Early crustal evolution in the western Yangtze Block: Evidence from U–Pb and Lu–Hf isotopes on detrital zircons from sedimentary rocks

Precambrian Research 222–223 (2012) 368–385

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Early crustal evolution in the western Yangtze Block: Evidence from U–Pb and Lu–Hf isotopes on detrital zircons from sedimentary rocks Li-Juan Wang a,b,c,∗ , Jin-Hai Yu a,c , W.L. Griffin c , S.Y. O’Reilly c a b c

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China Geological Survey of Jiangsu Province, Nanjing 210018, PR China GEMOC National Key Centre, Department of Earth and Planetary Science, Macquarie University, NSW 2109, Australia

a r t i c l e

i n f o

Article history: Received 3 December 2010 Received in revised form 3 August 2011 Accepted 6 August 2011 Available online 25 August 2011 Keywords: Yangtze Block Crustal evolution Zircon U–Pb ages Zircon Hf isotopes Zircon trace elements

a b s t r a c t In situ U–Pb and Lu–Hf data on detrital zircons from Paleo- to Neoproterozoic sediments have been used to gain a clearer picture of the regional tectonic background and crustal evolution in the western part of the Yangtze Block. The youngest concordant zircon ages for sedimentary units suggest maximum depositional ages of 1014 Ma for the Kunyang Group, 750–649 Ma for the Sinian sequences, and 525 Ma for the Cambrian sediments. The Yinmin Formation, previously assigned to the middle part of the Kunyang Group, was actually deposited after 1667 Ma and contains zircons whose ages are dominantly from late Archean to Paleoproterozoic (2.8–2.7 Ga, 2.5–2.3 Ga and ∼1.85 Ga). The Heishantou Formation in the lower part of the Kunyang Group has two major age populations of ∼1.0 Ga and 1.8–1.6 Ga. The Sinian and Cambrian sedimentary rocks are dominated by Neoproterozoic zircons with age peaks at ∼760 Ma and ∼825 Ma, consistent with the ages of widespread igneous rocks around the Yangtze Block. The Lu–Hf isotope data suggest that a significant juvenile input took place during Archean and Neoproterozoic times, respectively, while crustal reworking was dominant during the Paleoproterozoic time. The Archean–Paleoproterozoic detritus is isotopically distinct from the Archean–Paleoproterozoic basement exposed in the northern part of the Yangtze Block, suggesting that a subarea of old crust lies beneath the young sediments covering the craton. The presence of abundant ∼1.85 Ga zircons suggests that the Yangtze Block was probably part of the Columbia supercontinent during Paleoproterozoic time. Comparisons to the other parts of Columbia suggest that the Yangtze Block could have been adjacent to the North China Craton and/or Australia. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The Yangtze Block is separated from the North China Craton by the Qinling–Dabie–Sulu orogenic belt, and bounded by the Longmengshan Fault to the northwest, the Ailaoshan-Red River Fault to the southwest, and the Jiangnan Orogen to the southeast. The Yangtze Block consists of basement complexes overlain by a Neoproterozoic to Cenozoic cover. The oldest rocks exposed in the Yangtze Block are the 3.2–2.9 Ga Kongling gneiss (Qiu et al., 2000; Zhang et al., 2006a; Jiao et al., 2009), close to the Yangtze Gorge. Recent studies have suggested that Archean to Paleoproterozoic basement is probably more widespread with the Yangtze Block than previously thought (Zheng et al., 2006a; Zhang et al., 2006b,c;

∗ Corresponding author at: State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China. Tel.: +86 25 84368745. E-mail address: wang [email protected] (L.-J. Wang). 0301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2011.08.001

Wang et al., 2010b). The oldest supracrustal rocks are the late Paleoproterozoic volcanic and sedimentary rocks of the Dahongshan Group in the western Yangtze Block (Greentree and Li, 2008). Widespread Neoproterozoic igneous rocks and Proterozoic sedimentary rocks along the western margin of the Yangtze Block are targets for understanding the early history of the Yangtze Block (Zhao et al., 2010b). Extensive studies have focused on the tectonic setting and petrogenesis of typical Neoproterozoic igneous rocks, whereas the spatially associated sedimentary rocks are little known. Although a few recently reported detrital-zircon data provide age constraints on these sequences (Druschke et al., 2006; Li et al., 2006; Greentree et al., 2006; Greentree and Li, 2008; Sun et al., 2008; Zhao et al., 2011), fewer Lu–Hf isotopic data are available for discussing the crustal evolution (Sun et al., 2009a,b). In this study, U–Pb and Lu–Hf isotope systematics on detrital zircons from a series of Proterozoic sedimentary rocks in the western Yangtze Block are used to provide insights into the timing of major magmatic events in the region, the nature of the magmatism and the crustal evolution history of the Yangtze Block during Precambrian time.

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2. Regional geological setting South China is composed of the Yangtze craton to northwest and the Cathaysia Block to southeast. The Yangtze Block is separated from the Cathaysia Block by the Jiangnan Orogen (Fig. 1a). The southwestern margin of the Yangtze Block is marked by the Cenozoic Ailaoshan-Red River Shear Zone (Fig. 1a). The Paleoproterozoic to Neoproterozoic rocks exposed to the north of the Ailaoshan-Red River Fault include the Kunyang Group and the overlying Sinian and Cambrian sequences (Fig. 1b and c). No exposed contact has been observed between the Kunyang Group and an older crystalline basement. The Kunyang Group is subdivided into nine formations. However, the sequence of these formations is controversial. From the base up, the traditional opinion is including the Huangcaoling, Heishantou, Dalongkou, Meidang, Yinmin, Luoxue, Eshantou, Luzhijiang and Liubatang Formations (Greentree and Li, 2008), while the recently proposed sequence includes the Yinmin, Luoxue, Eshantou, Luzhijiang, Liubatang, Huangcaoling, Heishantou, Dalongkou and Meidang formations (Zhao et al., 2010b). The total thickness of the Kunyang Group is estimated to be 5000–9300 m. The Kunyang Group consists dominantly of carbonates in the upper part and silicic clastic units in the lower part, with rare volcanic rocks such as tuff and basalt in the Huangcaoling and Heishantou Formations. The sedimentary features suggest progressively shallowing water depth. The rock types in the basal units of the Kunyang Group indicate an anoxic deep water environment, while the interlayering of clastic rocks and carbonates is consistent with deposition in a shallow sea and fluctuating fluvial input. As the traditional sequence, the Huangcaoling Formation, the basal unit of the Kunyang Group, consists mainly of black or dark grey shale, with thin layers of siltstone. The overlying Heishantou Formation consists dominantly of fine-grained grey sandstone, with dark grey shale. The Dalongkou Formaiton largely consists of well-bedded carbonates with minor shale layers. The Meidang Formation, which overlies the Dalongkou Formation, consists of grey shale and sandstone with carbonate layers. The Yinmin Formation is composed of purple shale and feldspar-quartz sandstone. At the top of the Yinmin Formation, the clastic sediments grade into the dolomite of the Luoxue Formation, which hosts the stratiform Fe–Cu ore deposits of this region. The overlying Etouchang Formation includes grey shale and carbonate with minor sandstone. A red carbonate unit dominates the overlaying Luzhijiang Formation. The Liubatang Formation represents the top of the Kunyang Group and contains mainly of siliciclastic rocks including conglomerate, quartz sandstone, black shale and minor carbonate. The overlying Sinian sequences, from bottom up, consist of the Chengjiang, Nantuo, Doushantuo and Dengying Formations. A conglomerate marks the base of the Chengjiang Formation. The upper layers of this formation consist of purple quartz sandstone. The overlying Nantuo Formation is characterized by tillite. The Doushantuo Formation overlying the tillite comprises dominant dolomite and carbonate with minor sandstone. The Dengying Formation on the top of the Sinian sediments consists only of dolomite. The Cambrian Qiongzhusi Formation consists mainly of purple shales with layered siltstone. In this study, one siltstone (KM-8-2), three sandstones (KM-20, KM-19-4, KM-20-1), three purple sandstones (KM-10, KM-223, KM-7), one sandstone (KM-4) and one siltstone (KM-3) were collected from the Heishantou Formation, Yinmin Formation, Chengjiang Formation, Doushantuo Formation and Qiongzhusi Formation, respectively. 3. Analytical methods Trace-element and isotopic (Lu/Hf and U/Pb) in situ analyses were carried out on 483 zircon grains. The full dataset is given

