Geochemical constraints on the origin of post-depositional fluids in sedimentary carbonates of the Ediacaran system in South China

Precambrian Research 224 (2013) 341–363

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Geochemical constraints on the origin of post-depositional fluids in sedimentary carbonates of the Ediacaran system in South China Yan-Yan Zhao ∗ , Yong-Fei Zheng CAS Key Laboratory of Crust-Mantle Materials and environments, School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, China

a r t i c l e

i n f o

Article history: Received 20 June 2012 Received in revised form 10 October 2012 Accepted 12 October 2012 Available online 22 October 2012 Keywords: Ediacaran carbonate Isotope Trace elements Post-depositional fluid Continental deglaciation

a b s t r a c t In order to evaluate the preservation of primary geochemical signatures in sedimentary carbonates and the effect of post-depositional fluids, a combined study of C O isotopes and trace elements was carried out for different microfacies of the Lantian carbonates in the southern Anhui, which is equivalent to the Doushantuo Formation of the Ediacaran system elsewhere in South China. Stratigraphically the Lantian carbonate can be subdivided into the Upper and Lower Units. For the Upper Unit (UU), petrographic observations indicate that the wallrock carbonates experienced only limited recrystallization. The REE + Y patterns of wallrock carbonates with CaO contents higher than 53% are consistent with those treated by the acetic acid solution, representing the primary records of carbonates deposition. The veins and wallrock carbonates in this unit exhibit consistent REE + Y patterns and similar ı13 C and ı18 O values, suggesting the same origin of their depositional fluids. In contrast, there are significant differences in the geochemical signatures between veins/cements and wallrock carbonates in the Lower Unit (LU), suggesting that different origins of their depositional fluid. The REE + Y patterns for calcite veins and cements in the both units indicate incorporation of terrigenous materials into post-depositional fluids. With respect to correlations between ı13 C and ı18 O values, negative one occurs in the UU, whereas positive one occurs in the LU. Unusually low ı18 O values of −28.6‰ to −22.0‰ for the veins and cements in the both units indicate that the post-depositional fluids were derived from negative ı18 O continental freshwater subsequent to the Gaskiers and Marinoan iceages. The REE + Y patterns and ı18 O variations for the veins in UU suggest the parent fluid contained a component of continental deglacial meltwater. The unusually low ı18 O values also occur in the veins and cements of LU, indicating a significant contribution from meteoric water. On the other hand, the ı13 C values for the veins and cements of the both units suggest that the same carbon sources as the wallrock. The present results provide insights into the preservation of primary geochemical records in sedimentary carbonates and into the effect of post-depositional processes on the Precambrian carbonates. In particular, the origin of post-depositional fluids can be distinguished from the depositional fluids by means of the combined trace element and stable isotope tracing. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The element and isotope compositions of sedimentary carbonate rocks can serve as useful proxies for the geochemical features of ambient water and environmental evolution. However, carbonates can be altered by post-depostional fluid, depending on water/rock (W/R) ratio, temperature and composition of infiltrating fluid (e.g., Bathurst, 1975). This may impart a secondary signal and therefore the measured geochemical data may not accurately reflect their primary features (e.g., Holser, 1997; Walter et al., 2007). In this regard, it is critical to evaluate whether the geochemical proxies

∗ Corresponding author. Tel.: +86 551 3601567. E-mail address: [email protected] (Y.-Y. Zhao). 0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2012.10.014

were altered by post-depositional processes. In addition, postdepositional microfacies, such as veins and cements, may record the signature of post-depositional fluids. Thus, a comparative study of different carbonate microfacies may provide important information about the diagenetic history of sedimentary rocks and thus about the preservation of primary signatures in sedimentary carbonates. In this paper, we refer the depositional fluid as the fluid from which the wallrock carbonate is precipitated, and the post-depositional fluids as the fluid that acts after the carbonate precipitation. The post-depositional processes of sedimentary carbonates include diagenetic lithification and burial metamorphism under the conditions of low temperatures or slightly elevated temperatures. These stages usually involve compaction dissolution by internally buffered fluids and the infiltration of external fluids, such as

