP U Pb zircon data for the taffaceous bed in the lowermost Niutitang Formation in Taoying section. Spot 206 Pb c (%) U (ppm) Th (ppm) 232 Th/ 238 U 206 Pb (ppm) 207 Pb / 206 Pb ±1r (%) 207 Pb / 235 U ±1r% 206 Pb / 238 U ±1r% 206 Pb/ 238 U Age ± 1r (Ma) AP # AP # AP # AP # AP AP AP AP AP AP AP # AP AP AP AP # AP Notes: 1. Errors are 1r; Pb c and Pb indicate the common and radiogenic portions, respectively. 2. Error in standard calibration was 0.40% (not included in above errors but required when comparing data from different mounts). 2. Error in standard calibration was 0.40% (not included in above errors but required when comparing data from different mounts). 3. Common Pb corrected by assuming 206 Pb/ 238 U 208 Pb/ 232 Th age-concordance. 4. Those data with symble # are rejected from mean-age calculation and are not show in concordia plot (Fig. 4). A B Fig. 4. (A) U Pb concordia diagram for the tuffaceous bed in the Taoying section; Analyses with 1r error ellipses are plotted as radiogenic ratios after 208 Pb correction. Weighted-mean 206 Pb 238 U dates at a 95% confidence level for the main group of zircon grains show the best estimated age for the dated sample. MSWD mean square of weighted deviates; (B) 206 Pb/ 238 U ages of 10 zircon analyses. Error bars represent 2r errors. 3. Analytical procedure and result Three packages of sample (AP-29.0) were collected from the tuffaceous bed in the basal Niutitang Formation. Zircons were hand picked from heavy mineral concentrates under binocular microscope. The representative grains, together with the several standard zircons TEM (417 Ma) (Black et al., 2003), were mounted in an epoxy and polished to approximately half of their thickness. Prior to dating, zircon grains were photographed under transmitted and reflected lights. Cathodoluminescence (CL) images were used to observe the internal structure of grains in order to choose the well-preserved spot for dating. The U, Th and Pb isotope compositions were analyzed using SHRIMP II at the Ion Microprobe Analysis Center of the Academy of Geological Sciences in Beijing, China, following the procedure described in Williams (1998). The data were processed using the SQUID and ISOPLOT programs of Ludwig (2001, 2003). The uncertainties in isotope ratios are 1r, and the weighted mean ages are quoted at 95% confidence level after 208 Pb correction. Zircon grains extracted from sample AP-29.0 are euhedral to subeuhedral (Fig. 3C and D), with sizes mostly in the range of lm. Some of them have clear oscillatory zones (Fig. 3C) indicative of contemporaneous volcanic origin. Sixteen zircons were chosen for dating and their isotope data are shown in Table 1. Among the analyzed grains, 10 zircons gave a narrow age range from Ma, yielding a weighted mean 206 Pb/ 238 U age of ± 4.9 Ma, with mean square of weighted deviates (MSWD) of 1.6 (Fig. 4A and B). Three zircons (AP-2.1, AP-3.1 and AP-5.1; Fig. 3D) gave discordant ages of 576 Ma, 546 Ma and 548 Ma, respectively. These ages are older than the main group and the E C boundary age of ± 0.3 Ma (Amthor et al., 2003). Thus we interpret that they may have an inherited origin. Zircon AP-4.1 has significantly younger age of 448 Ma, which is most likely affected by the fracture on zircon surface. The remaining two grains

5 X. Wang et al. / Journal of Asian Earth Sciences 48 (2012) (AP-12.1, AP-16.1) are relatively small and are very dark under CL, with high U (2485 ppm, 2250 ppm) and Th (1137 ppm, 1220 ppm) contents (Table 1). They yielded discordant ages of 307 Ma and 483 Ma possibly due to lead loss. These six discordant ages are rejected from mean-age calculation and are not shown in concordia plot (Fig. 4A). Hence we consider the age ± 4.9 Ma as the best estimate for depositional age of the tuffaceous bed. 4. Discussion 4.1. Age constrains for the polymetallic layer The Niutitang black shales (and its correlative units; Fig. 1C) are widespread across the early Cambrian Yangtze