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in Supplementary Tables 1 and 2. Zircons were separated from heavy mineral concentrates and were hand-picked under a binocular microscope fitted with a UV light. A representative number of each morphological population was mounted in epoxy blocks on petrographic slides, and polished down to about half of their thickness for analysis. Prior to analytical work, backscattered electron/cathodoluminescence (BSE/CL) images were taken on the electron microprobe (EMP) to examine internal structures. EMP was also used to analyze the zircons for major and minor elements. This examination was followed by analysis of zircon U–Pb ages, Hfisotope compositions and trace elements. All analyses were carried out in the Geochemical Analysis Unit (GAU) in the GEMOC Key Centre in the Department of Earth and Planetary Sciences, Macquarie University. 3.1. Electron microprobe BSE/CL images of zircon grains were taken on a CAMECA-SX100 electron microprobe (EMP) at GEMOC, Macquarie University with operating conditions of 15 kV accelerating voltage and 15–20 nA beam current. The images are a combination of BSE and CL phenomena obtained by operating the BSE detector at high gain amplification, where the BSE image reflects differences in the mean atomic number of elements in the mineral, and CL is produced by the irradiation of the zircon with the electron beam. The BSE/CL images have been used to classify each grain in terms of external morphology and internal structure (e.g. Fig. 2). Hf contents of the zircons were determined on the same electron microprobe to be used as the internal standard for trace element analysis by LAICPMS. An accelerating voltage of 15 kV and a beam current of 20 nA were used for all analyses. The spatial resolution of the electron microprobe is ∼2 ␮m. The detection level for Hf was 0.12% with a precision of 2.5% RSD at 1.5% HfO2 . 3.2. U–Pb dating U–Pb dating was performed on an Agilent 7500 ICPMS, coupled to a New Wave Research 213 nm laser microprobe at GEMOC. A very fast scanning protocol was used, and data acquisition for each analysis took 3 min (1 min on background, 2 min on signal). Ablation was carried out in He to improve sample transport efficiency, provide more stable signals and give more reproducible U/Pb fractionation. Samples were analyzed in “runs” of ca. 16 analyses, which included 12 unknowns, bracketed by two analyses of the standard analyses of red GJ zircon standard at the beginning and end of the run. The “unknowns” include two well-characterized standard zircons, 91500 and Mud Tank, which are analyzed in every run as an independent control on reproducibility and instrument stability. The spot size for the laser analyses was about 40–50 ␮m. U–Pb ages were calculated from the raw signal data using the on-line software package GLITTER (Griffin et al., 2008; www.mq.edu.au/GEMOC). The detailed analytical procedure and its precision and accuracy are described by Jackson et al. (2004). Unless otherwise stated, the age data shown in the figures and discussions are 207 Pb/206 Pb ages for grains older than 1.0 Ga, and 206 Pb/238 U ages for younger grains. Ages with discordance >10% are excluded from the discussion. 3.3. Trace element determinations The trace-element analysis was performed simultaneously with the U–Pb analysis. Quantitative results for trace elements reported here were obtained through calibration of relative element sensitivities using the NIST-610 standard glass as the external calibration standard, and normalization of each analysis to the electron-probe data for Hf as an internal standard. The precision and accuracy of the NIST-610 analyses are 2–5% for REE, Y, Sr, Nb, Hf, Ta, Th and U at

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Fig. 1. (a) and (b) Geological map of South China and sample locations in the Kunming area in Yunnan Province; stratigraphy of sedimentary rocks in the Kunming area (Zhao et al., 2010b).

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Fig. 2. Representative CL images of detrital zircons from the Kunming area.

the ppm concentration level, and from 8% to 10% for Mn, P, Ti and Pb (Norman et al., 1996). Belousova et al. (2002) found broad correlations between the trace-element patterns of zircons and the composition of the magmatic host rocks. These correlations were used to create a classification tree that is useful for classifying an individual zircon grain in terms of its parental igneous rock type. According to the various trace-element patterns, zircons from kimberlites, carbonatites, mafic rocks (diabases + basalts), granitic rocks, syenitic rocks (syenites, larvikites) and Ne-syenites are recognized at a

probability of correct classification >80%. This scheme greatly enhances the use of detrital zircons in studying provenance. A more detailed system of classification also was presented by Belousova et al. (2002). 3.4. Hf-isotope analyses The Hf-isotope analyses reported here were carried out in situ using a Merchantek/New Wave Research LUV213 laser-ablation microprobe, attached to a Nu Plasma multi-collector ICPMS. Typical

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Fig. 3. Concordia plots for zircons from the Kunyang Group.

