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SIMS U–Pb zircon geochronological constraints on upper Ediacaran stratigraphic correlations, South China

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Fossiliferous Ediacaran successions of South China, the Doushantuo and Dengying formations and their equivalents, are key to understanding bio- and geological evolution at the Neoproterozoic–Cambrian transition. However, their absolute ages, especially the upper Ediacaran successions, are poorly constrained. SIMS zircon U–Pb dating results in this study suggest that ash beds at the basal and middle parts of the Jiucheng Member (middle Dengying Formation) in eastern Yunnan Province were deposited at 553.6 ± 2.7/(3.8) Ma and 546.3 ± 2.7/(3.8) Ma, respectively. These new dates indicate that the age for the base of Dengying Formation in eastern Yunnan Province is similar to the 550.55 ± 0.75 Ma date, which is from an ash bed at the top of the Miaohe Member and has been regarded as the age for the base of Dengying Formation in Yangtze Gorges area. These dates do not permit a clear test of the two correlation models for the chronostratigraphic position of the Miaohe Member (uppermost Doushantuo Formation vs. middle Dengying Formation), implying that further integrated intra-basinal stratigraphic correlations and more high-resolution chronological data from the upper Ediacaran deposits of South China are required. New dates of the Jiucheng Member constrain the age of the fossil biotas in the middle Dengying Formation and extend the stratigraphic range of Rangea , Hiemalora and Charniodiscus to 546.3 ± 2.7/(3.8) Ma. The geochronology of the Dengying Formation implies that Ediacaran-type fossils preserved in this formation are younger than the White Sea Assemblage and temporally overlapping with the Nama Assemblage.
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Geol. Mag. 154 (6), 2017, pp. 1202–1216. c
Cambridge University Press 2016 1202
doi:10.1017/S0016756816001102
SIMS U–Pb zircon geochronological constraints on upper
Ediacaran stratigraphic correlations, South China
CHUAN YANG†‡, XIAN-HUA LI§, MAOYAN ZHU¶ & DANIEL J. CONDON
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,
Chinese Academy of Sciences, Beijing 100029, China
College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth NG12 5GG, UK
¶State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese
Academy of Sciences, Nanjing 210008, China
(Received 13 August 2016; accepted 1 November 2016; first published online 13 December 2016)
Abstract Fossiliferous Ediacaran successions of South China, the Doushantuo and Dengying
formations and their equivalents, are key to understanding bio- and geological evolution at the
Neoproterozoic–Cambrian transition. However, their absolute ages, especially the upper Ediacaran
successions, are poorly constrained. SIMS zircon U–Pb dating results in this study suggest that ash
beds at the basal and middle parts of the Jiucheng Member (middle Dengying Formation) in east-
ern Yunnan Province were deposited at 553.6 ±2.7/(3.8) Ma and 546.3 ±2.7/(3.8) Ma, respectively.
These new dates indicate that the age for the base of Dengying Formation in eastern Yunnan Province
is similar to the 550.55 ±0.75 Ma date, which is from an ash bed at the top of the Miaohe Member and
has been regarded as the age for the base of Dengying Formation in Yangtze Gorges area. These dates
do not permit a clear test of the two correlation models for the chronostratigraphic position of the
Miaohe Member (uppermost Doushantuo Formation vs. middle Dengying Formation), implying that
further integrated intra-basinal stratigraphic correlations and more high-resolution chronological data
from the upper Ediacaran deposits of South China are required. New dates of the Jiucheng Member
constrain the age of the fossil biotas in the middle Dengying Formation and extend the stratigraphic
range of Rangea,Hiemalora and Charniodiscus to 546.3 ±2.7/(3.8) Ma. The geochronology of the
Dengying Formation implies that Ediacaran-type fossils preserved in this formation are younger than
the White Sea Assemblage and temporally overlapping with the Nama Assemblage.
Keywords: Zircon U–Pb age, Ediacaran, Dengying Formation, Ediacaran fossil, South China.
1. Introduction
The Ediacaran Period spans a critical time interval
during the Earth history, beginning with the termina-
tion of global Cryogenian glaciation and ending at the
earliest occurrence of marine bilaterian animals on a
global scale at the beginning of the Phanerozoic Eon
(Knoll et al.2004). The successions during this inter-
val record the dramatic fluctuations in the composi-
tions of the ocean and atmosphere, such as the large
excursions of the carbon isotope in seawater (e.g. Ji-
ang, Kennedy & Christie-Blick, 2003; Le Guerroué,
Allen, & Cozzi, 2006; Zhu, Zhang & Yang, 2007)
and increasing oxygen levels (e.g. Och & Shields-
Zhou, 2012; Lyons, Reinhard & Planavsky, 2014; Chen
et al.2015). Broadly coincident is the rapid radi-
ation of the complex, macroscopic multicellular organ-
isms on the eve of the Cambrian explosion, includ-
ing Ediacara-type soft-bodied fossils (e.g. Narbonne,
2005; Droser & Gehling, 2015), megascopic multicel-
lular algal fossils (e.g. Xiao et al.2002), trace fossils
(e.g. Jensen et al.2000; Chen et al.2013) and weakly
§Author for correspondence: lixh@gig.ac.cn
calcified metazoans (e.g. Hofmann & Mountjoy, 2001;
Cai et al. 2015). Stratigraphic successions spanning
the Ediacaran–Cambrian transition are well developed
and exposed in South China (Zhu, Zhang & Yang,
2007). Previous studies on the litho-, chemo- and
biostratigraphy of these successions have advanced
significantly in understanding the evolution of life and
environment during the Ediacaran Period (e.g. Jiang
et al.2011; Zhu et al.2013; Liu et al.2014). However,
the Ediacaran stratigraphy of South China is complic-
ated because of facies variation, hampering develop-
ment of a global integrated stratigraphic model for life
evolution and environmental changes at this critical in-
terval.
Absolute dating (i.e. radio-isotopic geochronology)
is the fundamental method for the development and
testing of models of intra-basinal and global strati-
graphic inter-comparison, and is the only way to
quantify the rates of the geological and biological pro-
cesses. The zircon 207Pb–206 Pb age 550.55 ±0.75 Ma
from the top of the Miaohe Member in the west
Huangling Anticline (Condon et al.2005), which has
been traditionally correlated to the Doushantuo Mem-
ber IV in the Yangtze Gorges area, constrains the age
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Zircon U–Pb dating of the Dengying Formation 1203
of the Miaohe biota (Xiao et al.2002) and is regarded
as the age for the top of the Doushantuo Formation,
that is, the top of the DOUNCE (Doushantuo negat-
ive carbon isotope excursion; Zhu, Strauss & Shields,
2007)/Shuram δ13C negative excursion and the base of
the Dengying Formation (e.g. Ding et al.1996; Wang
et al.1998; Condon et al.2005; Zhu, Zhang & Yang,
2007;Luet al.2013). This stratigraphic correlation
was recently challenged by An et al.(2015), who sug-
gested that Miaohe Member in the west Huangling
Anticline is significantly younger than the Doush-
antuo Member IV in the Yangtze Gorges area, most
likely time-equivalent with the lower Shibantan Mem-
ber of the Dengying Formation which yields abund-
ant Ediacara-type fossils (Chen et al.2014). Improve-
ment of the chronostratigraphic framework is therefore
required for understanding of the DOUNCE/Shuram
δ13C negative excursion and its causal relation with
the Miaohe biota and other Ediacaran fossils re-
corded in upper Ediacaran successions. One of the
key issues to test those two competing models is to
obtain absolute ages from the Dengying Formation.
However, no radiometric ages within the Dengying
Formation or its equivalents have yet been reported.
Meanwhile, the lack of the robust age constraints in the
Dengying Formation hinders the biostratigraphic cor-
relations between the Ediacara-type fossils discovered
in South China and the three Ediacara-type fossil as-
semblages in other palaeocontinents: the Avalon As-
semblage (c. 575–560 Ma); the White Sea Assemblage
(c. 560–550 Ma); and the Nama Assemblage (c. 550–
541 Ma) (Grotzinger et al.1995; Martin et al.2000;
Waggoner, 2003; Bowring et al.2007; Xiao & La-
flamme, 2009; Noble et al.2015).
To achieve a better understanding of the geochrono-
logical framework of the Dengying Formation and
the biostratigraphic correlations of the Ediacara-type
fossils in South China, we scrutinize the SIMS zir-
con 238U–206 Pb and 207Pb–206 Pb data in order to de-
rive robust age constraints. We present two new SIMS
zircon U–Pb datasets with a weighting towards the
207Pb–206 Pb dates due to issues of open-system beha-
viour (i.e. radiogenic Pb-loss) of the ash beds in the
middle part of the Dengying Formation in eastern Yun-
nan Province, South China.