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meteoric water at low temperatures without alteration of carbonates (e.g., Jacobsen and Kaufman, 1999). However, at relatively high temperatures, external fluids can significantly modify the primary geochemical records of carbonate minerals (e.g., Mahon et al., 1998; Thyne, 2001; Morrill and Koch, 2002; Kennedy et al., 2008; Knauth and Kennedy, 2009). Nevertheless, the post-depositional alteration of sedimentary carbonates could also occur when neither temperature nor isotope composition of porewater is different from that of depositional water (e.g., Wilson et al., 2000; Garzione et al., 2004; Jaffrés et al., 2007). In this regard, the exchange of elements and isotopes between carbonate minerals and porewater can be considered as the internal processes, with little or no change during the rise of temperature and the overflow of water. Therefore, it is important to constrain the origin of post-depositional fluids and their relationships to adjacent host carbonates. The vein and cement carbonates occur widely in sedimentary carbonate rocks and their geochemical compositions can provide constraints on the origin of post-depositional fluids (Sample, 1996; Fayek et al., 2001; Wang et al., 2010), the scale of porewater flow (Campbell et al., 2002; Garzione et al., 2004), the temperature of diagenetic alteration (Tritlla and Cardellach, 1997; Yao and Demicco, 1997; Tritlla et al., 2001; Suchy et al., 2002; Suchy´ et al., 2010), and the relative time of fluid infiltration (Campbell et al., 2002; Hood et al., 2003; Wanas, 2008). Thus, the study of vein and cement carbonates is essential to deciphering the post-depositional history of carbonates (Fantle and DePaolo, 2006, 2007; Fantle et al., 2010; Marfil et al., 2005). Various geochemical proxies, such as ı18 O value and its relationship with ı13 C values and Mn/Sr ratio have been used to evaluate the preservation of primary signatures during carbonate diagenesis and the origin of post-depositional fluids (e.g., Jacobsen and Kaufman, 1999; Knauth and Kennedy, 2009; Derry, 2010). For example, carbonates deposited in shallow marine and continental shelf environments are susceptible to diagenetic alteration by meteoric water. As a consequence, Sr is expelled from the marine carbonate, whereas Mn is incorporated in it (e.g., Le Guerroué et al., 2006a). The O isotope composition of marine carbonates is sensitive to the meteoric diagenesis because of the difference in ı18 O value between seawater and meteoric water (e.g., Jacobsen and Kaufman, 1999). The C isotope composition of post-depositional fluid generally changes with that of dissolved CO2 in the fluid, which generally has lower ı13 C values than seawater (thus marine carbonates). Therefore, the water–rock interaction is evident when the post-depositional fluid of different element concentrations and isotope compositions was infiltrated into the sedimentary carbonates (e.g., Derry, 2010). In this paper, we present a combined study of stable isotopes and trace elements on different microfacies such as vein, cement and host carbonates from the Lantian Formation of the Ediacaran system in South China. We also performed high-resolution analyses of these geochemical parameters for wallrock adjacent to the veins and cements, and through the veins themselves. We aim to obtain the primary geochemical records of sedimentary carbonates and constrain the origin of post-depositional fluids.

Fig. 1. Simplified geological map showing the occurrence of late Neoproterozoic sedimentary rocks in southern Anhui (insert) between the Yangtze and Cathaysia Blocks in South China. Asterisk denotes the sample locality.

microcrystalline or sandy dolomite (Zhou et al., 2001; Jiang et al., 2003b; Zhao et al., 2009). The second unit is characterized by interbedded black organic-rich shale and calcareous mudstone, with diverse metaphytes in its upper part (Yuan et al., 1999). The third is the upper carbonate unit (Upper Unit, UU), which was deposited on the upper part of the Lantian Formation. It is composed of gray to black medium-bedded, micritic dolomitic limestone and thinly bedded interlayered micritic limestone and argillaceous limestone (Zhao et al., 2009). The fourth unit consists of silty slate and carbonaceous chert (Fig. 2). The LU and UU carbonates of the Lantian Formation are probably equivalent