ablation times were 80–120 s, resulting in pits 40–55 m across and 40–50 m deep. The Mud Tank standard zircon was analyzed before the unknowns to check reliability and stability of the instrument. The detailed analytical procedure and its precision and accuracy are described by Griffin et al. (2000, 2006). Model ages (TDM ) are calculated based on a depleted-mantle source with 176 Hf/177 Hf = 0.28325 and 176 Lu/177 Hf = 0.0384. TDM ages, which are calculated using the measured 176 Lu/177 Hf of the zircon, can only give a minimum age for the source material of the magma from which the zircon crystallized. Therefore, we have C ) which also calculated, for each zircon, a “crustal” model age (TDM assumes that its parental magma was produced from an average continental crust (176 Lu/177 Hf = 0.015) that originally was derived from the depleted mantle. εHf (t) values were calculated using 176 Hf/177 Hf = 0.282772 as the chondritic value and the decay constant 176 Lu = 1.865 × 1011 /a, proposed by Scherer et al. (2001). 4. Results The analytical data, and observations on external morphology and internal structure of each grain, are given in Supplementary Tables 1 and 2 and Fig. 2. Zircons have been considered to be possibly of metamorphic (rather than magmatic) origin when they display a combination of anhedral or rounded morphology, little or no internal structure, low Th and U contents and Th/U < 0.1 (Rubatto et al., 1999; Hoskin and Black, 2000). However, most zircons from this study display oscillatory zoning and high Th/U (>0.4), indicative of magmatic origin. Most zircons have concordant ages (discordance <10%), which are obvious on the concordia plots in Figs. 3 and 4. We have rejected some strongly discordant grains (discordance >50%). 4.1. Kunyang Group 4.1.1. Heishantou Formation Sample KM-8-2 comes from Baohe Town, where the Heishantou Formation, the lowest part of the Kunyang Group, is exposed

beneath younger cover rocks. The U–Pb data define a broad range of ages between 1014 Ma and 2526 Ma. The cumulative probability histogram shows a large peak around 1.05 Ga, with smaller peaks at 1.25 Ga, 1.35 Ga and 1.75 Ga. All zircons in the age window 1.0–1.05 Ga have negative εHf from −0.45 to −11.93 (Figs. 5 and 6, Supplementary Table 2). Zircons with ages between 1.1 Ga and 1.75 Ga have dominantly positive εHf ; and only 4 grains have negative εHf . Five zircons give ages of 1.75–1.99 Ga and have εHf from −0.93 to −6.48. One grain with a 2526 Ma age has εHf of +2.1 and a TDM = 2.76 Ga; this is the only indication of older crust in this sample.

4.1.2. Yinmin Formation Three samples (KM-15-1, KM-19-4, KM-20-1) were taken from different parts of this formation; their relative stratigraphic positions are not clear in the field. 186 detrital zircons were selected for U–Pb dating, trace element and Lu–Hf isotope analysis. Sample KM-15-1 shows two dominant peaks in age at 1.8–1.9 Ga and 2.3–2.4 Ga, with smaller age peaks at 2.4–2.5 Ga and 2.7–2.8 Ga. Several discordant grains form a discordia with an upper intercept age of 2766 Ma. All 1.6–2.0 Ga zircons have negative εHf , ranging from −13.0 to −1.2. Most zircons in the age window of 2.3–2.5 Ga have negative εHf ; but one grain has extremely high εHf (+12.1). The C model ages of 2.7–3.0 Ga grains have εHf from −5.3 to +4.8 and TDM 3.0–3.5 Ga. Three zircons older than 3.0 Ga (3011–3111 Ma) show a narrow range in εHf (+2.1 to +2.8) and TDM (3.17–3.23 Ga). The youngest concordant detrital zircon is dated at 1699 ± 74 Ma. Sample KM-19-4 defines similar age populations (1.8–1.9 Ga, 2.3–2.5 Ga, 2.7–2.8 Ga) to KM-15. Several grains lie on a discordia with an upper intercept at 2.8 Ga. All 1.7–2.0 Ga zircons have εHf < 0, C model ages for except for one grain (KM-19-4-32, εHf = +18.2). TDM these grains range from 2.5 Ga to 3.5 Ga. εHf for those zircons with ages of 2.3–2.5 Ga is distributed with the larger group from +0.4 to +5.9, and the others from −0.1 to −7.6. Most zircons older than 2.7 Ga have very high εHf , even above the depleted mantle line; only four grains have negative εHf from −7.9 to −1.4. The oldest

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Fig. 4. Concordia plots for zircons from the Sinian and Cambrian sediments.

C model ages for these grains are around 3.7 Ga. The youngest TDM concordant age is 1723 ± 30 Ma. Sample KM-20-1 contains a large concentration of zircons (n = 21) with 207 Pb/206 Pb ages of 1.85–1.90 Ga, and a peak at C 1875 Ma. They have negative εHf , even down to −19.4, and TDM model ages from 2.7 Ga to 3.7 Ga. This sample also shows three smaller age populations of 2.4–2.5 Ga, 2.6–2.7 Ga and 2.8–2.9 Ga. The youngest zircon in this sample has a 207 Pb/206 Pb age of

1842 ± 27 Ma. Three discordia lines have been identified with upper intercept ages of ∼1850 Ma, ∼2500 Ma and ∼2800 Ma. The εHf of most zircons lies in narrow range from −1 to −9, and the maxC imum TDM model age is older than 3.8 Ga; only four grains have positive εHf , two of which are close to the depleted mantle line. The C model oldest analyzed zircon gives an age of 3030 Ma and a TDM C = 3.2 Ga, age only 170 Ma older than the crystallization age (TDM TDM = 3.1 Ga).

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Fig. 5. Probability density plots of U–Pb ages and plots of 176 Hf/177 Hf vs. U–Pb age for the detrital zircons; CHUR: Chondritic Uniform Reservoir; dashed lines show evolution of crustal volumes with 176 Lu/177 Hf = 0.015, corresponding to the average continental crust. The data are plotted by formation.

4.2. Sinian sediments 4.2.1. Chengjiang Formation Three samples (KM-7, KM-10-1, KM-22-3) were collected from the Chengjiang Formation in different widely separated areas (ca. 25 km spacing); their relative stratigraphic positions are therefore obscure. Their zircons show similar age populations and Hf-isotope signatures. Detrital zircons from KM-7 define a tight cluster of ages between 750 Ma and 850 Ma, with a few grains older than 1000 Ma. Some Neoproterozoic and Mesoproterozoic zircons are quite discordant (>20%), they are thus excluded from this discussion. The Neoproterozoic zircons have approximately the same proportion of positive and negative εHf , ranging from −9.4 to +6.3. Most zircons with ages between 1018 Ma and 2084 Ma have positive εHf ; only one grain has slightly negative εHf (−2.3). The four oldest grains, C with ages of 2748–3220 Ma, have mainly negative εHf , and TDM model ages from 3.1 Ga to 3.9 Ga. The youngest zircon in this sample has a 206 Pb/238 U age of 750 ± 9 Ma. Sample KM-10-1 shows a major peak in age at 750–900 Ma, with smaller peaks at 1800–1900 Ma and ∼2300 Ma. The 750–900 Ma zircons show a wide spread in εHf (t), from −6.70 to +35.1 (these grains have zoning and high Th/U, suggesting a magmatic origin).