2. Geological background and sampling
As a part of the Rodinia supercontinent, the South
China Block was formed by amalgamation of Yangtze
and Cathaysia blocks along the early Neoproterozoic
Sibao Orogen (Fig. 1), although the timing of the am-
algamation is still controversial (e.g. Li et al.2002,
2009a; Zhou et al.2002). Mantle plume was formed
beneath the Rodinia supercontinent about 50 million
years after the completion of its assembly, leading
to the development of the continental rifting and the
break-up of the supercontinent (Li et al.2008b). Rift-
related sedimentary successions and bimodal mag-
matic rocks dated at 0.85–0.72 Ga are widespread in
South China, especially around the periphery of the
Yangtze Block (e.g. Wang & Li, 2003;Liet al.2008a;
Yang et al.2015). Overlying the rift-related sequences
are the glacial and interglacial deposits, recording the
two Neoproterozoic worldwide glaciations in South
China (Shields-Zhou, Porter & Halverson, 2016). The
early Ediacaran Doushantuo Formation is overlying
the Nantuo (Marinoan-equivalent) glacial deposit, with
the characteristic ‘cap carbonates’ at the base (Jiang,
Kennedy & Christie-Blick, 2003; Zhu, Zhang & Yang,
2007). The thin ash layer at the top of the cap carbon-
ate was dated at 635.2 ±0.6 Ma, constraining the ter-
minal timing of the Nantuo glaciation in South China,
which reflects the global Marinoan glaciation (Con-
don et al.2005). The Doushantuo Formation is com-
posed mainly of carbonate rocks and black shale. Large
acanthomorphic acritarchs, multicellular algae, animal
embryos and few animal fossils are present in the
Doushantuo Formation (e.g. Xiao, Zhang & Knoll,
1998; Chen et al.2004; Liu et al.2013;Yinet al.
2015). As the uppermost part of the Ediacaran strata
in South China, the Dengying Formation (and its equi-
valents) consists mainly of carbonate rocks in the shal-
low water basin and cherts in the slope and deepwater
basin (Zhu et al.2003,2007). The Dengying Form-
ation is known to contain abundant fossils, such as
Ediacara-type soft-bodied fossils, trace fossils, mac-
roalgal fossils and some tubular fossils (e.g. Hua, Chen
& Yuan, 2007; Chen et al.2013,2014;Caiet al.2015).
These terminal Ediacaran fossils are central to our un-
derstanding of the animal evolution on the eve of the
Cambrian explosion.
The Ediacaran–Cambrian transitional strata with
shallow-marine facies are extensively developed and
well exposed in eastern and northeastern Yunnan
Province, South China (Zhu et al.2001). We studied
and sampled the middle part of the Dengying Form-
ation in the Xiaolantian and Yinchangpo sections in
the eastern Yunnan Province (Figs 1,2). The Xiaolan-
tian section is located to the NE of the Fuxian Lake,
about 6 km to the east of the Chengjiang county town.
This section is near the well-described Feidatian–
Dongdahe section (Zhu, Zhang & Yang, 2007) and
the lithostratigraphy of the Dengying Formation at the
two sections is similar, so the horizon of the dated
ash bed from Xiaolantian section is marked on the
column of Feidatian–Dongdahe section (Fig. 2). The
Xiaolantian section consists of four formations span-
ning upper Ediacaran – lower Cambrian stratigraphy
(from bottom to top): Dengying, Zhujiaqing, Shiyan-
tou and Yu’anshan formations. The Dengying Form-
ation is divided into three members including (from
bottom to top) the Donglongtan, Jiucheng and Baiy-
anshao members. The Donglongtan and Baiyanshao
members are dominated by dolostone. The Jiucheng
Member mainly consists of sandstone and muddy
siltstone interbedded with laminated silty dolomite
at the basal part. Overlying the Dengying Forma-
tion is the Zhujiaqing Formation which is composed
of dolostone, phosphorite and interbedded phosphatic
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1204 C. YANG AND OTHERS
Figure 1. (Colour online) Simplified palaeogeographic map of the Yangtze Block during the Precambrian–Cambrian transition interval
(modified after Zhu et al.2003). The dots represent the cities or the areas, and the triangles represent the sections.
limestone. Numerous small shelly fossils (SSFs) were
discovered from this formation (Qian et al.1996). Un-
conformably overlying the Zhujiaqing Formation is the
Shiyantou and Yu’anshan formations which are dom-
inated by siltstone and shales. The famous Chengjiang
biota occurs in the middle part of the Yu’anshan Form-
ation (Zhang & Hou, 1985). The Yinchangpo section
is located about 10 km to the NW of the Huize county
town. This section consists of two formations includ-
ing Dengying and Zhujiaqing formations (Fig. 2). The
Donglongtan Member of the Dengying Formation is
c. 460 m thick and is composed of thickly bedded to
massive dolostone. The Jiucheng Member is c. 20 m
thick and consists of muddy siltstone and silty dolo-
stone. The Baiyanshao Member consists of c. 260 m of
medium- to thickly bedded, laminated dolostone. Only
the lower part of the Zhujiaqing Formation crops out
in this section, composed of interbedded dolostone and
chert (Fig. 2).
Two ash samples from the middle part of the Dengy-
ing Formation were collected for SIMS zircon U–
Pb dating (Figs 2,3). Sample 14CJ07 was collec-
ted c. 3.8 m above the phosphorite layer at the base
of the Jiucheng Member, Dengying Formation in the
Xiaolantian section, and sample 14YCP02 was col-
lected from the middle part of the Jiucheng Member,
471 m above the base of the Dengying Formation in
the Yinchangpo section.
3. SIMS zircon U–Pb dating method
Zircon crystals were separated from c. 3 kg of each
sample using conventional density and magnetic sep-
aration techniques. Together with zircon standards
Plešovice, 91500, Penglai and Qinghu, zircon grains
were mounted in an epoxy resin which was then pol-
ished to section the crystals in half for analysis. All
zircon grains were documented with transmitted and
reflected light photomicrographs and cathodolumines-
cence (CL) images to reveal their external and internal
structures, and the mount was vacuum-coated with
high-purity gold prior to secondary ion mass spectro-
metry (SIMS) analysis.
Measurements of U, Th and Pb isotopes were con-
ducted using a Cameca 1280-HR SIMS at the Insti-
tute of Geology and Geophysics, Chinese Academy of
Sciences in Beijing. The O2primary ion beam was
accelerated at –13 kV with an intensity of 5–12 nA.
The ellipsoidal spot is c. 20 μm×30 μm in size. Pos-
itive secondary ions were extracted with a 10 kV po-
tential. Oxygen flooding was used to increase the O2
pressure to c. 5×106Torr in the sample chamber,
enhancing the secondary Pb+sensitivity to a value
of 35 cps (nA ppm)–1 for zircon. In the secondary ion
beam optics, a 60 eV energy window was used to re-
duce the energy dispersion. To achieve a higher pre-
cision of SIMS U–Pb zircon geochronology, a dy-
namic multi-collector U–Pb dating method was used
to take advantages of both mono-collector mode (high-
precision determination of the 238U–206 Pb age) and
multi-collector mode (high-precision determination of
the 207Pb–206 Pb age). This new analytical protocol is
able to achieve a higher precision by a factor of two
for the 207Pb–206 Pb age than the conventional mono-
collector mode within the same working time, allow-
ing the 207Pb–206 Pb and 238U–206 Pb ages to be obtained
simultaneously with comparable quality to effectively
evaluate the concordance of the U–Pb system for
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Zircon U–Pb dating of the Dengying Formation 1205
Figure 2. (Colour online) Stratigraphic column of the Yinchangpo section in the eastern Yunnan Province and its lithostratigrahic correlation to the Ediacaran–Cambrian transition successions in
eastern Yunnan Province (Zhu, Zhang & Yang, 2007;Liet al.2013a), southern Shaanxi Province (Zhu, Zhang & Yang, 2007) and Yangtze Gorges area (Condon et al.2005;Jianget al.2007;Luet al.
2013;Zhuet al.2013;Anet al.2015). The biostratigraphy is based on Tang et al.(2006), Hua, Chen & Yuan (2007), Zhu (2010), Chen et al.(2013,2014) and Zhang, Hua & Zhang (2015). The dotted
lines represent the lithostratigraphic boundaries. KY – Kunyang Group; NT – Nantuo Formation; ZJQ – Zhujiaqing Formation; KCP – Kuanchuanpu Formation; YJH – Yanjiahe Formation; SJT –
Shuijingtuo Formation; JC – Jiucheng Member; DB – Daibu Member; GJS – Gaojiashan Member; Fm – formation; Mb – member.
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1206 C. YANG AND OTHERS
Figure 3. (Colour online) Field photographs showing the sampling site of the ash beds (a) 14CJ07 and (b) 14YCP02. Photomicrographs
of samples (c) sample 14CJ07 and (d)14YCP02, showing they are mainly composed of mud, epidote and quartz.
Phanerozoic and late Precambrian samples (Liu et al.
2015). Each measurement consists of seven cycles, and
the total analytical time is c. 14 min. More details of
this method are provided by Liu et al.(2015).
Analyses of the standard zircons were interspersed
with those of unknown grains. Two successive sessions
(sessions A and B) were conducted within a short time
period and the errors of the U–Pb calibration curve fit-
ted by the standard zircons, 0.75 % (1 SD, session A)
and 0.55 % (1 SD, session B), were propagated to the
unknowns of the respective session (Fig. 4). The U–
Th–Pb ratios were determined relative to the Plešovice
standard zircon (Sláma et al.2008), and the abso-
lute abundances were calibrated to the standard zir-
con 91500 (Wiedenbeck et al.1995). Measured Pb iso-
topic compositions were corrected for common Pb us-
ing the 204Pb method. Corrections are sufficiently small
to be insensitive to the choice of common Pb composi-
tion, and an average of present-day crustal composition
(Stacey & Kramers, 1975) is used for the common Pb
assuming that the common Pb is largely surface con-
tamination introduced during sample preparation. To
be consistent with the published dates, the 238U/235U
ratio of 137.88 (Steiger & Jaeger, 1977) and 238U and
235U decay constants of Jaffey et al.(1971) are adop-
ted in this study. Data reduction was carried out us-
ing the Isoplot/Exv. 4.15 program (Ludwig, 2003). De-
tails of the calibration method are provided by Li et al.