2. Geological setting The Lantian Formation sharply overlies the Marinoan Leigongwu Formation (Fig. 1) and is equivalent to the Doushantuo Formation of the Ediacaran system elsewhere in South China (e.g., Jiang et al., 2003a, 2006; Condon et al., 2005). It can be divided into four lithological units (Zhou et al., 2001; Zhao et al., 2009; Zhao and Zheng, 2010). The first is the lower carbonate unit (LU) that directly overlies the Leigongwu Formation diamictite, representing the Marinoan cap carbonate. It is composed of gray and thinly bedded

Fig. 2. Carbon and oxygen isotope compositions of bulk-analyzed carbonate from the Upper and Lower Units of the Lantian Formation in southern Anhui, South China. Master stratigraphic column is shown in the left; only the main rock types are shown for each formation. Approximate stratigraphic positions for ı13 C and ı18 O profiles are indicated. The ages are cited from Condon et al. (2005). Abbreviations: LGW: the Leigongwu Formation, LU: the Lower Unit in the Lantian Formation, UU: the Upper Unit in the Lantian Formation, LT: the Lantian Formation, PYC: the Piyuancun Formation.

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The sedimentary carbonate often contains some non-carbonate materials such as siliciclastics, oxide and sulfide, which can influence the element concentrations of carbonate during analyses. The acetic acid cannot release the element of non-carbonate materials and thus it is the popular method for carbonate analysis. But, the acetic acid is not proper for the high-resolution analysis of different microfacies in carbonate (such as spar, mineral, vein or cement). LA-ICP-MS has been applied to the microfacies in the present study. The in situ analysis of trace elements in carbonates can be quantitatively conducted by LA-ICP-MS. This is exclusively performed by the external calibration combined with the internal standard (generally Ca), which is determined either by independent methods (e.g. electron microprobe), constant stoichiometry of the mineral (e.g., Mertz-Kraus et al., 2009), or internal standard-independent calibration strategy of bulk components as 100% (e.g., Chen et al., 2011). Because the carbonates of the Lantian Formation contain a few non-carbonate materials (Zhao et al., 2009), only the external calibration strategy can be used in this study. 3.1. XRD analysis Mineralogical characterization of the sedimentary carbonates was made using X-ray diffractometry (XRD) techniques (Tucker, 1988), which was carried out at State Key Laboratory of Mineral Deposits at Nanjing University, Nanjing. The samples were scanned with a Rigaku D/max-IIIa diffractometer equipped with a Cu-target tube and a curved graphite monochromator, operating at 37.5 kV and 20 mA. Samples were step-scanned from 3◦ to 70◦ with a step size of 0.02◦ (2). A side-packing method proposed by National Bureau of Standards (NBS) was used to prepare the XRD samples. 3.2. Elemental concentration

Fig. 3. Occurrence of carbonate vein and wallrock in the Upper Unit (a) and Lower Unit (b) of the Lantian Formation. The red spots are the microsampling locations for the carbon and oxygen isotope analyses. Note the laminations with different colors develop in the wallrock of sample 05WN167. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to the post-Marinoan and post-Gaskiers deposits, respectively (Hoffmann et al., 2004; Zhou et al., 2004; Condon et al., 2005; Zhang et al., 2005; Zhao and Zheng, 2010). There are different occurrences of vein across different laminations in the Lantian carbonates (Fig. 3). Our carbonate samples were collected from the UU and LU carbonates at Shiyu section, and both vein and carbonate samples were analyzed for their major-trace element contents and C O stable isotope compositions. 3. Analytical methods It is known that different measurement methods of trace elements can yield somewhat different results (e.g., Baily et al., 2000; Li et al., 2011). For example, the abundances of trace elements may not be the same if they are measured by the leaches in HNO3 and HF solutions (de Alvarenga et al., 2008, 2004; Frimmel and Lane, 2005), HCl solution (Singh et al., 1998), H3 PO4 solution (Azmy et al., 2001), acetic acid solution (Ling et al., 2007), procedure-sequential leaching (Zhao et al., 2009; Li et al., 2011), or by laser ablationinductively coupled plasma-mass spectrometer (LA-ICP-MS) (Chen et al., 2011). Thus, it is necessary to find a proper method of analysis for our samples.