C ranges from 0.83 Ga to 2.13 Ga. Zircons in the age popTheir TDM ulation of 1800–1900 Ma have dominantly negative εHf ; only one grain with an age of 1801 Ma has significantly higher εHf (+6.04). The dominant source rock type for these zircons is a low-Si granitoid. Zircons older than 2000 Ma have both positive and negative εHf ; four grains nearly lie on the depleted-mantle line. The oldest C of 3.1 Ga. The youngest zircon is dated at 2849 ± 29 Ma, and has TDM concordant age is 769 ± 9 Ma, which is much younger than that in the Kunyang Group. The largest single population of zircons from sample KM-223 has an age range of 750–850 Ma. The youngest zircon is dated at 754 ± 9 Ma. Zircons in this population have both positive and negative εHf , ranging from −10.0 to + 10.8, and have the oldest C of 2.3 Ga. Only two Mesoproterozoic zircons have been idenTDM tified, and they have positive εHf (+7.3, +3.4). Five zircons with ages of 1845–1877 Ma have a weighted mean 207 Pb/206 Pb age of 1858 ± 25 Ma (MSWD = 0.22). Four zircons with ages ranging between 1902 Ma and 2205 Ma do not form a single age population. All zircons with ages between 1845 Ma and 2205 Ma have signifiC model cantly negative εHf , even down to −13.8, and maximum TDM ages of 3.35 Ga. A population (n = 3) of 2311–2357 Ma grains has a weighted mean 207 Pb/206 Pb age of 2344 ± 29 Ma (MSWD = 0.65) and negative εHf ranging from −1.6 to −6.7. Two zircon grains give

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Fig. 6. Probability density plots of U–Pb ages and plots of 176 Hf/177 Hf vs. U–Pb age for the detrital zircons; CHUR: Chondritic Uniform Reservoir; dashed lines show evolution of crustal volumes with 176 Lu/177 Hf = 0.015, corresponding to the average continental crust. The data with zircons are classified in terms of source-rock type.

ages of 2472 Ma and 2478 Ma and have slightly negative εHf (−3.3, C model ages of ∼3.1 Ga. The oldest two grains ana−2.9), with TDM lyzed in this sample have ages of 2702 ± 37 Ma and 2988 ± 27 Ma. Both grains have positive εHf , one of which plots close to the depleted-mantle line. Their TDM model ages are only 200 Ma older than their crystallization ages.

2492 Ma have mainly negative εHf ; only two grains have positive εHf . The three oldest grains have ages of 2624–2731 Ma and show C negative εHf and TDM model ages older than 3 Ga. Some highly discordant zircon grains are rounded and have high U contents (U > 250 ppm), suggesting radiation-induced microstructural damage.

4.2.2. Doushantuo Formation Zircons from sample KM-4-1 have a young age population of 824–861 Ma. Three younger grains have ages between 385 Ma and 719 Ma. However, the youngest grain is extremely discordant (>30%); the youngest concordant age is 649 ± 9 Ma. All Neoproterozoic zircons have negative εHf , from −2.6 to −15.9, and most have C TDM model ages around 2 Ga. One grain with an age of 1177 Ma C has positive εHf (+4.6) and TDM model age of 1.7 Ga. Three zircons concentrated at 1648–1670 Ma, have a wide range of εHf from −7.7 to +4.5. The largest population has an age range from 1834 Ma to 1885 Ma, and a weighted mean age of 1863 ± 13 Ma (MSWD = 0.26). All zircons in this age window have a narrow range C model ages around 2.9 Ga. of εHf , mostly from −5 to −7, and TDM C model Four 1906–1967 Ma grains also have negative εHf and TDM ages of 2.6–2.9 Ga. Zircons with ages ranging from 2221 Ma to

4.3. Cambrian Qiongzhusi Formation Fifty zircons were selected from sample KM-3 in the lower part of the Qiongzhusi Formation for U–Pb dating. The U–Pb data show a major peak in age at 720–850 Ma, but a large number of these grains have discordant ages. These discordant zircons lie on a discordia with an upper intercept age at 975 Ma and lower intercept age at 433 Ma. Forty grains were analyzed for Lu–Hf isotope. Most zircons with ages of 525–866 Ma have positive εHf , mostly between +5 and +10; only 5 grains exhibit negative εHf . Seven Neoproterozoic grains with ages from 877 Ma to 1059 Ma have both positive C and negative εHf , and the oldest TDM model ages are 2.3 Ga. Four Proterozoic zircons do not form an age population, and have ages ranging from 1595 Ma to 2240 Ma. Only three Archean zircons have been identified, and one of them has slightly positive εHf (+1.9).

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4.4. Trace element analysis of zircons These detrital zircons are derived from a variety of rock types, ranging from mafic rocks and carbonatites to granitoids (Fig. 7). In the Kunyang Group, the detrital zircons from the Yinmin Formation were derived mainly from dolerite and low-Si granitoids, while the zircons in the Heishantou Formation were dominantly sourced from low-Si and high-Si granitoids. In the Sinian and Cambrian Formations, the inferred host rock types of zircons consist mainly of both mafic rocks and low-Si granitoids.

5. Discussion 5.1. Depositional ages of sedimentary rocks Previous studies on the age of the Kunyang Group used traditional U–Pb and Pb-evaporation methods, whole rock Sm–Nd isotopes, and Rb–Sr and K–Ar dating (e.g. Wu et al., 1990; Chen and Ran, 1992; Ma and Bai, 1998; Zhang et al., 2003). However, the analytical techniques have developed, and these methods are not considered as reliable means for dating the depositional ages of sediments. More recent studies apply SHRIMP or LA-ICPMS methods for zircon dating to constrain the maximum depositional age of the sedimentary rock. Greentree et al. (2006) identified the youngest zircon age of 1193 ± 15 Ma in the Laowushan Formation, which is their newly defined unit below the Kunyang Group, and a weighted mean age of 968 ± 15 Ma for zircons from the Liubatang Formation at the top of the Kunyang Group. Therefore, the depositional age of the Kunyang Group was constrained between ca. 1000 and 960 Ma. U–Pb dating results for detrital zircons from the Heishantou Formation, which is the lower part of the Kunyang Group, suggested a maximum depositional age of the Kunyang Group to be 960–1000 Ma (Zhang et al., 2007; Sun et al., 2009a,b). This study has found similar age constraints (1014 ± 48 Ma) for the Heishantou Formation of the Kunyang Group; this is quite close to previously determined ages for the volcanic interbeds from the Heishantou Formation (1032 ± 9Ma; Zhang et al., 2007) and for felsic volcanic rocks in the Tianbaoshan Formation of the coeval Huili Group (958 ± 16 Ma, Mou et al., 2003; 1028 ± 9 Ma, Geng et al., 2007). The contemporary sedimentary rocks also have been found in the Huili Group (Sun et al., 2009a). However, the youngest concordant age obtained from the Yinmin Formation, which was assigned to be the middle part of the Kunyang Group, is 1699 ± 74 Ma, similar to the 1742 ± 13 Ma obtained from a quartz trachyte sample of the Yinmin Formation within error (Zhao et al., 2010b). Although this age is only a maximum age for the deposition of the Yinmin Formation, all zircons in three samples collected from three different locations in this formation are essentially identical, an we did not find any younger grains from 186 representive zircons. Comparing to the abundant late-Mesoproterozoic zircons in the Heishantou and Luzhijiang Formations of the Kunyang Group (Greentree et al., 2006), it is suggested that the Yinmin Formation is much older than the other formations, and probably deposited at late Paleoproterozoic. A recent study reported a similar formation age (1675 ± 8 Ma) of the Dahongshan Group in the Xinping Country, Yunnan Province (Greentree and Li, 2008), confirming the presence of old sedimentary basements in the southwestern part of the Yangtze Block. Therefore, the sedimentary rocks mapped in the Yinmin Formation probably belong to an older sedimentary unit equivalent to the Dahongshan Group to the southwest, rather than to the Kunyang Group. The Sinian Chengjiang Formation has a maximum depositional age of 750 ± 9 Ma, which is consistent with sedimentary sequences around the periphery of the Yangtze Block such as the Bikou