(2009b). In order to monitor the external uncertain-
ties of SIMS U–Pb measurements, analyses of zircon
standard Qinghu were interspersed with unknowns.
Twelve analyses yielded a weighted mean 238U–206Pb
age of 159.8 ±0.6 Ma (2 SE, MSWD =1.2), identical
within errors to the reported age of 159.5 ±0.2 Ma (Li
et al.2013b).
SIMS zircon U–Pb data are given in Table 1, and
uncertainties on individual analysis are reported at
a1σlevel. Noteworthy is that the precision of the
weighted mean ages is the standard error (SE), which
will decrease with an increasing quantity of zircon data
included in the weighted mean age calculation. The
external uncertainty of 1 % (2 SD) for SIMS zircon
238U–206 Pb age (e.g. Ireland & Williams, 2003; Yang
et al.2014) should be taken into consideration when
comparing the SIMS zircon 238U–206 Pb age with other
published dates (e.g. the ID-TIMS date from Condon
et al.2005). The multi-collector SIMS determination
of the 207Pb–206 Pb age for the latest Neoproterozoic
zircon has a comparable precision as the 238U–206 Pb
age (Li et al.2009b), and the zircon 207Pb–206 Pb age
by SIMS measurement is independent of the matrix-
matched U–Pb calibration. We assume that the external
reproducibility for the multi-collector SIMS determin-
ation of latest Neoproterozoic zircon 207Pb–206 Pb age
is c. 0.5 % (2 SD) (Li et al.2009b,2010; Liu et al.
2015). The reported errors of the weighted mean ages
in this study include two components, for example,
Age ±X/(Y), where Xrepresents the analytical error
and (Y) represents the analytical +external reprodu-
cibility. The calculation of (Y) follows the uncertainty
propagation workflow of Horstwood et al.(2016). The
MSWD (mean square of the weighted deviates) of the
weighted mean age is calculated prior to the addition
of the external uncertainties.
4. Results
4.a. Sample 14CJ07 at the base of the Jiucheng Member
Most zircons from this sample are 100–200 μmin
length and have aspect ratios of 3–5. They are eu-
hedral and long-prismatic in morphology, with weakly
oscillatory zoning under CL images (Fig. 5). A total
of 50 analyses were conducted on 50 zircons. U and
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Zircon U–Pb dating of the Dengying Formation 1207
Table 1. SIMS zircon U–Pb data of ash samples from Dengying Formation in eastern Yunnan Province
Sample/spot no. Session no. U (ppm) Th/U 206Pb/204 Pb 207Pb/206 Pb ±σ(%) 207Pb/235 U±σ(%) 206 Pb/238U±σ(%) 207 Pb/206Pb ±σ207 Pb/235U±σ206 Pb/238 U±σDisc. % conv.
14CJ07/01 A 163 0.66 29291 0.0585 0.50 0.719 1.13 0.0891 1.01 549.1 10.9 550.0 4.8 550.2 5.3 0.2
14CJ07/02 A 87 0.59 12145 0.0590 0.57 0.722 0.97 0.0887 0.79 568.4 12.4 552.0 4.2 548.0 4.1 –3.8
14CJ07/03 A 73 0.42 20706 0.0587 0.79 0.727 1.09 0.0898 0.75 555.8 17.1 554.6 4.7 554.3 4.0 –0.3
14CJ07/04 A 218 0.28 50149 0.0589 0.36 0.736 0.84 0.0906 0.76 563.7 7.8 560.1 3.6 559.2 4.1 –0.8
14CJ07/05 A 76 0.45 32927 0.0588 0.63 0.733 1.03 0.0904 0.82 559.3 13.7 558.0 4.4 557.7 4.4 –0.3
14CJ07/06 A 545 0.75 73313 0.0588 0.23 0.714 0.79 0.0881 0.75 559.9 5.1 547.3 3.3 544.3 3.9 –2.9
14CJ07/07 A 88 0.89 19430 0.0586 0.62 0.706 0.98 0.0874 0.76 554.1 13.4 542.6 4.1 539.9 3.9 –2.7
14CJ07/08 A 115 0.62 16979 0.0583 0.67 0.708 1.01 0.0880 0.75 540.9 14.6 543.4 4.3 543.9 3.9 0.6
14CJ07/09 A 237 0.83 43849 0.0584 0.57 0.714 0.99 0.0886 0.80 546.6 12.5 547.3 4.2 547.4 4.2 0.2
14CJ07/10 A 130 0.62 21554 0.0587 0.47 0.720 1.02 0.0891 0.91 554.5 10.3 550.9 4.4 550.0 4.8 –0.8
14CJ07/11 A 278 0.78 63783 0.0587 0.32 0.730 0.85 0.0901 0.79 557.1 7.0 556.4 3.7 556.3 4.2 –0.2
14CJ07/12 A 85 0.59 25308 0.0587 0.70 0.718 1.03 0.0887 0.76 554.7 15.2 549.4 4.4 548.1 4.0 –1.2
14CJ07/13 A 120 0.78 20451 0.0584 0.49 0.714 0.96 0.0887 0.83 543.2 10.7 547.2 4.1 548.1 4.4 0.9
14CJ07/14 A 82 0.59 13003 0.0591 0.57 0.738 0.97 0.0905 0.78 570.6 12.3 561.0 4.2 558.6 4.2 –2.2
14CJ07/15 A 150 0.72 29693 0.0590 0.46 0.730 1.00 0.0898 0.89 566.8 10.0 556.6 4.3 554.2 4.7 –2.3
14CJ07/16 A 178 0.63 12595 0.0582 0.47 0.704 0.91 0.0877 0.78 537.4 10.1 541.0 3.8 541.9 4.1 0.9
14CJ07/17 A 90 0.60 19189 0.0593 0.62 0.728 1.03 0.0890 0.82 577.6 13.5 555.2 4.4 549.7 4.3 –5.0
14CJ07/18 A 57 0.59 14968 0.0582 0.72 0.701 1.10 0.0873 0.82 538.3 15.7 539.4 4.6 539.7 4.3 0.3
14CJ07/19 A 276 0.99 51605 0.0585 0.34 0.713 0.84 0.0884 0.77 546.7 7.3 546.4 3.6 546.3 4.0 –0.1
14CJ07/20 A 174 0.68 32148 0.0584 0.45 0.718 0.90 0.0892 0.79 544.5 9.7 549.4 3.8 550.6 4.2 1.2
14CJ07/21 B 158 0.72 42537 0.0583 0.47 0.711 0.79 0.0884 0.64 542.4 10.2 545.3 3.4 546.0 3.4 0.7
14CJ07/22 B 314 1.13 44559 0.0590 0.29 0.763 0.64 0.0938 0.57 567.7 6.4 575.8 2.8 577.8 3.1 1.9
14CJ07/23 B 76 0.60 19871 0.0588 0.62 0.726 0.88 0.0895 0.63 560.6 13.5 554.4 3.8 552.8 3.3 –1.4
14CJ07/24 B 127 0.61 27966 0.0585 0.49 0.711 0.75 0.0882 0.57 549.7 10.6 545.6 3.2 544.7 3.0 –1.0
14CJ07/25 B 103 0.57 11814 0.0589 0.54 0.725 0.91 0.0893 0.73 562.0 11.8 553.4 3.9 551.3 3.9 –2.0
14CJ07/27 B 266 0.73 42690 0.0585 0.36 0.719 0.66 0.0891 0.56 547.7 7.7 549.8 2.8 550.3 2.9 0.5
14CJ07/28 B 227 0.63 16860 0.0590 1.06 0.726 1.20 0.0892 0.57 565.7 23.0 554.0 5.2 551.1 3.0 –2.7
14CJ07/29 B 141 0.61 37134 0.0587 0.42 0.735 0.70 0.0909 0.56 555.3 9.2 559.5 3.0 560.6 3.0 1.0
14CJ07/31 B 303 0.84 31211 0.0585 0.37 0.718 0.67 0.0890 0.55 549.5 8.2 549.4 2.8 549.4 2.9 0.0
14CJ07/32 B 183 0.74 41113 0.0584 0.38 0.712 0.70 0.0884 0.58 546.3 8.4 545.9 3.0 545.8 3.1 –0.1
14CJ07/33 B 295 0.88 13812 0.0588 0.30 0.713 0.67 0.0880 0.60 559.4 6.5 546.6 2.8 543.5 3.1 –3.0
14CJ07/34 B 191 0.75 23098 0.0586 0.38 0.712 0.67 0.0882 0.56 551.9 8.2 546.1 2.9 544.7 2.9 –1.4
14CJ07/35 B 545 1.03 32548 0.0587 0.24 0.730 0.61 0.0903 0.55 555.2 5.3 556.8 2.6 557.2 3.0 0.4
14CJ07/36 B 166 0.75 15392 0.0589 0.41 0.723 0.79 0.0890 0.68 564.2 8.9 552.3 3.4 549.4 3.6 –2.7
14CJ07/39 B 179 2.06 18990 0.0589 0.39 0.730 0.75 0.0899 0.64 562.0 8.5 556.5 3.2 555.1 3.4 –1.3
14CJ07/40 B 367 0.83 23684 0.0586 0.27 0.735 0.62 0.0909 0.56 553.7 6.0 559.4 2.7 560.7 3.0 1.3
14CJ07/41 B 238 0.53 44655 0.0585 0.54 0.712 0.77 0.0884 0.55 546.8 11.7 546.1 3.3 545.9 2.9 –0.2
14CJ07/42 B 67 0.60 10299 0.0590 0.64 0.721 0.85 0.0887 0.56 566.9 13.8 551.5 3.6 547.7 2.9 –3.5
14CJ07/43 B 187 1.03 41999 0.0585 0.39 0.720 0.70 0.0893 0.58 546.9 8.6 550.5 3.0 551.3 3.0 0.8
14CJ07/44 B 128 0.62 27603 0.0586 0.49 0.719 0.75 0.0890 0.57 552.6 10.6 550.3 3.2 549.8 3.0 –0.5
14CJ07/45 B 179 0.66 36561 0.0586 0.38 0.726 0.74 0.0898 0.63 553.8 8.4 554.1 3.2 554.2 3.4 0.1
14CJ07/46 B 454 0.94 17212 0.0585 0.26 0.720 0.78 0.0892 0.73 549.8 5.8 550.7 3.3 550.9 3.9 0.2
14CJ07/47 B 301 0.64 63740 0.0585 0.30 0.719 0.63 0.0891 0.56 549.9 6.6 550.0 2.7 550.0 2.9 0.0
14CJ07/48 B 87 0.56 17976 0.0583 0.63 0.701 0.84 0.0872 0.55 540.3 13.7 539.2 3.5 538.9 2.9 –0.3
14CJ07/49 B 154 0.68 21791 0.0582 0.46 0.705 0.72 0.0879 0.56 536.9 10.0 542.0 3.0 543.2 2.9 1.2
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1208 C. YANG AND OTHERS
Table 1. Continued
Sample/spot no. Session no. U (ppm) Th/U 206Pb/204 Pb 207Pb/206 Pb ±σ(%) 207Pb/235 U±σ(%) 206 Pb/238U±σ(%) 207 Pb/206Pb ±σ207 Pb/235U±σ206 Pb/238 U±σDisc. % conv.