Major elements were analyzed by electron microprobe (EMP) at State Key Laboratory of Lithospheric Evolution in Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS), Beijing. A JEOL-JXA 8100 automated microanalyzer was used with a beam current of 10 nA, an accelerating voltage of 15.0 kV. Counting time was 50 s on peak and 20 s each on two background measurements, with a probe beam size of 1 ␮m. Trace element concentrations in vein and wallrock carbonates were measured in situ on thin sections by LA-ICP-MS at CAS Key Laboratory of Crust-Mantle Materials and Environments in University of Science and Technology of China, Hefei. Laser sampling was performed using a Geolas 2005 (MicroLas, Germany) system equipped with a 193 nm ArF-excimer laser. Helium was used as the carrier gas to enhance transport efficiency of the ablated material. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. An Agilent 7500a ICP-MS was used to acquire ion-signal intensities. Sampling pits have diameter of about 90 ␮m. The laser frequency was 8 Hz and the energy density was 14 J cm−2 , and the laser depth was about 10–20 ␮m. Each analysis includes an approximately 20 s background acquisition followed by 50 s data acquisition from the sample. Data reduction was made using the GLITTER 4.4 software (Macquarie University; Achterbergh et al., 2001). Calibration was performed using NIST SRM 610 as an external standard in conjunction with internal standardization using Ca following the method of Longerich et al. (1996) and Günther et al. (1999). Concentration values of NIST SRM 610 used for external calibration were taken from Pearce et al. (1997). Analyses of USGS rock standards (BCR2G and BHVO-2G) indicate that the precision (1, relative standard deviation) is better than ±10% for rare earth and other trace elements. Limits of detection (LOD) for these USGS standards were described by Gao et al. (2002) in detail. LOD for each element and analysis were calculated individually as three times the standard

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deviation of the background signal (taken before ablation) divided by element sensitivity during the respective ablation. Before the trace element analyses by LA-ICP-MS, minerals were analyzed by EMP on the same thin sections. In general, the LA-ICPMS spots overlapped the EMP spots in order to make the internal calibration of trace element concentrations. If not, the minerals were identified and their chemical compositions were estimated from known studies. In this case, it did not affect the calculation of trace element compositions greatly because the CaO contents of all individual laminations and veins vary within a limited range. Since the EMP results exhibit consistent CaO contents for individual microfacies in the wallrock and vein carbonates, the internal calibration using the average CaO contents for the same microfacies in the wallrock is acceptable for the trace element analyses. Although this protocol could affect the absolute concentrations of the all trace elements to some extent, it has no influence on the shape of normalized patterns in the REE + Y distribution diagram. It was noted that carbonate standards for the trace element analysis of carbonates are not readily available. The standard used in this study was a NIST SRM 610 silicate glass, with trace element concentrations reported by Hinton (1999). While previous studies have suggested that the matrix effect is essential for the LA-ICPMS analysis, there are increasing observations to suggest that the matrix effect of matching the standards is not essential when using the ArF excimer laser for the analysis of REE in carbonates (Strand et al., 2009). As a result, relatively accurate and reproducible results can be obtained from different matrices using the ArF excimer laser (193 nm) with the NIST glass standards (Fallon et al., 2002; Wyndam et al., 2004; Strand et al., 2009). Moreover, Chen et al. (2011) reported that the accuracy of REE calibrations against different strategies at large spot sizes (>44 ␮m) are consistent with each other. This indicates that our samples can be calibrated against NIST SRM610. This is also confirmed by a comparison of the present LAICP-MS data with the previous analyses by the solution method on the same samples of wallrock carbonates (Zhao et al., 2009). Thus, the results from the present LA-ICP-MS analyses are applicable to geochemical tracing. 3.3. Isotope composition Carbon and oxygen isotopes were analyzed for both bulk carbonates and subsamples by microdrilling. The samples were broken into chips and pulverized for the bulk analyses. In order to do isotopic analysis of individual microfacies, we polished the carbonates into thin slices for subsampling. A high-speed drill of Leica Micromill was used to acquire 20–50 ␮g powders of vein and wallrock carbonates on the polished slices. The C and O isotope compositions were analyzed by the GasBench II technique in the continuous flow mode and the extracted CO2 gases were measured on a Finnigan MAT253 mass spectrometer at CAS Key Laboratory of Crust-Mantle Materials and Environments in University of Science and Technology of China in Hefei. Details of this procedure and analytical techniques were described by Zha et al. (2010). Carbon and oxygen isotope data are both reported relative to the standards Pee Dee Belemnite (PDB) in the conventional ı13 C and ı18 O notations, respectively. Precisions and reproducibilities of both ı13 C and ı18 O analyses are better than ±0.2‰ based on replicate analyses of internal standards and samples. Two carbonate reference materials were used to monitor daily isotope analyses, with ı13 C = 1.95‰ and ı18 O = −2.20‰ for International Standard NBS-19, and ı13 C = −6.06‰ and ı18 O = −24.12‰ for National Standard of China GBW04417 (Zheng et al., 1998). While the PDB standard was used throughout for the both ı13 C and ı18 O values of sedimentary carbonates, the VSMOW standard was only used for denoting the ı18 O values of water and magmatic rocks as well as associated calculations involving water/fluid in discussion