Group, Lieguliu Formation and Guanyinya Formation in the southern Gansu Province in northwestern Yangtze Block (Sun et al., 2009a), the Xiajiang Group in the Fanjingshan area in the southeastern Yangtze Block (Wang et al., 2010b), and the Liantuo Formation and Wudangshan Group in the northern Yangtze Block (Liu et al., 2008; Zhang et al., 2006c; Ling et al., 2008). The Doushantuo Formation, which overlies the glacial sediments, is dated at 649 ± 9 Ma, providing the closest upper limit for a correlative of the Marinoan glaciation in southwestern China. This is identical to the age of glacial sediments in the Yangtze George in the northern Yangtze Block (Zhang et al., 2005) and in the Guizhou Province on the southeastern margin (Zhou et al., 2004; Wang et al., 2010b), suggesting that glaciers were widespread in the Yangtze Block during the global Neoproterozoic ice age. The Cambrian sedimentary rocks overlaying the Dengying dolomite in this study are dated at 525 ± 7 Ma, which is reasonable for its sampling location. Therefore, it appears that the southwestern margin of the Yangtze Block was a basin which received weathered material from the uplifted basement during the Paleoproterozoic and Neoproterozoic times. The formation of the Paleoproterozoic basin was probably related to the breakup of the Columbia Supercontinent (Zhao et al., 2004). The basin that formed in Neoproterozoic has been proposed as evidence for models in which continuous subduction (1000–740 Ma) (Sun et al., 2009a) or continental rifting (Li et al., 2003) occurred during the breakup of Rodinia. 5.2. Early crustal evolution of the Yangtze Block Zircon U–Pb ages can record the timing of magmatic and metamorphic events, but cannot provide conclusive information on the nature of such magmatic events (juvenile or reworked). However, combining the Hf model ages with the U–Pb crystallization ages allows investigation of the possible relationships between the igneous activity and the evolution of the crust. This study has found six major age peaks: 750–850 Ma (Chengjiang Formation, Doushantuo Formation and Jiezhusi Formaiton), 1000–1300 Ma (Heishantou Formation), 1600–1800 Ma (Heishantou Formation), ∼1850 Ma (Yinmin Formaiton, Heishantou Formaiton and Chengjiang Formation), 2300–2450 Ma (Yinmin Formation) and 2700–2850 Ma (Yinmin Formation), suggesting six important periods in the crustal-evolution history of the Yangtze Block. As described above, the Kongling complex represents the only known outcrops of Archean rocks within the Yangtze Block (Gao et al., 2001; Qiu et al., 2000; Zhang et al., 2006a). The 2.9–3.2 Ga trondhjemitic and migmitic gneisses have been dated by LA-ICPMS analysis on magmatic zircons (Zhang et al., 2006a; Jiao et al., 2009). The Kongling metapelites contain 2.8–3.3 Ga detrital zircons and their whole-rock Nd-isotope compositions indicate a derivation from 3.0 to 3.3 Ga crust (Qiu et al., 2000). A few Archean detrital zircons from sedimentary rocks in the Kongling area and the Fanjingshan area in the southeastern Yangtze have 3.6–4.3 Ga Hf model ages, which is indicative of Eoarchean and even Hadean crust (Liu et al., 2008; Zhang et al., 2006c; Wang et al., 2010b). On the basis of U–Pb and Lu–Hf data on detrital zircons (Liu et al., 2008; Zhang et al., 2006a,b), Liu et al. (2008) proposed four periods of Archean crustal growth in the northern Yangtze: 2.35–2.5 Ga, 2.6–2.7 Ga, 2.95–3.0 Ga and 3.2–3.8 Ga. Recent studies of detrital zircons from the Dahongshan Group in the southwestern Yangtze Block reported age populations of 2.1–2.4 Ga and 2.7–2.9 Ga grains, but did not include Hf data (Greentree and Li, 2008). This study has found similar age populations at 2.3–2.5 Ga and 2.7–2.9 Ga from sandstone samples in the Yinmin Formation, and their 176 Hf/177 Hf data help to unravel the Archean evolution history. As in the northern Yangtze Block (Liu et al., 2008), most 2.3–2.5 Ga zircons in the western Yangtze Block lie above or around the CHUR line, indicating derivation from juvenile sources.

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Fig. 7. Relative abundance of rock types derived from zircon compositions.