14YCP02/01 A 180 1.50 35519 0.0583 0.47 0.708 0.89 0.0881 0.75 539.6 10.3 543.5 3.8 544.5 3.9 0.9
14YCP02/02 A 129 1.26 24768 0.0580 0.79 0.694 1.10 0.0868 0.76 529.2 17.2 535.1 4.6 536.4 3.9 1.4
14YCP02/03 A 132 1.09 60566 0.0583 0.52 0.700 0.92 0.0871 0.75 540.0 11.4 538.7 3.8 538.4 3.9 –0.3
14YCP02/04 A 89 1.73 29577 0.0586 0.84 0.707 1.14 0.0875 0.77 551.3 18.1 542.7 4.8 540.6 4.0 –2.0
14YCP02/05 A 149 1.48 35862 0.0585 0.51 0.713 0.94 0.0883 0.78 549.3 11.2 546.3 4.0 545.5 4.1 –0.7
14YCP02/06 A 459 0.09 83406 0.0584 0.35 0.714 0.84 0.0888 0.77 543.0 7.5 547.3 3.6 548.4 4.0 1.0
14YCP02/07 A 83 1.48 14430 0.0587 0.73 0.720 1.06 0.0889 0.77 556.1 15.9 550.6 4.5 549.3 4.1 –1.3
14YCP02/08 A 149 2.45 27030 0.0587 0.55 0.712 0.93 0.0880 0.75 554.6 11.9 545.8 3.9 543.7 3.9 –2.0
14YCP02/09 A 243 2.28 16148 0.0586 0.52 0.707 0.91 0.0875 0.75 553.7 11.3 543.1 3.8 540.6 3.9 –2.5
14YCP02/11 A 191 1.70 28771 0.0586 0.69 0.717 1.02 0.0889 0.76 550.7 15.0 549.2 4.4 548.8 4.0 –0.4
14YCP02/12 A 275 2.81 59470 0.0585 0.39 0.718 0.84 0.0889 0.75 549.8 8.4 549.4 3.6 549.3 3.9 –0.1
14YCP02/13 A 714 0.48 114013 0.0586 0.22 0.724 0.78 0.0897 0.75 551.3 4.8 553.3 3.3 553.8 4.0 0.5
14YCP02/15 A 236 0.36 44621 0.0585 0.39 0.714 0.87 0.0885 0.77 549.7 8.6 547.1 3.7 546.5 4.0 –0.6
14YCP02/16 A 171 1.99 33233 0.0584 0.48 0.694 0.91 0.0861 0.77 545.9 10.4 535.0 3.8 532.5 4.0 –2.6
14YCP02/18 A 171 2.06 34179 0.0584 0.44 0.720 0.89 0.0895 0.77 543.8 9.7 550.9 3.8 552.7 4.1 1.7
14YCP02/19 A 560 2.04 68824 0.0584 0.25 0.712 0.80 0.0885 0.76 543.1 5.4 546.1 3.4 546.8 4.0 0.7
14YCP02/20 A 187 1.33 27453 0.0583 0.43 0.715 0.87 0.0889 0.76 541.6 9.3 547.5 3.7 549.0 4.0 1.4
14YCP02/21 A 241 1.01 53854 0.0585 0.42 0.718 0.86 0.0889 0.75 550.1 9.2 549.5 3.7 549.3 4.0 –0.1
14YCP02/22 A 74 0.94 20550 0.0584 0.67 0.700 1.01 0.0870 0.75 544.5 14.7 539.1 4.2 537.8 3.9 –1.3
14YCP02/23 A 198 0.76 33761 0.0585 0.41 0.708 0.87 0.0878 0.76 548.2 9.0 543.7 3.7 542.6 4.0 –1.1
14YCP02/24 A 69 0.91 21565 0.0579 0.69 0.693 1.10 0.0869 0.85 525.7 15.1 534.8 4.6 536.9 4.4 2.2
14YCP02/25 A 144 1.35 49199 0.0584 0.50 0.688 1.07 0.0854 0.94 545.8 10.9 531.6 4.4 528.3 4.8 –3.3
14YCP02/26 A 280 1.63 27889 0.0585 0.61 0.699 1.06 0.0867 0.87 547.5 13.3 538.3 4.5 536.1 4.5 –2.2
14YCP02/27 A 251 1.67 66634 0.0597 0.36 0.781 0.86 0.0948 0.79 594.0 7.7 585.9 3.9 583.8 4.4 –1.8
14YCP02/28 A 256 1.45 63527 0.0584 0.36 0.714 0.84 0.0886 0.76 545.1 7.8 546.8 3.6 547.2 4.0 0.4
14YCP02/29 A 140 1.29 26676 0.0584 0.59 0.713 0.96 0.0886 0.76 543.5 12.9 546.3 4.1 547.0 4.0 0.7
14YCP02/30 A 241 1.35 40168 0.0585 0.38 0.708 0.85 0.0879 0.76 546.9 8.3 543.9 3.6 543.1 4.0 –0.7
14YCP02/32 A 114 1.35 28170 0.0585 0.52 0.710 0.94 0.0881 0.78 548.9 11.4 545.0 4.0 544.0 4.1 –0.9
14YCP02/33 A 152 2.54 29242 0.0580 0.50 0.678 0.94 0.0849 0.80 528.7 11.0 525.7 3.9 525.0 4.0 –0.7
14YCP02/34 A 347 2.53 55309 0.0586 0.30 0.726 0.82 0.0898 0.76 553.6 6.6 554.4 3.5 554.6 4.0 0.2
14YCP02/35 A 236 0.80 61510 0.0583 0.42 0.697 1.16 0.0868 1.08 540.0 9.1 537.1 4.8 536.4 5.6 –0.7
14YCP02/36 A 176 1.93 17264 0.0586 0.41 0.728 0.88 0.0901 0.78 551.5 9.0 555.1 3.8 556.0 4.2 0.9
14YCP02/37 A 460 1.57 74944 0.0584 0.26 0.705 0.79 0.0876 0.75 544.4 5.7 541.7 3.3 541.1 3.9 –0.6
14YCP02/38 A 183 10.56 47554 0.0582 0.41 0.699 0.86 0.0871 0.75 538.3 9.0 538.3 3.6 538.3 3.9 0.0
14YCP02/39 A 109 0.99 27207 0.0583 0.61 0.706 0.97 0.0879 0.75 539.9 13.3 542.5 4.1 543.1 3.9 0.6
14YCP02/40 A 91 1.26 25455 0.0585 0.70 0.710 1.03 0.0880 0.75 547.9 15.3 544.6 4.3 543.7 3.9 –0.8
14YCP02/41 A 136 2.00 32791 0.0586 0.48 0.714 0.91 0.0884 0.77 552.1 10.4 547.4 3.8 546.3 4.0 –1.1
14YCP02/42 A 121 1.50 29885 0.0579 0.53 0.687 0.93 0.0861 0.75 524.5 11.7 530.9 3.8 532.4 3.9 1.6
14YCP02/43 A 302 0.66 81276 0.0582 0.37 0.700 0.84 0.0872 0.75 536.9 8.2 538.7 3.5 539.2 3.9 0.4
14YCP02/44 A 853 1.24 50917 0.0586 0.20 0.719 0.78 0.0890 0.76 552.3 4.4 550.0 3.3 549.5 4.0 –0.5
14YCP02/45 A 224 1.56 49367 0.0582 0.41 0.691 0.86 0.0862 0.75 536.3 8.9 533.7 3.6 533.1 3.9 –0.6
14YCP02/46 A 81 1.23 29362 0.0589 0.66 0.702 1.03 0.0864 0.79 564.5 14.3 539.8 4.3 533.9 4.0 –5.6
14YCP02/48 A 348 0.84 71703 0.0587 0.33 0.713 0.85 0.0882 0.78 554.3 7.2 546.7 3.6 544.9 4.1 –1.8
14YCP02/49 A 123 1.80 37266 0.0584 0.63 0.707 1.10 0.0879 0.89 543.7 13.8 543.2 4.6 543.1 4.7 –0.1
14YCP02/50 A 268 1.84 47745 0.0584 0.33 0.716 0.82 0.0889 0.75 543.8 7.3 548.1 3.5 549.1 4.0 1.0
Age discordance in conventional concordia space.