platform, extending over some 1000 km from southeast in the basin to northwest in the platform interior. The basal Niutitang Formation consists of an organic-rich sulfide layer with extreme enrichment of redox-sensitive elements such as Ni, Mo, V, U, and PGE (e.g., Mao et al., 2002; Jiang et al., 2006b; Lehmann et al., 2007) that has been used as a marker bed close to the E C boundary (e.g., Steiner et al., 2001; Yang et al., 2004; Jiang et al., 2007a). This polymetallic (Ni Mo PGE) layer has been interpreted as recording an unusual oceanographic event, i.e., large-scale hydrogen sulfide (H 2 S) release from the deep ocean (Wille et al., 2008), or alternatively as sea-floor hydrothermal venting event (Steiner et al., 2001; Jiang et al., 2006b, 2007a, 2009) at the E C transition. Earlier Re Os isochron ages of 542 ± 11 Ma (Li et al., 2002), 541 ± 16 Ma (Mao et al., 2002) and 537 ± 10 Ma (Jiang et al., 2003) have been repeatedly reported from the polymetallic (Ni Mo PGE) layer and seemly confirmed its depositional age close to the E C boundary. A more recent SHRIMP U Pb age of ± 0.7 Ma (Jiang et al., 2009) from a volcanic ash bed a few meters below the polymetallic (Ni Mo PGE) layer in Songlin, northeastern Guizhou Province, however, indicated that the polymetallic layer was deposited at least 10 Ma after the E C boundary. Interestingly, a much younger U Pb age of 518 ± 5 Ma has been also reported from a volcanic ash bed below the polymetallic layer in an adjacent section in the same area (Zhou et al., 2008). The new SHRIMP zircon U Pb age of ± 4.9 Ma from the deep-water Taoying section is from a tuffaceous bed that is approximately 1 m below the polymetallic (Ni Mo PGE) sulfide layer. It provides a direct age constraint for the polymetallic sulfide layer, which is ca. 523 Ma. This age is consistent with the newly reported Re Os age of 521 ± 5 Ma from the polymetallic (Ni Mo PGE) layer in northeastern Guizhou (Xu et al., 2011). These new ages indicate that the paleoceanographic (e.g., Wille et al., 2008) or hydrothermal (e.g., Steiner et al., 2001; Jiang et al., 2006b, 2007a) event recorded by the polymetallic layer happened during the early Tommotian (or upper Meishucunian) rather than at the E C transition (e.g., lower Nemakit-Daldynian or lower Meishucunian) The duration of the Liuchapo Formation The Liuchapo Formation in South China rests conformably on the Ediacaran Doushantuo Formation and is overlain by black shales of the lower Cambrian Niutitang Formation (or its correlative units). It consists mainly of black cherts and siliceous shales. In some sections, the upper part of the Liuchapo Formation contains Horodyskia and Palaeopascichnus fossils (Dong et al., 2008). Traditionally the Liuchapo Formation is considered as the deep-water equivalent of the Ediacaran Dengying Formation that has an age from ca. 551 Ma to 542 Ma. Recent research implies that the Liuchapo Formation may be diachronous and the upper part of this unit may be early Cambrian (e.g., Chang et al., 2009; Chen et al., 2009). The exact duration, however, is unknown. Chen et al. (2009) reported a SHRIMP zircon U Pb age of ± 5.5 Ma from a tuff bed immediately overlying the Liuchapo black chert in the Ganziping section of western Hunan Province. This age constrains the duration of the Liuchapo Formation as approximately 6 Ma from the E C boundary. However, in Ganziping section the Liuchapo Formation is only 6 m thick and is sandwiched between two carbonate units assigned to the Dengying Formation (Chen et al., 2009). The age of ± 4.9 Ma present here is obtained from the basal Niutitang Formation, which is 20 cm above the top of the Liuchapo Formation. No exposure or erosional surface is observed at the Liuchapo/Niutitang boundary in the Taoying section. Thus it is very likely that the majority of the Liuchapo Formation was deposited during the early Cambrian rather than the late Ediacaran and the E C boundary should be located within the