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However, zircons with ages of 2.7–2.9 Ga, which are absent in the northern Yangtze Block, have positive εHf , indicative of a local input of Archean mantle-derived materials in the western Yangtze Block. In the present study, all zircons older than 3 Ga (except KM-7-113) have 176 Hf/177 Hf close to the depleted mantle, suggesting an important episode of crustal growth. Therefore, the northern and western parts of the Yangtze Block show similar magmatic signatures at 2.3–2.5 Ga, but they have followed different evolutionary paths during the Archean time. The largest Paleoproterozoic zircon population of ∼1850 Ma, found in the present study, is not well represented in the known Yangtze Block. Xiong et al. (2009) recently reported a similar age for the Quanyishang granite (∼1.85 Ga) that intrudes the Kongling terrane in the northern Yangtze Block. The Paleoproterozoic zircons C model ages in this granite show very low 176 Hf/177 Hf and old TDM (∼3.8 Ga), indicating reworking of an ancient crust. To the east of the granite, 1.9–2.0 Ga trondhjemite (Qiu et al., 2000), migmatites (Zhang et al., 2006b), amphibolite and metapelite (Zhang et al., 2006) have been identified in the Kongling complex. Zircons from these rocks have εHf (t) of −13.3 to −1.0 and Hf crustal model ages of 2.7–3.4 Ga, suggesting probable remelting of the Archean basement exposed in the Kongling area. In this study, detrital zircons with ages of ∼1.85 Ga also have negative εHf (t), indicating reworkC model ing of Archean crust (mostly 2.7–3.3 Ga). However, their TDM ages are much younger than those of ∼1.8 Ga grains in the Kongling granites, suggesting different sources. Zircons in the age population of 1.0–1.3 Ga have both negative and positive εHf (t), indicating that both reworking of older crust and juvenile input took place in the late Mesoproterozoic. The ages of about 830–740 Ma represent an important period of magmatism in South China. Based on available Hf isotope studies of magmatic zircon, previous studies (Zheng et al., 2007) proposed that two generations of crustal formation are linked to the middleNeoproterozoic magmatism on the periphery of the Yangtze Block. The older one (∼820 Ma) is characterized by negative εHf (t) (−9.1 to −1.6) and old Hf model ages of 1.81–2.25 Ga (weighted mean 1.97 ± 0.13 Ga). This magmatism occurred in the Nanhua Rift of western part of the Jiangnan Orogen (Zheng et al., 2007) and in the Dabie–Sulu Orogen (Zheng et al., 2005, 2006b; Chen et al., 2003). In contrast, zircons from the younger magmatism (800–760 Ma) have positive εHf (t) (+1.1 to +9.9) and Hf model ages of 0.94–1.30 Ga (a weighted mean of 1.15 ± 0.06 Ga). This type of magmatism occurred at this time in the eastern part of the Jiangnan Orogen (Zheng et al., 2007), the Dabie–Sulu Orogen (Zheng et al., 2006b; Chen et al., 2003) and the Kangding Rift along the western margin of the Yangtze Block (Zheng et al., 2007; Zhao et al., 2008c). The occurrence of several periods of crustal magmatism on the northern margin of the Yangtze Block suggests that this was a weak zone, and thus would be a natural locus for rifting during supercontinent breakup. Although Neoproterozoic igneous rocks that involved reworking of old crust have not been found on the western margin of the Yangtze Block, this study shows that the Neoproterozoic detrital zircons have the characteristics of both periods discussed above, suggesting the existence of both crustal reworking and juvenile contribution at that time in the western Yangtze.

5.3. Relationships with ancient supercontinents Worldwide coeval orogenic belts are one of the key features used in piecing together ancient supercontinents. Two of the age peaks (∼1850 Ma, ∼1050 Ma) shown in our data are consistent with the assembly times of the Paleoproterozoic Columbia and late Mesoproterozoic Rodina supercontinents (Zhao et al., 2002a; Li et al., 2002), respectively.

5.3.1. Rodinia Previous studies have compared the Grenvillian orogenic belt (Jiangnan Orogen) in South China with those in other continents to unravel the relationship between South China and circumjacent blocks (Li et al., 2002; Yu et al., 2008). So far, two main models have been proposed for the position of South China in the Rodinia supercontinent. One school of thought holds that South China was situated between southeastern Australia and western Laurentia (Li et al., 2002), whereas the other believes that the South China Block was on the periphery of Rodinia, close to Western Australia and India (Zhao and Cawood, 1999; Jiang et al., 2003; Zhang and Piper, 1997; Yang et al., 2004; Zheng, 2004; Wang et al., 2007, 2008, 2010b; Yu et al., 2008). The Jiangnan Orogen between the Cathaysia and Yangtze Blocks was recognized as a Grenville-aged orogen, based on ages of 968 ± 23 Ma and 1007 ± 14 Ma obtained from an ophiolite in northeastern Jiangxi Province (eastern section of the Jiangnan Orogen) (Li et al., 1994), and granitic gneiss in the Kangding Group (western edge of the Yangtze Block) (Li et al., 2002), respectively. Two younger Grenvillian ages (913 ± 15 Ma, 905 ± 14 Ma) were obtained by SHRIMP dating of zircon from arc-related tonalite and granodiorite in northeastern Zhejiang Province (Ye et al., 2007; Chen et al., 2009; Li et al., 2009). In addition, Ar–Ar ages of 1042 ± 7 Ma and 1015 ± 4 Ma for the Tianli schists were reported by Li et al. (2007), suggesting the time of metamorphism and deformation in the eastern section of the Jiangnan Orogen. The abundant S-type granites previously thought to have formed in the Grenville orogen have been redated at 830–800 Ma (Li, 1999; Wang et al., 2006). The “Mesoproterozoic” basement (Sibao Group-Lengjiaxi Group) which was thought to be folded intensively during the Sibao (≈Grenville) orogenesis along the Jiangnan Orogen actually formed after 860 Ma (Wang et al., 2007). Therefore, it is still controversial whether the Jiangnan Orogen is in fact a Grenvillian orogen. Instead, abundant Grenvillian age (1050–900 Ma) detrital zircons have been found in Proterozoic sediments such as the Yanbian Group and the Kunyang Group along the western margin of the Yangtze Block, and the Zengcheng and Xunwu gneiss in the southern part of the Cathaysia Block (Sun et al., 2009a; Greentree and Li, 2008; Li et al., 2006; Wang et al., 2008; Yu et al., 2008). This distribution suggests that some Grenville orogens were probably close to the western Yangtze Block and southern Cathaysia, respectively. These identified orogens are younger about 100–300 Ma by than the typical Grenville orogen in Texas, the Grenville Province in southern Laurentia and the Albany-Fraser and Musgrave orogeny in central Australia (1300–1050 Ma), but similar to the Eastern Ghats (∼960 Ma) in India and Northern Prince Charles Mountains (990–960 Ma) in East Antarctica. Therefore it is more reasonable to place South China on the margin of Rodinia, matching the identified orogens to those in East India and East Antarctica (Fig. 8).

5.3.2. Columbia An older and larger age population identified from this study is ∼1850 Ga, which is consistent with the global-scale 2.0–1.8 Ga collisional events that led to the assembly of the Columbia supercontinent (Zhao et al., 2004). However, the Yangtze Block, which was long considered to be a younger continental fragment, has not been positioned in this Paleo-Mesoproterozoic supercontinent. The presence of the abundant ∼1850 Ma detrital zircons in this study, coupled with a few Paleoproterozoic granites and metamorphic rocks (1.8–2.0 Ga) in the northern part of the Yangtze Block, could have important implications for the possible location of the Yangtze Block in Columbia. Furthermore, the age population of ∼2.3 Ga, which is rarely identified elsewhere in the world, could be good indicator for tracing its possible neighbors.