(Zhao and Zheng, 2010). The ı18 O conversion relationship between the PDB-VSMOW scales is ı18 O(VSMOW) = 1.03091 ı18 O(PDB) + 30.91 according to Hoefs (2009). 4. Results 4.1. Petrographic observation The micrite carbonates in the Upper Unit are calcitic and those in the Lower Unit are dolomitic. The veins, cements, and adjacent wallrocks in these samples were selected for the geochemical analyses. However, the veins distributing along the dissolution gaps or laminations, which obviously formed recently, are excluded in sampling. 4.1.1. The Upper Unit Microfacies are mostly composed of micrite wallrock, and veins, with few sparry cements (Fig. 4). The wallrock is composed of the interlocking mosaic, ghosts of clots and peloids. The mosaic ghosts of clots and peloids are micron-sized, equant calcite crystals with the rather uniform crystal size, shape and low porosity (Fig. 4). The minerals are mainly calcite, quartz and illite as indicated by the XRD analyses (Fig. 5a–c). The peloids or clots show heterogeneous distribution in the size without recrystallization, but the replacement occurs on the interface of vein and wallrock. The minerals exhibit no petrographic evidence of diagenetic alteration such as dissolution, recrystallization and overgrowth (Fig. 4). Pyrite occurs in a few samples as scattered, small (up to 100 ␮m), euhedral grains. The veins are across the different laminations, and their shapes look somewhat like arborization, tube or grain (Fig. 3a and b). Some of the veins consist of radial cluster and acicular sparry calcite and some are composed of obviously allotriomorphic to hypautomorphic granular sparry calcite with a diameter about 1 mm (Fig. 4). The acicular calcite is common for early diagenetic, porosity-occluding phase (Tobin et al., 1999). Elongated veins are filled with two generations of acicular and granular calcite crystals in sample 05WN167 (Fig. 4a). Tiny crystals often occur in many single calcite crystals in the vein of sample 05WN164 (Fig. 4c and f), which were probably formed by the interlocked crystals. The gaps tearing calcite crystals also occur in the veins of sample 05WN164 (Fig. 4f and g). 4.1.2. The Lower Unit The LU carbonate is composed of microcrystalline to mesocrystalline minerals as interlocking mosaics of micron-sized, equant dolomite crystals (Fig. 6a and b). The fine matrix is mainly composed of dolomite, calcite, quartz, chlorite, and few non-carbonate materials as indicated by the XRD analyses (Fig. 5d and e). The partial replacement can be observed in the minerals of wallrock (Fig. 6g). The fracture lines in sample 05WN155 were led by the surfacial exposure and the voids were completely filled with calcite cements (Fig. 6c). The veins often occur as granular sparry calcite fragment or tube. Especially in sample 05WN155, the cement and wallrock are interpenetrated (Fig. 6d and f). The fragment of both wallrock and cement are granular and angular, with a diameter of about 3 mm (Fig. 6d and f). Two vein generations with different colors under the plane-polarized light can be distinguished from the cements of sample 05WN155 (Fig. 6e). However, the sequence of cementation cannot be distinguished because these cements share the same calcite grains (Fig. 6e). 4.2. Major elements 4.2.1. The Upper Unit For the UU, the wallrock and vein carbonates exhibit similar CaO and MgO contents of 54.73–56.83 wt.% and 0.08–0.32 wt.%,