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Zircon U–Pb dating of the Dengying Formation 1209
Figure 4. (Colour online) Logarithmic U–Pb calibration graphs. Curve (a) is applied to samples 14YCP02 and 14CJ07/01-20, and
curve (b) is applied to 14CJ07/21-50.
Figure 5. (Colour online) Photomicrographs of representative zircons analysed in this study. The ellipses indicate the SIMS U–Pb
analytical spots, each 30 μm in length for scale. Zircon 207Pb–206 Pb ages are quoted.
Th contents are within the range 57–545 ppm and
31–559 ppm, respectively, with Th/U ratios of 0.28–
2.06 (mostly within 0.28–1.03), suggesting a mag-
matic origin. With the exception of five sets of data
from zircons with high common lead, all the others
are considered to be reliable. The main population
includes 44 grains, and their 238U–206 Pb ages range
from 539 ±3 Ma to 561 ±3Ma (Fig. 6), yielding a
weighted mean 238U–206Pb age of 549.5 ±1.1/(5.6) Ma
(2σ,n=44, MSWD =2.6). Their weighted mean
235U–207 Pb and 207Pb–206 Pb ages are 550.4 ±1.0 Ma
(2σ,n=44, MSWD =2.6) and 553.6 ±2.7/(3.8) Ma
(2σ,n=44, MSWD =0.77), respectively. The spot
no. 22 yielded a 238U–206 Pb age of 578 ±3 Ma and a
207Pb–206 Pb age of 568 ±6 Ma. Its 238 U–206Pb age is
outwith the main population and it is a little reverse
discordant (discordance =1.9 %). To be prudent, this
grain was excluded from the weighted mean age cal-
culation.
4.b. Sample 14YCP02 at the middle part of the Jiucheng
Member
Zircons from sample 14YCP02 are 100–200 μmin
length and have aspect ratios of 2–4. They are euhedral
and prismatic in morphology, showing oscillatory zon-
ing under CL images (Fig. 5). A total of 50 analyses
were conducted on 50 zircons, which are characterized
by moderate U (47–853 ppm) and Th (42–1934 ppm)
contents with Th/U ratios ranging over 0.09–10.56
(mostly within 0.36–2.81). The morphology and the
Th/U ratios suggest that they are of magmatic ori-
gin. With the exception of five sets of data from zir-
cons with high common lead, all the others are con-
sidered to be reliable. The major population consists of
44 analyses, with their 238U–206Pb ages ranging from
525 ±4Mato556±4 Ma. They yield a weighted
mean 238U–206 Pb age of 543.1 ±1.2/(5.6) Ma (2σ,n=
44, MSWD =2.9, Fig. 6). Their weighted mean 235U–
207Pb and 207 Pb–206Pb ages are 543.8 ±1.2 Ma (2σ,n
=44, MSWD =2.9) and 546.3 ±2.7/(3.8) Ma (2σ,n
=44, MSWD =0.58), respectively. Spot no. 27 gives
a clearly older 238U–206Pb age of 584 ±4 Ma and older
207Pb–206 Pb age of 594 ±8 Ma than the major popula-
tion, suggesting a xenocrystal origin.
5. Discussion
5.a. Ages of the ash beds from middle Dengying Formation
Zircon U–Pb geochronology is the most widely used
radiometric dating tool for determining geological
ages and establishing time scales of processes in the
Earth history. However, the Pb-loss in zircons, a result
of the crystal lattice being damaged by the emission
of alpha particles and alpha recoil processes, has often
been recognized in U–Pb systematics of zircon (e.g.
Nasdala et al.2005). In micro-beam zircon U–Pb dat-
ing techniques, such as the LA-ICPMS and SIMS, it
is possible to recognize the significant Pb-loss effect
in the high-U zircons or zircons with old ages since
their 238U–206 Pb and 207Pb–206 Pb dates can be resolved.
However, Pb-loss can be cryptic and impossible to
identify in the zircons with younger ages without
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1210 C. YANG AND OTHERS
Figure 6. (Colour online) U–Pb concordia diagrams of samples (a) 14CJ07 and (b) 14YCP02. Weighted mean 238U–206 Pb ages versus
their MSWD versus the number of the oldest grains included in the weighted mean age calculation for samples (c) 14CJ07 and (d)
14YCP02. The grey area represents the range of the acceptable MSWD (Wendt & Carl, 1991).
reference to other techniques such as CA-ID-TIMS or
40Ar–39 Ar dating (e.g. Kryza et al.2012;Crowleyet al.
2014; Watts et al.2016). Thermal annealing and chem-
ical abrasion (‘CA’) can minimize the effect of Pb-loss
by removing the radiation-damaged parts of zircons,
and have been widely used in thermal ionization mass
spectrometry zircon U–Pb dating method (Mattinson,
2005). The SIMS 238U–206 Pb ages of the CA-treated
zircons are generally older than the results obtained
from the untreated zircons from the same magmatic
rock, even though no discordance can be resolved from
the latter analytical data set (Kryza et al.2012; Watts
et al.2016).
The dynamic multi-collector U–Pb dating method
we adopted in this study has the ability to measure the
high-precision 207Pb/206 Pb ratio as in the static multi-
collector mode without trade-off in the analytical pre-
cision of the 238U/206 Pb ratio of the conventional peak-
hopping mono-collector mode (Liu et al.2015). This
new method has potential to simultaneously produce
higher-precision 238U–206Pb and 207Pb–206 Pb ages of
zircon, providing reliable evaluation on the concord-
ance of the zircon U–Pb system as young as 500 Ma
(Liu et al.2015). For the two samples in this study,
their weighted mean 238U–206Pb age <235 U–207Pb age
<207Pb–206 Pb age, indicating that they are discord-
ant and most likely resulted from subtle radiogenic
Pb-loss. The MSWD of the two weighted mean 238 U–
206Pb ages (2.6 and 2.9) are large and fall outside of
the acceptable range of the MSWD values for a given
population included in weighted mean age calcula-
tion (44 grains for each sample in this study, Fig. 6),
implying that they do not represent a single popu-
lation and their weighted mean 238U–206Pb ages are
meaningless (Wendt & Carl, 1991). If the assumption
that the analysed zircons have suffered subtle radio-
genic Pb-loss is correct, we can explore a MSWD-
based model to exclude the outliers. The model is
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Zircon U–Pb dating of the Dengying Formation 1211
that using the largest population of the oldest grains
that yield an acceptable MSWD value to calculate the
weighted mean 238U–206Pb ages. The MSWD-based
model weighted mean 238U–206Pb ages for the two
samples are therefore 551.9 ±1.2/(5.6) Ma (Fig. 6c)
and 545.5 ±1.4/(5.6) Ma (Fig. 6d). We note however
that the calculation of a MSWD with a value that is
acceptable for ndoes not confirm a single age popula-
tion.
Present-day radiogenic Pb-loss of zircon will not af-
fect the accuracy of the 207Pb–206 Pb age. The zircon
207Pb–206 Pb age by SIMS measurement is independent
of the matrix-matched U/Pb calibration, and its uncer-
tainty is derived predominantly from the uncertainty of
the measured 207Pb/206 Pb ratio and the common Pb cor-
rection. All the zircons included in the weighted mean
age calculation have 206Pb/204Pb ratios >10 000, in-
dicating that the common Pb correction only makes
a small contribution to the final 207Pb–206 Pb age un-
certainty (Li et al.2009b). As mentioned above, the
new analytical protocol we adopted is able to achieve a
higher precision by a factor of two for the 207Pb–206Pb
age than the conventional mono-collector mode within
the same working time (Liu et al.2015). We there-
fore suggest that the weighted mean 207Pb–206Pb ages
of 553.6 ±2.7/(3.8) Ma and 546.3 ±2.7/(3.8) Ma are
the best estimates of the crystallization ages for the two
ash samples in the middle Dengying Formation in east-
ern Yunnan Province, South China. This suggestion
is also supported by the fact that the weighted mean
207Pb–206 Pb age is identical within errors to its cor-
responding MSWD-based model weighted mean 238U–
206Pb age.