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Fig. 8. Position of South China in Rodinia (Yu et al., 2008). Global reconstruction of Rodinia (inset) after Hoffman (1991). The arrows denote transport directions for detritus. Abbreviations: SC, South China; NPCMs, Northern Prince Charles Mountains; In, India; Aus, Australia; Ka, Kalahari; E-Ant, East Antarctica; Lau, Laurentia; Sib, Siberia; Con, Congo; Amz, Amazonia; Bal, Baltica; W-Afr, West Africa.

Fig. 9. A reconstruction of the Columbia supercontinent modified after Zhao et al. (2002a,b), showing age distributions of zircons from the continents (Yang et al., 2009; Mapeo et al., 2006; Griffin et al., 2004, 2006; Condie et al., 2009).

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Fig. 10. Possible position of the Yangtze Block in the Columbia Supercontinent.

Based on the available lithostratigraphic, tectonothermal, geochronological and paleomagnetic data from 2.1–1.8 Ga collisional orogens, it has been possible to establish links between South America and West Africa; between Western Australia and South Africa; between Laurentia and Baltica; between Siberia and Laurentia; between Laurentia and Central Australia; between East Antarctica and Laurentia, and between North China and India (Zhao et al., 2002a,b, 2003a,b). Comparing three older age populations of ∼2300 Ma, 2450–2500 Ma and 2700–2900 Ma in the Paleoproterozoic sediments in the Yangtze Block to those from the other blocks in Columbia supercontinent indicates that only Australia and North China have all three tectonic events, suggesting a link between the Yangtze Block and these two blocks. In North China, the Paleoproterozoic Trans-North China Orogen has been considered to be one of the global orogens which marked the assembly of Columbia (Zhao et al., 2002a, 2008a,b, 2010a; Guan et al., 2002; Wang et al., 2010a,c). In this orogen, Paleoproterozoic lithologies have been identified in the Luliang Complex, including pre-collisional 2375 ± 10 Ma and 2199–2173 Ma granitoid gneisses, ∼2360 Ma basalts, andesites, dacites and rhyolites, syn-collisional 1832 ± 10 Ma garnet-bearing gneissic granites and pos-orogenic 1815 ± 5 Ma charnockites, 1807 ± 10 Ma porphyritic granite and 1798–1790 Ma massive granites (Geng et al., 2000; Wan et al., 2000; Lu et al., 2006, 2008; Zhao et al., 2008a). A recent study (Liu et al., 2009) reported magmatic zircon from the tonalite with ages from 2829 ± 18 Ma to 2832 ± 11 Ma and ages from 2838 ± 35 Ma to 2845 ± 23Ma in amphibolite from Lushan in the far south of the Trans-North China Orogen of the North China Craton. Subsequently,

metamorphic zircon growth occurred between 2772 ± 17/22 in the tonalites and 2776 ± 20 and 2792 ± 12 Ma in the amphibolites. Detrital zircons in sand samples from the Yellow River, the Luan River and the Yongding River in North China Craton have major age groups of 1.6–2.0 Ga, 2.1–2.5 Ga and a Hf model age peak at 2.7–2.8 Ga, which agree well with the coeval igneous and metamorphic rocks that are widespread in the North China Craton (Yang et al., 2009). Paleoproterozoic orogens are widespread in Australia, such as the ∼1850 Ga Pine Creek Orogen (Carson et al., 2008), 1860–1850 Ma Halls Creek Orogen (Bodorkos et al., 2000), 1844 Ma Granites in Tanami Orogen (Smith, 2001) and 1880–1730 Ma Arunta Orogen (Collins and Shaw, 1995). Swager and Nelson (1997) noted gneissic granites with igneous emplacement ages of ∼2675 Ma that postdate the bulk of greenstone volcanism (2720–2675 Ma) in the Eastern Goldfields Province of the Yilgarn Craton. Nelson (1997a,b) also mentioned a 2738 Ma deformed granitic gneiss in the northern part of the province. Detrital zircons from modern drainages across the northern part of the Yilgarn Craton show similar age peaks at ∼1.8 Ga, ∼2.33 Ma and 2.7–2.8 Ga (Griffin et al., 2004). Murgulov et al. (2007) reported ∼1.8 Ga and ∼2.3 Ga detrital zircons from the Georgetown Inlier, North Queensland, Australia. Some inherited zircons with ages of ∼2.3 Ga and 2.4–2.5 Ga were found in the 1.8 Ga basement rocks in the western Mt Isa Inlier, northeastern Australia (Bierlein et al., 2008). Although both ∼2300 Ma and 2700–2900 Ma events took place in Africa and America (Condie et al., 2009), the 2450–2500 Ma magmatism is absent. In India, ∼2300 Ma magmatism has not been

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recognized. In other blocks, none of these three age populations has been found (Fig. 9). In the adjacent Cathaysia Block, which lies to the south of the Yangtze Block (Fig. 1), Paleoproterozoic magmatism and metamorphism took place at 1888–1855 Ma in the Wuyishan area (Yu et al., 2009, 2010). All the 1.86–1.90 Ga zircons from these granites show similar Hf-isotope compositions, with Hf model ages clustering at ∼2.8 Ga. Coupled with inherited zircons (ca. 2.5–2.7 Ga) and geochemical characters of Paleoproterozoic granitic rocks and the metamorphic rocks, these authors proposed that Paleoproterozoic overprint on an Archean basement in the Wuyishan terrane, which was a part of Columbia supercontinent. The detrital zircon dating results for sedimentary rocks in the Cathaysia Block indicate an Archean crust (∼2.5 Ga) similar to those of North China, India and Antarctica (Yu et al., 2008). The similarity of Paleoproterozoic crust in South China, North China and Australia suggests that South China could be positioned in the western part of the supercontinent, close to North China or/and Australia, in the reconstruction of Columbia (Fig. 10). However, this possibility needs more evidences to confirm in our future work. 5.4. Provenance of the sedimentary rocks Our new zircon U–Pb age dating, together with recently published data (Zhang et al., 2006c), indicate that Proterozoic sedimentary rocks around the Yangtze Block are probably derived from different sources. The three samples from the Yinmin Formation contain more Archean (2700–2850 Ma) to Paleoproterozoic (2300–2450 Ma, ∼1850 Ma) age populations, broadly similar to those found in the magmatic and metamorphic rocks in the northern Yangtze Block. Nevertheless, in detail, some differences have been identified between the age populations of detrital zircons in this study and those from igneous and metamorphic rocks. In the northern Yangtze Block, the best-represented age populations are 2.9–3.0 Ga, 1.9–2.0 Ga and ∼1.85 Ga (Zhang et al., 2006c; Qiu et al., 2000), slightly older than those obtained from the western Yangtze Block. The 2300–2450 Ma magmatism is absent in the northern Yangtze Block (Fig. 11). Although the Quanyishang granites in the Yichang area have a crystallization age of 1854 Ma, they have much lower 176 Hf/177 Hf (Xiong et al., 2009) than the ∼1.8 Ga zircons in this study, suggesting different origins of the magmas in the two areas. Therefore, the currently exposed Archean to Paleoproterozoic basement was not a major source of these Paleoproterozoic sediments. If the Yangtze Block was a part of Columbia, the adjacent North China Craton and Australia could be potential sources for these sediments. Consequently, the Paleoproterozoic sediments would be largely sourced from an exotic or now totally concealed source region. However, the euhedral morphology of some Paleoproterozoic zircons suggests a short-distance transport, favoring the now-covered Yangtze Block as the major source region. Similar age populations also have been identified in the Dahongshan Group to the southwest (Greentree and Li, 2008), providing further evidence for the existence of the Archean to Paleoproterozoic crust in the western Yangtze Block. The Heishantou Formation has a younger age population of 1.0–1.3 Ga, equal to the global Grenvillian orogenic period. A previous study on the Kunyang Group (Greentree et al., 2006) reported a similar age population in the coherent sedimentary sequence (Laowushan Formation), which was considered to be sourced from an uplifted Grenvillian orogenic belt and deposited in a foreland basin over an already-thinned continental crust. However, based on the U–Pb and Lu–Hf analysis of zircons from the Kunyang Group and coeval Huili Group, Sun et al. (2009a) argued that these sediments were derived from an arc source and formed in a subductionrelated setting and infer that they were most likely associated with E–W subduction along the western margin of the Yangtze Block.