Liuchapo Formation. If the black shale at the top of the Doushantuo Formation in the Taoying section is time equivalent to that of the Doushantuo Formation in the Yangtze Gorges area, which has an age of ca. 551 Ma (Condon et al., 2005; Zhang et al., 2005), the duration of the Liuchapo Formation in deep-water sections may be as long as 30 million years, much longer than the carbonate-dominated Ediacaran Dengying Formation (ca Ma) The Ediacaran Cambrian boundary and stratigraphic correlation The E C boundary (542.0 ± 0.3 Ma, Amthor et al., 2003) in South China has been highly debated due to the absence of precise radiometric ages. It is traditionally defined by the first occurrence of small shelly fossils (SSF) (e.g., Chen, 1984; Qian, 1999) and Micrhystridium-like acritarchs (Ding et al., 1992; Yin, 1997; Dong et al., 2009) which is located m above the Dengying-Yanjiahe boundary in the Yangtze Gorges area (Fig. 5). However, owing to the lithologically dependent fossil preservation, this idea has been challenged by carbon isotope chemostratigraphy that shows a prominent negative d 13 C carb excursion below the lowest occurrence of SSFs (Zhou et al., 1997; Shen and Schidlowski, 2000; Zhu et al., 2003; Jiang et al., 2007b; Jiang et al., 2012; Ishikawa et al., 2008; Li et al., 2009). In the basinal facies, the first appearance of sponge spicules was usually taken as the base of the Cambrian. Without additional constraints, the stratigraphic correlation between shallow-water platform and deep-water basin has been difficult and in cases, arbitrary. If the deposition of Liuchapo Formation began at ca. 551 Ma (Condon et al., 2005) and ended at ca. 523 Ma, as discussed above, the E C boundary in the basinal facies must be located within the Liuchapo Formation. The exact position of the E C boundary requires additional ages and/or paleontological data, but considering the simple lithology and the lack of obvious stratigraphic discontinuity, it is more likely located at the lower part of the Liuchapo Formation (Fig. 5). In combination with available absolute ages and biostratigraphic data, regional stratigraphic correlation across the early Cambrian Yangtze platform indicates strong stratigrapgic diachrony and condensation (Fig. 5). The late Ediacaran Dengying Formation is only time-equivalent with the lower Liuchapo Formation in the basin; whereas the middle and upper Liuchapo Formation is probably deposited during the Nemakit-Daldynian or lower-middle Meishucunian. In this case, the majority of the Liuchapo Formation should be correlated with the Zhujiaqing Formaiton in Meishucun section, eastern Yunnan Province, the Kuanchuanpu Formation in Shatan section, Sichuan Province, and the Yanjiahe Formation in the Yangtze Gorges area. In the Zunyi area represented by the Songlin section in northeastern Guizhou Province, the majority of the Nemakit-Daldynian strata is missing, either due to no deposition at subaerial exposure or due to stratigraphic truncation associated with the unconformity at the Dengying-Niutitang boundary. The overlying Niutitang Formation is of

6 6 X. Wang et al. / Journal of Asian Earth Sciences 48 (2012) 1 8 Fig. 5. Late Ediacaran to early Cambrian stratigraphic correlation across the Yangtze platform, based on integrated geochronology and biostratigraphy in South China. Fossil records are mainly from Steiner et al.(2007) and Dong et al.(2009); U Pb ages in Meishucun section, Songlin section and Ganziping section are from Compston et al.(2008), Jiang et al.(2009) and Chen et al.