5.b. Geochronological constraints on the Dengying
Formation
The Dengying Formation is divided into three parts
in South China, although they have been called dif-
ferent names in different areas (Fig. 2). The three
lithostratigraphic units of the Dengying Formation
in eastern Yunnan Province (in upwards order: Don-
glongtan, Jiucheng and Baiyanshao members) are tra-
ditionally correlated to those in other areas of the
Yangtze Platform (Zhu, Zhang & Yang, 2007) such
as the Yangtze Gorges area (Hamajing, Shibantan and
Baimatuo members) and southern Shaanxi Province
(Algal Dolostone, Gaojiashan and Beiwan members).
Although numerous studies have been carried out on
the lithostratigraphy, chemostratigraphy and palaeon-
tology within the Dengying Formation, less achieve-
ment on geochronology has been made. No zircon U–
Pb data have been directly obtained from the Dengying
Formation or its equivalents, hampering attempts to
verify the chronostratigraphic framework. The Miaohe
Member, which consists of black siliceous and car-
bonaceous shales and contains the famous Miaohe
biota in west Huangling Anticline, Yangtze Gorges
area, has long been considered as the uppermost part
of the Doushantuo Formation (‘traditional correlation
model’, e.g. Ding et al.1996; Wang et al., 1998; Zhu,
Zhang & Yang, 2007). The zircon 207 Pb–206Pb age of
550.55 ±0.75 Ma for the ash bed at the top of the
Miaohe Member from the Jijiawan section in the west
Huangling Anticline has therefore been widely accep-
ted as the maximum age for the base of the Dengy-
ing Formation (Condon et al.2005). However, some
researchers recently argued that the Miaohe Member
is most likely time-equivalent to the lower Shibantan
Member (equivalent to the Jiucheng Member in east-
ern Yunnan, Fig. 2) of the Dengying Formation in
the Yangtze Gorges area, suggesting that the base of
the Dengying Formation, that is, top of the Doush-
antuo Formation and the top of the DOUNCE/Shuram
carbon isotope excursion, should be much older than
550.55 ±0.75 Ma (An et al.2015).
In this study, the ash beds from the basal
and middle parts of the Jiucheng Member of the
Dengying Formation in the Xiaolantian and Yin-
changpo sections are dated at 553.6 ±2.7/(3.8) Ma
and 546.3 ±2.7/(3.8) Ma, respectively, indicating that
the base of the Dengying Formation is older than
553.6 ±2.7/(3.8) Ma in eastern Yunnan Province. It
seems that the age for the base of the Dengying Forma-
tion there is older than the age for the top of the Miaohe
Member in the Yangtze Gorges area (zircon 207 Pb–
206Pb age 550.55 ±0.75 Ma; Condon et al.2005), con-
tradicting the traditional correlation model but agree-
ing with that of An et al.(2015). There are three
possible interpretations for the controversial correla-
tions. (1) Considering the sequence boundary at the
base of the Jiucheng Member and the thick Don-
glongtan dolostone unit in the lower part of the Dengy-
ing Formation, it is possible that the base of Dengy-
ing Formation in eastern Yunnan Province is older
than 550.55 ±0.75 Ma. That is not unexpected as the
lithostratigraphic boundaries in different sedimentary
basins can be diachronous, and the base of the Dengy-
ing Formation in the Yangtze platform represents ini-
tiation of a highstand system tract. (2) The tradi-
tional correlation model is incorrect and implies a lat-
eral facies changes of the Dengying Formation among
the basins of the Yangtze Platform, therefore support-
ing the correlation model of An et al.(2015) which
states that, the base of the Dengying Formation is be-
low the Miaohe Member in the west Huangling Anti-
cline, and 550.55 ±0.75 Ma for the top of the Miaohe
Member in this area represents the age of the middle
Dengying Formation (Fig. 2). This interpretation is
least likely because this correlation model violates
the field observations, the chemostratigraphy and sed-
imentary model of the Dengying Formation over the
entire Yangtze Platform (Zhu, Zhang & Yang, 2007;
Lu et al.2013; Zhu et al.2013), and the stratigraphic
correlations between South China, Namibia and Oman
(Grotzinger et al.1995; Bowring et al. 2007). (3) It is
beyond the precision of these two SIMS zircon 207 Pb–
206Pb ages to exactly determine the age for the base
of Dengying Formation in eastern Yunnan Province.
Within errors, the age 553.6 ±2.7/(3.8) Ma for the
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1212 C. YANG AND OTHERS
Figure 7. (Colour online) Temporal distribution of the Ediacara-type fossils and the representative biomineralized animals in South
China (Hua, Chen & Yuan, 2007;Chenet al.2013,2014), Namibia (Grotzinger et al.1995;Bowringet al. 2007), Oman (Amthor
et al.2003;Bowringet al.2007) and White Sea (Martin et al.2000). The thicknesses of the lithostratigraphic units are not to scale.
DST – Doushantuo Formation; N – Nomtsas Formation; Fm – formation; Gr – group. Red bars represent the locations of the ash beds
in the columns.
base of the Jiucheng Member is similar to the age
550.55 ±0.75 Ma for the top of the Miaohe Member
in west Huangling Anticline in Yangtze Gorges area.
Further, because of the update of the U–Pb tracer calib-
ration (Condon et al.2015; McLean et al.2015), there
may be a bias between the new 238U–206 Pb date derived
from new parameters and the legacy ID-TIMS data
in the published literatures (e.g. Burgess, Bowring &
Shen, 2014). The analyses above demonstrate that the
two SIMS zircon 207Pb–206Pb ages from the Jiucheng
Member in eastern Yunnan Province do not allow a
clear test of the two correlation models for the chro-
nostratigraphic position of the Miaohe Member in the
Yangtze Gorges area (uppermost Doushantuo Forma-
tion vs. middle Dengying Formation; Fig. 2). Higher-
precision geochronology on the upper Doushanuo and
Dengying formations, including recalibration of the
published dates, is therefore needed to further test and
refine the upper Ediacaran stratigraphic correlation in
South China.
5.c. Geochronological constraints on the biostratigraphic
correlation
Abundant fossils have been discovered from the Edi-
acaran successions in South China, especially from
the Dengying Formation and its equivalents (e.g. Hua,
Chen & Yuan, 2007; Chen et al.2013,2014). Most
of them, such as the Gaojiashan biota in the southern
Shaanxi Province, the Xilingxia biota in the Yangtze
Gorges area and the Jiangchuan biota in the Yun-
nan Province (Fig. 2), are preserved in the middle
part of the Dengying Formation with some fossils
extending to the upper part of the formation. They
are mainly composed of macroscopic multicellular al-
gae, Ediacara-type fossils, tubular fossils and trace
fossils (Zhu, 2010). Our new zircon 207Pb–206Pb age
of 553.6 ±2.7/(3.8) Ma for the ash bed from the basal
Jiucheng Member in eastern Yunnan Province provides
constraints on the maximum age of the biotas in the
middle part of the Dengying Formation. The zircon
207Pb–206 Pb age of 555.3 ±0.3 Ma for the ash bed
from the lower part of sequence B of the Ust-Pinega
Formation represents the minimum age for the oldest
Ediacara-type fossils in the White Sea region (Fig. 7;
Martin et al.2000). The SIMS zircon 207Pb–206 Pb age
for the basal Jiucheng Member is slightly younger
than (or within errors identical to) the age from the
White Sea region, indicating that the Ediacara-type
fossil assemblage preserved in the Dengying Forma-
tion is younger than the White Sea Assemblage.
The zircon 207Pb–206 Pb age of 546.3 ±2.7/(3.8) Ma
for the ash bed from the middle part of the Jiucheng
Member constrains the ages of the Ediacara-type
fossils, trace fossils and some tubular fossils in the
middle part of the Dengying Formation (Fig. 7).
This age is identical within errors to the zircon
207Pb–206 Pb age of 549.34 ±0.82 Ma for the ash bed
from the middle part of the Kuibis subgroup of the
Nama Group, which provides the minimum age for
the Ediacara-type fossils and Cloudina in Namibia
(Grotzinger et al.1995; Bowring et al. 2007). The δ13C
negative excursion at the lowermost Cambrian strata
starts at the top of the Dengying Formation in eastern
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Zircon U–Pb dating of the Dengying Formation 1213
Yunnan Province (Li et al.2013b), and the onset of this
globally significant biogeochemical event was con-
strained at 541.00 ±0.13 Ma in Oman (Amthor et al.
2003; Bowring et al.2007). If this δ13C negative excur-
sion can be correlated between South China and Oman,
the age for the top of the Dengying Formation in east-
ern Yunnan Province would be c. 541 Ma, which is
similar to the age for the top of the Spitskopf Member
of the Nama Group (bracketed by zircon 207Pb–206Pb
ages 543.3 ±1 Ma and 539.4 ±1 Ma; Grotzinger et al.
1995). Comparison between the newly obtained zir-
con 207Pb–206 Pb ages and those from the Nama Group
indicates that the Ediacara-type fossils in the middle
Dengying Formation in South China can be tempor-
ally correlated to the Nama Assemblages in Namibia.
This result is consistent with the affinity between the
Dengying and Nama assemblages based on the same
Ediacara genera such as the Pteridinium and Rangea,
the presence of the Cloudina and Sinotubulites, and
the abundance of trace fossils in the two assemblages
(Chen et al.2014). Pteridinium is one of the youngest
Ediacara fossils, extending to the uppermost Ediacaran
deposits (younger than 543.3 ±1 Ma; Grotzinger et al.