Fig. 11. Probability density plots comparing U–Pb ages for detrital zircons in western Yangtze Block with rock ages in the Yangtze Block and the Cathaysia Block.

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Fig. 12. “Event signature” curves combining zircon ages with calculated crustal model ages for the study area. Source crustal residence time is the time between the separation of the source from the depleted mantle and the crystallization of the zircon. In this plot, an upward trend with decreasing age indicates juvenile input, while a downward trend implies reworking of older crust.

Grenville-aged igneous rocks are rarely exposed in the Yangtze Block. To the north of our study area, zircons from granitic gneiss in the Kangding Complex of Sichuan Province form a concordant population with a mean 207 Pb/206 Pb age of 1007 ± 14 Ma (Li et al., 2002). In the reconstruction models of the Rodinia supercontinent (Li et al., 2002; Yu et al., 2008), those adjacent blocks could be potentially possible source regions for these Grenvillian zircons. However, the euhedral character of the detrital zircons implies a short transport. Therefore, the clastic sediments of the Mesoproterozoic sequences were probably derived from these nearby coeval igneous rocks in the western margin of the Yangtze Block. Slightly younger sedimentary sequences such as the Yanbian Group have a dominant age population ranging from 860 Ma to 1000 Ma as well, providing further evidence for the occurrence of the widespread Grenvillian material along the western margin of the Yangtze Block. In general, the Sinian sediments in this study have essentially uniform zircon age distributions ranging from 750 Ma to 850 Ma, with peaks at ∼760 Ma or ∼800 Ma, similar to those of the Bikou, Lieguliu and Guanyinya Formations, suggesting uniform and proximal sources. Mafic–ultramafic intrusions (820 Ma Wangjiangshan, 814 Ma Beiba, 780 Ma Bijigou, 746 Ma Luojiaba and 762 Ma Tianpinghe), 792 Ma granites and 782 Ma adakites are found in the Hannan area on the northwestern margin of the Yangtze Block (Zhao and Zhou, 2009a,b; Pei et al., 2009). Zircons from the Dadukou (746 ± 10 Ma, Zhao and Zhou, 2007), Tongde (813 ± 14 Ma, Sinclair, 2001; 820 Ma, Li et al., 2003), Gaojiacun (806 ± 4 Ma, Zhou et al., 2006) and Shaba mafic plutons (751 ± 12 Ma, Li et al., 2003) in the Panzhihua area have εHf (t) ranging from +1.9 to +11.6. Their TDM model ages range from 0.86 to 1.28 Ga. The adjacent Datian and Dajianshan adakitic plutons have zircons with ages of 760 ± 4 Ma and εHf (t) ranging from +2.26 to +11.7 and TDM model ages from 0.86 to 1.24 Ga (Zhao and Zhou, 2007). Most zircons from these igneous rocks have features similar to those of detrital zircons in this study. Therefore, the widespread Neoproterozoic igneous rocks along the western margin could

be the sources for these detrital zircons in the Neoproterozoic sediments. Fig. 12 shows the similarity of event signatures during Archean and Paleoproterozoic times for the source areas of the Kunyang Group and the younger sandstones in this study and Sun et al. (2008), suggesting that the old materials in the Sinian and Cambrian sediments were sourced from the Paleoproterozoic sedimentary rocks of the Yinmin Formation or from the same basement rocks. Some Paleoproterozoic zircons in the Yinmin Formation show euhedral characters, while the Paleoproterozoic zircons from the younger sandstones have rounded morphology. It is suggested that the old materials in the younger sediments could be derived from reworking of Paleoproterozoic sediments. 6. Conclusions In this study, six major U–Pb age peaks are defined by zircons from Proterozoic sedimentary rocks: 750–850 Ma, 1000–1300 Ma, 1600–1800 Ma, ∼1850 Ma, 2300–2450 Ma and 2700–2850 Ma, suggesting six important periods of crustal magmatic activity in the Yangtze Block. Significant juvenile input took place during Archean and Neoproterozoic times, while the Proterozoic events are dominated by crustal reworking. Comparison with the timing of magmatic and metamorphic events in the possible adjacent blocks within previous supercontinents suggests that the Yangtze Block was probably close to North China or/and Australia in the Paleoproterozoic, and to India and the Antarctic during the MesoNeoproterozoic. Most of the detrital material was derived from the now-covered parts of the Yangtze Block, with minor components from exotic source regions such as North China and/or Australia. Acknowledgements This research was supported by an ARC Discovery and Linkage grants (SYO’R and WLG). It used analytical instruments and

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laboratories at GEMOC, Department of Earth and Planetary Sciences funded by ARC LIEF, and DEST, Systemic Infrastructure Grants, industry partners and Macquarie University. We thank Norman Pearson, Mei-Fei Chu, Elena Belousova, Shelley Allchurch and Justin Payne for their assistance when Lijuan Wang was undertaking the LAM-ICPMS and LAM-MC-ICPMS analyses. The study was also supported by National Natural Science Foundation of China (Grant Nos. 40972127 and 41102123) and a grant (no. 2008-I-01) from the State Key Laboratory for Mineral Deposit Research (Nanjing University). Liang Luo and Zhenyu He are thanked for help with the field investigations and zircon separation. This is contribution number 728 from the ARC National Key Center for Geochemical Evolution and Metallogeny of Continents (http://www.gemoc.mq.edu.au). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.precamres.2011.08.001. 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