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Fig. 4. Optical photomicrographs of carbonate vein and wallrock. (a) Acicular and granular calcite crystals in the vein of sample 05WN167 (scale bar is 100 ␮m). (b) Optical photomicrographs of carbonate vein and wallrock in sample 05WN75. Acicular or fibrous calcite crystals fill the veins. The red numbers are the locations for in situ major and trace element analyses. (c) Optical photomicrographs of vein calcites in sample 05WN164. It should be noted that the calcite crystal is broken by a gaps and fine crystals occur in the large crystal. (d) Brocken interior surfaces of the wallrock in sample 05WN68. BSE image of carbonate vein and wallrock in samples 05WN164 (e) and (f), 05WN167 (g) and 05WN73 (h). The red numbers in (e)–(h) are the location for in situ major element analyses and shown in Fig. 8. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. XRD patterns for sample of (a), (b) and (c) the Upper Unit, (d) and (e) the Lower Unit. The signs are abbreviations of the mineral: Ab, NaAlSi3 O8 ; C, calcite; Chl, chlorite; D, dolomite; I, illite; Py, pyrite; Q, quartz.

respectively (Fig. 7b, Table 1), except sample 05WN164. The minerals in the wallrock and vein are mainly calcite, with few siliciclastic materials. The siliciclastic contents are much higher in the wallrock than those in the veins as suggested by the parallel trends of SiO2 , Al2 O3 and Fe2 O3 (Table 1). For instance, the SiO2 contents in the wallrock vary between 0.23 and 11.5 wt.%, whereas those in

the veins are only between 0.01 and 0.08 wt.% (Fig. 8). In general, the veins are depleted in Mg, Fe, Al, Ba, Rb, Pb, Na, Th and U, but enriched in Sr relative to the adjacent wallrock (Fig. 7a–d). Sample 05WN164 is different from other samples, which is mainly composed of quartz, calcite and dolomite (Fig. 5b). The wallrock of this sample have lower CaO, but higher MgO contents of

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Fig. 6. Optical photomicrographs of carbonate vein and wallrock in samples (a) 05WN58, (b) 05WN155, (c) 05WN155. It should be not that the stylolite lines occur in sample 05WN155. (e) The wallrock of sample 05WN157 and (e) the vein of sample 05WN155. The red numbers are the locations for in situ major and trace element analyses. (f) BSE image of vein and wallrock in sample 05WN155. The red number is the spot of Fig. 11a. (g) and (h) Brocken interior surfaces of the wallrock in sample 05WN157. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 7. The concentrations of major and trace elements in vein and wallrock carbonates of (a)–(d) the Upper Unit and (e)–(f) the Lower Unit.

28.22– 47.75 wt.% and 17.56–18.77 wt.%, respectively, than that of other samples(Fig. 7a). The bright wallrock matrix has much lower SiO2 contents than the dark peloid and clots (Fig. 7a). The major element contents of veins in this sample are consistent with those of the bright wallrock matrix.

4.2.2. The Lower Unit The wallrock has CaO contents of 29.07–30.47 wt.% and MgO contents from 17.79 to 20.56 wt.% (Fig. 8a, Table 1). It consists of dolomite (Fig. 5d and e), with Mg/Ca ratios of about 0.6. The other minerals include quartz, chlorite, calcite, and albite (Fig. 5d and e).

Granular sparry calcite is the main composition of veins, with CaO contents of 53.05–54.17 wt.% (Fig. 8a, Table 1). There are few other chemical components such as MnO and SiO2 that are lower than 0.06 wt.% and 0.03 wt.%, respectively (Table 1). Nevertheless, the veins have much higher CaO, but lower SiO2 than the wallrock (Fig. 8c). The veins are depleted in Mg, Fe, Al, Rb, Na, Th and U, but enriched in Sr relative to the wallrock on the whole (Fig. 7e and f). 4.3. Trace elements The trace element concentrations of carbonates from the two units of the Lantian Formation are presented in Table 2. REE + Y

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Table 1 EMP analyses of major elements in vein and wallrock carbonates from the Lantian Formation in southern Anhui. No.