(2009), respectively. The E C boundary in deep-water facies should be within the Liuchapo Formation, but its exact position requires additional age and/or paleontological data. Tommotian or upper Meishucunian age and is much more widespread across the Yangtze platform. Time equivalent units may include the Shiyantou Formation in Yunnan, the Guojiaba Formation in Sichuan, the Shuijingtuo Formation in Hubei and the Xiaoyanxi Formation in Hunan (Fig. 5) Uncertainties and future research The existing age data from the Niutitang Formation and its equivalent units also raise questions on the precision of age dating and the duration of the Liuchapo Formation. In the Zunyi section, zircon grains from almost the same ash bed produced different ages of ± 0.7 Ma (Jiang et al., 2009) and 518 ± 5 Ma (Zhou et al., 2008). Considering the better precision of the older age, it is more likely that the younger age (518 ± 5 Ma) was due to the lead loss that commonly resulted in younger ages (e.g., Kooijman et al., 2011). A few zircon grains from the ash bed in this study produced younger ages and there is a possibility that the age of ± 4.9 Ma from the Taoying section has also been altered to some degree from lead loss. There is a possibility that most early Cambrian ages from lead loss. This raises the possibility so far (Fig. 5), including ± 0.7 Ma (Jiang et al., 2009) and 518 ± 5 Ma (Zhou et al., 2008) ages from Zunyi, northern Guizhou, ± 4.9 Ma from Taoying, northeastern Guizhou (this study), and ± 5.5 Ma from Ganziping, northwestern Hunan (Chen et al., 2009), were from the same, contemporaneous ash bed widely distributed across the early Cambrian Yangtze platform. The age differences among sections and authors are due to the bean location during SHRIMP U Pb analyses and the degree of lead loss in analyzed zircon grains. The ages of ± 1.5 Ma (Jenkins et al., 2002), ± 2.9 Ma (Compston et al., 2008), and ± 3.9 Ma (Zhu et al., 2009) from the Meishucun section in eastern Yunnan could be also from the same ash bed as those of Guizhou and Hunan provinces. However, in the Meishucun section, bed 5 and bed 9 bentonites have different age groups from ca. 540 Ma to ca. 526 Ma (Compston et al., 2008). It is uncertain which ash bed is more likely correlatable with the ones in Guizhou and Hunan provinces. Elucidating uncertainties on the potential synchrony of ash beds and age differences across the early Cambrian Yangtze platform requires more precise TIMS age dating of the zircon grains collected from these sections in the same lab following the same procedure. 5. Conclusion We report a new SHRIMP zircon U Pb age of ± 4.9 Ma from a tuffaceous bed in the basal Niutitang Formation in Taoying, Guizhou Province, South China. This age is significantly younger than the E C boundary (542 Ma) and suggests that: (1) the polymetallic (Ni Mo PGE) layer within the Niutitang Formation has a depositional age of ca. 523 Ma; (2) the paleoceanographic or hydrothermal event recorded by the polymetallic sulfides happened during the early Tommotian stage rather than the Nemakit-Daldynian stage; (3) the Liuchapo Formation in deep-water sections may last as long as 30 million years; (4) the E C boundary may be located in the lower pat of the Liuchapo Formation and the majority of the Liuchapo Formation may have been deposited during the early Cambrian rather than late Ediacaran, and (5) the stratigraphic units across the Yangtze platform at the E C transi-

7 X. Wang et al. / Journal of Asian Earth Sciences 48 (2012) tion are strongly diachronous and highly condensed. However, additional study is required in the future to test whether the existing ages from the basal Niutitang Formation and its time equivalent units in the Yangtze platform were from the same, contemporaneous ash bed. Acknowledgements We are grateful to Mingshuai Zhu and Zhiqing Yang for data analysis and processing. Professor Linzhi Gao is thanked for his valuable comments on the earlier version of the manuscript. We appreciate Dr. Michel Faure and an anonymous reviewer for their constructive comments that helped to improve the manuscript. This research was supported by the Ministry of Science and Technology of China (2011CB808800), the National Natural Science Foundation of China ( ), and Fundamental Research Funds for the Central Universities (2011YYL018). 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