1995). The Rangea are present in strata of age 558–
549 Ma, and the Hiemalora and Charniodiscus are
usually discovered in strata older than 550 Ma (Xiao
& Laflamme, 2009; Noble et al.2015). The discov-
ery of these fossils in the middle Dengying Formation
(Chen et al.2014) extends their stratigraphic ranges to
as young as 546.3 ±2.7/(3.8) Ma.
Assigning the Ediacara-type fossils in the Dengy-
ing Formation to the Nama Assemblage significantly
extends their taphonomic ranges. The Ediacara-type
fossils in Namibia were mostly preserved as casts and
moulds in siliciclastic rocks, similar to the preserva-
tion in the Flinders Ranges area in Australia, where
the fossils were restricted to the Ediacara Member of
the Rawnsley Quartzite (e.g. Gehling & Droser, 2013).
The Ediacara-type fossils with a limestone taphonomic
window in the Dengying Formation in South China
represent a distinct taphonomic pathway, extend their
ecological range and prove that these Ediacara or-
ganisms were marine organisms rather than terrestrial
lichens or microbial colonies (Xiao et al. 2005; Chen
et al.2014).
6. Conclusions
The following conclusions about the upper Ediacaran
geochronology in South China can be made by SIMS
zircon U–Pb dating of the ash beds from the Jiucheng
Member, Dengying Formation in the eastern Yunnan
Province, South China.
(1) Excess ‘scatter’ in U–Pb datasets is inter-
preted to reflect subtle radiogenic Pb-loss in the ana-
lysed zircons. The two weighted mean 207Pb–206 Pb
ages, 553.6 ±2.7/(3.8) Ma and 546.3 ±2.7/(3.8) Ma,
are considered as the best estimates of the crystalliz-
ation ages for the two ash samples from the basal and
middle part of the Jiucheng Member, Dengying Form-
ation in eastern Yunnan Province, South China.
(2) The age of the base of the Dengying Formation
in eastern Yunnan Province is older than, or within er-
rors identical to, that in the Yangtze Gorges area. The
two SIMS zircon 207Pb–206Pb ages from the Jiucheng
Member do not permit a clear test of the two correl-
ation models for the chronostratigraphic position of
the Miaohe Member (uppermost Doushantuo Forma-
tion vs. middle Dengying Formation).
(3) The Ediacara-type fossils preserved in the
Dengying Formation in South China are temporally
correlated to the Nama Assemblage. Their exceptional
limestone taphonomic window in South China sheds
new light on the diversity and palaeoecology of the
macroscopic Ediacaran life forms.
Acknowledgements. We thank Qiu-Li Li, Yu Liu, Guo-
Qiang Tang and Xiao-Xiao Ling for assistance with SIMS
zircon U–Pb analysis, Zhongwu Lan and Zhi Chen for field
assistance. This work was supported by the Chinese Ministry
of Science and Technology (grant no. 2013CB835000) and
the Strategic Priority Research Program (B) of the Chinese
Academy of Sciences (grant no. XDB18030300).
Declaration of Interest
None
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... Litho-chemo-biostratigraphic data are from Zhu et al. (2001) and D. . Radioisotopic dates are from Compston et al. (2008) and Yang et al. (2017Yang et al. ( , 2018 was siliciclastic-dominated, shallow-marine successions (Yao et al., 2014). The eastern Yunnan Province is tectonically located in the platform interior of the Yangtze Block, at the western part of the SCB. ...
... No magmatic rocks with comparable ages have yet been identified in the SCB, except ash layers intercalated in the strata (e.g., Compston et al., 2008;Yang et al., 2017). It is noteworthy that tuff layers started to appear in western SCB Ediacaran-Cambrian strata from ca. 0.55 Ga (Figure 1c). ...
... Detrital zircon U-Pb data presented in this study illustrate that the provenance of clastic sediments in western SCB changed dramatically during the late Ediacaran (0.56-0.54 Ga), as shown by the appearance of both the unique 1.20-Ga zircon population from a local uplift and the 0.55-to 0.52-Ga tuff layers (Figure 2a). This sudden reappearance of volcanic ash deposits after tens of million years of nonvolcanic record (Yang et al., 2017; close to the western margin of the SCB, along with the lack of same-aged volcanic activity in the SCB, can be best explained by the approaching of a magmatic arc to the SCB during that time. Together, these sudden changes in the sedimentary record indicate a significant change in the tectonic and paleogeographic environment for the western SCB; here, we interpret as reflecting the collision of the SCB with other Gondwanan blocks. ...
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The South China Block (SCB) has been regarded by many as an integral part of Gondwana, but proposed timing and processes for its accretion to Gondwana vary and remain contentious, largely owing to the lack of reliable Pan‐African age paleomagnetic data and tectono‐magmatic records from the SCB. Integrated in situ U‐Pb ages and Hf‐O isotope analyses of detrital zircons from geochronologically well calibrated Ediacaran–Cambrian sedimentary rocks of western SCB reveal age populations of 2.51 Ga, 1.85 Ga, 1.20 Ga, 0.80 Ga, and 0.52 Ga. Detrital zircon age spectra indicate a major tectonic transition for the SCB during 0.56–0.54 Ga, interpreted to reflect the beginning of the collision between SCB‐Indochina and NW India blocks. The collisional event lasted until early Ordovician, leading to the suturing of the SCB‐Indochina to the northern margin of East Gondwana.
... Ash beds in the eastern Yunnan province have yielded a U-Pb zircon age of 546 ± 3 Ma for Member 3 within the Dengying Formation . Using δ 13 C isotope correlations, Yang et al. (2017) propose that the top of the Dengying Formation could be ca. 541 Ma. ...
... The Dengying Formation spans ca. 551-541 Ma, with an age of 546 Ma recorded for Member 3 in equivalent South China outcrops (Condon et al., 2005;Yang et al., 2017). The duration of the two unconformities is assumed to be on the order of ~1 Myr (Figure 3). ...
... Until recently it was assumed that platform and lagoon facies dominated throughout the Sichuan Basin (e.g. Yang et al., 2017), however, on a smaller scale within the Sichuan Basin, significant variations in the Dengying Formation facies has also been noted in the well data drilled during hydrocarbon exploration and production (e.g. Gu et al., 2016;Zhou et al., 2017;Zou et al., 2014) and in particular within the trough where the Dengying Formation is much reduced in thickness and consists of mudstones interbedded with cherts and dolomites deposited in a slope environment (Figure 3). ...
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The Upper Ediacaran to Lower Cambrian of the Sichuan Basin in South China has long been considered to be dominated by shallow‐water deposition. Hydrocarbon exploration, however, has revealed that a NW‐SE trending intraplatform trough formed in the basin during the same period. Although different models have been proposed, the formation and evolution of the trough are still not fully understood. In this study, we investigate both the origin of the intraplatform trough and the formation of the Sichuan Basin by integrating seismic interpretation, well correlation, and tectonic subsidence analysis. The seismic and well data clearly show three stages of development of the trough. The first stage, in the early Late Ediacaran, is characterized by considerable thinning of the lower two members of the Upper Ediacaran from the platform margins to the trough. In the second stage, in the late Late Ediacaran, the platform margins backstepped and the extent of the trough expanded significantly to a width of ~400 km. The third stage, in the early Early Cambrian, was dominated by gradual filling of the trough and onlapping of the platform margins. Backstripped tectonic subsidence curves show one, or two closely spaced episodes of linear subsidence starting at ~550 Ma and then decreasing exponentially until ~450 Ma. The shape of the subsidence curves is consistent with formation of the Sichuan Basin by low, and slow amounts of lithospheric stretching of thickened cratonic lithosphere. The tectonic subsidence increases from the centre to the NW of the basin. Interestingly the margins of the trough do not correlate with contoured values of increased tectonic subsidence and we infer that the trough was a palaeogeographic embayment in a large carbonate platform that developed in a broad, ramp‐like area of slow and low subsidence tilting down to the proto‐Tethyan ocean located to the NW of the basin.
... The Dengying Fm exposed in the Gaojiaxi-Yanjiahe section is subdivided into three Mb's (Hamajing, Shibantan, and Baimatuo). The Hamajing Mb is a c. 20 m thick interval, which consists of massive intraclastic and oolitic dolomite grainstone and oncoids, and correlates stratigraphically with the Donglongtan Mb at the Xiaotan section (Yang et al., 2016). The overlying Shibantan Mb is c. ...
... 50 m thick and comprises dark grey laminated limestone, which contains Ediacaran-type fossils, tubular and vermiform metazoans, and trace fossils (Xiao et al., 2005;Shen et al., 2009;Chen et al., 2013;Chen et al., 2014;Duda et al., 2014). The Baimatuo Mb comprises of c. 40 m thick massive dolostones and correlates stratigraphically with the Baiyanshao Mb at the Xiaotan section (Yang et al., 2016). ...