Type

SiO2

MgO

FeO

CaO

P2 O5

MnO

ZrO2

SrO

CO2

Total

Upper Unit 05WN69-1 05WN69-2 05WN69-3 05WN69-4 05WN73-1 05WN73-2 05WN73-3 05WN73-4 05WN73-5 05WN73-6 05WN75-5 05WN75-1 05WN164-1 05WN164-2 05WN164-3 05WN164-4 05WN164-5 05WN164-6 05WN164-3 05WN167-1 05WN167-2 05WN167-3 05WN167-4 05WN167-5 05WN167-6

Wallrock, light grey Wallrock, light grey Wallrock, light grey Wallrock, dark grey Vein Vein Vein Vein Vein Wallrock, light grey Vein Wallrock Wallrock Wallrock, dark grey Vein Vein Vein Wallrock, light grey Vein Vein Vein Vein Vein Wallrock

1.00 1.23 0.86 11.05 0.05 0.05 0.12 0.01 0.00 0.83 0.79 4.17 1.15 4.09 0.02 0.03 0.01 1.53 0.23 0.01 0.00 0.00 0.00 0.00 0.02

0.29 0.26 0.29 0.67 0.08 0.08 0.08 0.06 0.08 0.29 0.28 0.41 0.33 18.08 0.28 0.30 0.27 18.77 18.62 0.07 0.07 0.09 0.03 0.05 0.07

0.17 0.20 0.12 0.25 0.09 0.14 0.05 0.13 0.12 0.10 0.08 0.18 0.18 0.74 0.13 0.07 0.11 0.52 1.71 0.00 0.08 0.11 0.01 0.03 0.02

54.94 53.56 53.92 40.56 54.98 55.80 54.87 54.78 53.97 54.71 54.57 50.37 54.63 29.91 53.77 53.61 54.39 31.77 31.33 54.30 54.30 54.97 54.39 54.39 53.54

0.06 0.07 0.10 0.07 0.07 0.05 0.05 0.04 0.08 0.05 0.10 0.08 0.96 0.07 0.07 0.07 0.06 0.06 0.06 0.06 0.04 0.04 0.06 0.08 0.06

0.13 0.12 0.09 0.12 0.17 0.15 0.15 0.14 0.13 0.19 0.13 0.11 0.18 0.35 0.31 0.47 0.38 0.14 0.74 0.06 0.10 0.09 0.11 0.10 0.08

0.05 0.06 0.01 0.00 0.00 0.00 0.05 0.01 0.07 0.06 0.00 0.00 0.01 0.14 0.00 0.00 0.05 0.00 0.08 0.08 0.06 0.00 0.01 0.00 0.00

0.07 0.00 0.05 0.01 0.02 0.04 0.06 0.05 0.10 0.04 0.03 0.04 0.11 0.00 0.06 0.10 0.21 0.00 0.06 0.11 0.04 0.00 0.12 0.04 0.06

45.24 44.47 44.20 49.07 43.57 44.22 43.58 43.33 42.81 44.77 44.59 46.40 46.69 50.07 42.92 42.91 43.51 48.15 46.93 42.94 42.93 43.43 42.94 42.95 42.31

101.95 99.97 99.64 101.77 99.02 100.52 98.99 98.55 97.35 101.03 100.57 101.76 104.25 103.44 97.54 97.54 98.98 100.93 99.75 97.63 97.62 98.74 97.67 97.62 96.17