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Cadmium (Cd) isotope signatures (expressed as δ¹¹⁴Cd values) in seawater and in suitable modern and ancient sediments have been proposed as a useful tracer for paleobioproductivity and for constraining Cd as a micronutrient. This study contributes to the calibration of Cd isotope compositions in ancient shallow-water carbonates and proposes a combination with chromium (Cr) isotopes to link bioproductivity and ocean redox condition. We analysed 64 carbonate samples from the Jiulongwan, Gaojiaxi-Yanjiahe and Xiaotan sections of the Yangtze Platform, South China, covering the Shuram negative carbon isotope excursion (SCE) and Ediacaran-Cambrian transition (ECT), for Cd concentrations [Cd] and Cd isotopes. The results of this study show δ¹¹⁴Cd values ranging from −0.9‰ to +0.6‰, averaging −0.20 ± 0.65‰ (2σ, n = 56) and [Cd] with substantial variations from 0.003 μg/g to 2.78 μg/g. Using the experimental fractionation factor for Cd into calcite (−0.45‰), we calculate an average ambient surface seawater δ¹¹⁴Cd during the entire studied period of +0.24 ± 0.65‰, largely covering today's surface and deep ocean water compositions. The δ¹¹⁴Cd of ambient seawater during the SCE is reconstructed to an average value of +0.48 ± 0.40‰. This elevated signature possibly reflects the result of increased nutrient upwelling and concomitant enhanced bioproductivity levels in the upper photic surface waters during this period, leading to an increased oxygenation of the surface water. The carbonates deposited during the post-Shuram-Wonoka interval reveal a strong correlation between [Cd] and [Zn] and a correlation between degree of negative Ce anomalies and positive δ¹¹⁴Cd excursions, which point to strongly oxidized surface waters and concomitant biotic removal of isotopically light Cd isotopes into phytoplankton. It remains unclear why the strongly oxidized surface waters during the Late Ediacaran—Early Cambrian did not exert more pronouncedly positively fractioned Cd isotope signatures in shallow-water carbonates as expected from elevated bioproductivity during this transitional period.
... uncertainty; however, our approach of considering these within a stratigraphic framework can aid the development of an age thickness model. There is a temporal overlap between the Mistaken Point Ecological Reserve fossil-bearing section and that of Charnwood Forest, United Kingdom (bracketed to 569-556 Ma; Noble et al., 2015), and some sections in the Central Urals (Maslov et al., 2013), but even the youngest Mistaken Point Ecological Reserve fossils predate macrofossil-bearing strata in Siberia, China, Brazil, and Namibia (Grotzinger et al., 1995;Parry et al., 2017;Yang et al., 2017). Detailed studies from the ca. ...
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The rise of complex macroscopic life occurred during the Ediacaran Period, an interval that witnessed large-scale disturbances to biogeochemical systems. The current Ediacaran chronostratigraphic framework is of insufficient resolution to provide robust global correlation schemes or test hypotheses for the role of biogeochemical cycling in the evolution of complex life. Here, we present new radio-isotopic dates from Ediacaran strata that directly constrain key fossil assemblages and large-magnitude carbon cycle perturbations. These new dates and integrated global correlations demonstrate that late Ediacaran strata of South China are time transgressive and that the 575-to 550-Ma interval is marked by two large negative carbon isotope excursions: the Shuram and a younger one that ended ca. 550 Ma ago. These data calibrate the tempo of Ediacaran evolution characterized by intervals of tens of millions of years of increasing ecosystem complexity, interrupted by biological turnovers that coincide with large perturbations to the carbon cycle.
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陕西迹(Shaanxilithes)见证了埃迪卡拉纪–寒武纪之交生物演化及生态环境的变化, 是埃迪卡拉纪末期标志性的疑难化石, 具有全球对比意义。本文首次报道了云南曲靖会泽县大海乡朱家箐剖面灯影组旧城段泥质灰岩中新发现的宁强陕西迹(Shaanxilithes ningqiangensis Xing, Yue and Zhang, 1984)。所采集的标本整体形态上呈条带状, 边缘呈锯齿状参差不齐, 可见环状体分布。通过镜下观察, 可发现散落圆盘状单元。将大海乡朱家箐剖面与晋宁六街镇王家湾剖面渔户村组旧城段所产出的Shaanxilithes 化石对比后发现, 朱家箐剖面的标本宽度更窄, 长度更长, 碎片化更加明显。通过微区X 射线荧光光谱仪(μ-XRF)将朱家箐剖面和王家湾剖面的标本进行对比分析后可知, 前者形态在Si、Al、K 的元素分布图中显示清晰, 而后者的形态主要体现在Fe 元素的分布图上; 前者化石和围岩中Ca 元素的含量远远高于后者; S 元素仅在前者存在微量的分布, 在后者中未有信号显示。新的化石证据表明Shaanxilithes 并非只产出自碎屑岩中, 还可以在灰岩中保存。化石μ-XRF 的面扫描和半定量分析表明Shaanxilithes 可能为伊利石矿物交代, 并且说明在不同岩相背景下, 风化作用程度的强弱造成两个产地化石的差异性保存。随着研究区域不断扩大, Shaanxilithes 相继在华南板块滇东地区、印度西北缘、纳米比亚、华北板块西南缘等地区晚埃迪卡拉世地层中被发现。此次在滇东会泽地区发现的Shaanxilithes, 进一步表明该化石分布的广泛性, 可作为全球晚埃迪卡拉世地层对比的标准化石。本文结合形态学分析和微区X 射线荧光光谱分析, 讨论了Shaanxilithes 的埋藏学信息与亲缘关系, 为探索埃迪卡拉纪–寒武纪过渡时期的生物和生态特征提供了更多视角。
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Chapter
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The Cryogenian Period was first established in 1988 along with other Precambrian eon, era and period-level subdivisions that were defined numerically by Global Standard Stratigraphic Ages (GSSAs). As absolute age constraints have improved, some of these time intervals no longer bracket adequately the geological event(s), for which they were named. For example, the age discrepancy between the basal Cryogenian GSSA at 850 Ma and the onset of widespread glaciation ca. 717 Ma has rendered the 850 Ma boundary obsolete. The International Commission on Stratigraphy has now formally approved the removal of the Cryogenian GSSA from its International Chronostratigraphic Chart and supports its replacement with a rock-based Global Stratotype Section and Point (GSSP). The new Cryogenian GSSP will be placed at a globally correlative level that lies stratigraphically beneath the first appearance of widespread glaciation and is assigned in the interim a 'calibrated age' of circa 720 Ma. This new definition for the Tonian/Cryogenian boundary should be used in future publications until a formal Cryogenian GSSP can be ratified. The change marks progress towards establishment of a 'natural' (rock-based) scale for Precambrian time.
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Carbonaceous compression fossils in shales of the uppermost Doushantuo Formation (ca. 555-590 Ma) at Miaohe in the Yangtze Gorges area provide a rare Burgess-Shale-type taphonomic window on terminal Proterozoic biology. More than 100 macrofossil species have been described from Miaohe shales, but in an examination of published and new materials, we recognize only about twenty distinct taxa, including Aggregatosphaera miaoheensis new gen. and sp. Most of these fossils can be interpreted unambiguously as colonial prokaryotes or multicellular algae. Phylogenetically derived coenocytic green algae appear to be present, as do regularly bifurcating thalli comparable to red and brown algae. At least five species have been interpreted as metazoans by previous workers. Of these, Protoconites minor and Calyptrina striata most closely resemble animal remains; either or both could be the organic sheaths of cnidarian scyphopolyps, although an algal origin cannot be ruled out for P. minor. Despite exceptional preservation, the Miaohe assemblage contains no macroscopic fossils that can be interpreted with confidence as bilaterian animals. In combination with other late Neoproterozoic and Early Cambrian body fossils and trace fossils, the Doushantuo assemblage supports the view that body-plan diversification within bilaterian phyla was largely a Cambrian event.
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Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust. However, for zircons compromised by open system behavior, achieving robust dates can be difficult. Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb. While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon. In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study. We present CA and non-CA 206Pb/238U ages and trace elements determined by SHRIMP-RG for zircons from three Caetano samples (Caetano Tuff, Redrock Canyon porphyry, and a silicic ring-fracture intrusion) and for R33 and TEMORA-2 reference zircons. We find that non-CA Caetano zircons have weighted mean or bimodal U-Pb ages that are 2–4% younger than CA zircons for the same samples. CA Caetano zircons have mean U-Pb ages that are 0.4–0.6 Myr older than the 40Ar/39Ar sanidine eruption age (34.00 ± 0.03 Ma; error-weighted mean, 2σ), whereas non-CA zircons have ages that are 0.7–1.3 Myr younger. U-Pb ages do not correlate with U (~ 100–800 ppm), Th (~ 50–300 ppm) or any other measured zircon trace elements (Y, Hf, REE), and CA and non-CA Caetano zircons define identical trace element ranges. No statistically significant difference in U-Pb age is observed for CA versus non-CA R33 or TEMORA-2 zircons. Optical profiler measurements of ion microprobe pits demonstrate consistent depths of ~ 1.6 μm for CA and non-CA Caetano, R33 and TEMORA-2 zircons, and do not indicate variations in secondary ion sputtering rates due to chemical or structural changes from the CA treatment. Our new data underscore the potential for cryptic Pb-loss to go unrecognized in other geologically young magmatic centers that do not have zircons with high U, statistically discordant isotope ratios, high common Pb, or metamict textures.
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The Sinian (Terminal Proterozoic) and Early Cambrian shallow- to deep-water sequences of the Yangtze Platform were investigated. Based on integrated lithostratigraphic, biostratigraphic, and other approaches, the shallow-water sequence from the base of the Sinian ( base of the Doushantuo Formation) to the top of the Qiongzhusian ( top of the Yu'anshan Formation) was Subdivided into 12 stratigraphic intervals. These 12 intervals were applied in turn to the Subdivision and correlation of the sequences present in various facies of the Yangtze Platform. The high-resolution stratigraphic framework here developed can serve as a time frame for ongoing multidisciplinary analyses of the "Cambrian explosion".