1.48 0.01 0.02 0.03 0.10 0.02

19.97 0.45 0.45 0.45 20.43 0.43

0.71 0.01 0.09 0.05 0.60 0.09

29.67 54.12 54.01 54.47 29.95 54.57

0.33 0.05 0.06 0.05 0.03 0.05

0.73 0.46 0.33 0.41 0.65 0.37

0.00 0.02 0.03 0.06 0.00 0.08

0.00 0.17 0.11 0.06 0.02 0.10

48.64 43.44 43.31 43.71 46.78 43.77

101.53 98.74 98.39 99.29 98.57 99.46

0.01 0.59 0.02 2.12 0.27 0.00 0.01 0.01 0.97 2.50 1.82

0.47 19.14 0.33 20.54 20.56 0.47 0.41 0.77 20.46 17.79 18.53

0.11 1.18 0.08 0.50 0.53 0.03 0.07 0.05 0.81 2.26 1.48

54.28 29.08 53.69 30.47 30.89 54.07 53.62 54.17 29.10 28.70 29.13

0.04 0.04 0.05 0.05 0.07 0.06 0.06 0.05 0.04 0.00 0.05

0.46 1.02 0.60 0.39 0.44 0.43 0.42 0.50 0.95 1.53 1.35

0.00 0.01 0.00 0.00 0.03 0.00 0.00 0.05 0.03 0.05 0.00

0.03 0.03 0.06 0.00 0.00 0.00 0.11 0.09 0.07 0.00 0.00

43.54 46.03 43.04 50.06 47.82 43.32 43.00 43.85 47.80 47.98 47.58

98.93 97.13 97.85 104.13 100.62 98.37 97.72 99.52 100.22 100.81 99.94

Lower Unit 05WN55-1 05WN55-2 05WN55-3 05WN55-4 05WN55-5 05WN55-6

Wallrock Wallrock Wallrock Wallrock Wallrock Wallrock

Lower Unit at Shiyu Wallrock 05WN55-7 Wallrock 05WN55-8 05WN55-9 Wallrock 05WN58-1 Wallrock, dark grey 05WN155-1 Wallrock 05WN155-2 Vein 05WN155-3 Vein Vein 05WN155-4 05WN155-5 Wallrock 05WN157-1 Wallrock 05WN157-1 Wallrock

concentrations were normalized to the Post-Archaean Australian Shale (PAAS) composite (Taylor and McLennan, 1985). Ratios of some elements after the PAAS-normalization are used to describe the element features of individual samples. It should be noted that 90 ␮m spot was used in the LA-ICP-MS analysis of trace elements for the precise measurement of enough gas amounts, which is beyond the size of several to 30 ␮m for individual crystals in the wallrock carbonate. As a consequence, all the LA-ICP-MS analyses on the wallrock have sampled mixtures of multicrystalline carbonate mineral and even other minerals that occur in grain boundaries within the spot of 90 ␮m. 4.3.1. The Upper Unit The REE concentrations of veins vary from 11.3 to 98.6 ppm (Table 2). The REE + Y patterns in all samples are similar to each other (Fig. 9a): (1) MREE enrichment with [Pr/Tb]PAAS of 0.2–0.8 and [Tb/Yb]PAAS of 1.5–3.6; (2) positive La anomalies with [La/La*]PAAS of 1.0–3.2; (3) negative Ce anomalies with [Ce/Ce*]PAAS of 0.7–0.9; (4) non-uniform Eu anomalies, and negative Eu anomalies in samples 05WN73 and 05WN75 with [Eu/Eu*]PAAS of 0.4–1.0 but positive Eu anomalies in samples 05WN164 and 05WN167 with [Eu/Eu*]PAAS of 1.2–1.8; (5) no obvious Gd and Er anomaly; (6) a large range of Y/Ho ratios from 32 to 48.

The REE concentrations of wallrock vary in a large range from 22 to 717 ppm. But uniformly low REE concentrations of 22–52 ppm occur in the samples except 05WN73 and 05WN164 (Table 2). The REE + Y patterns in these samples are characterized by (Fig. 9b): (1) MREE enrichment with [Pr/Tb]PAAS of 0.4–0.9 and [Tb/Yb]PAAS of 1.0–3.4; (2) positive La anomalies with [La/La*]PAAS of 1.2–1.9; (3) no obvious Ce, Gd, Er and Eu anomalies with [Ce/Ce*]PAAS close to 1.0, [Gd/Gd*]PAAS of 0.8–1.4, [Er/Er*]PAAS of 0.8–1.3 and [Eu/Eu*]PAAS of 0.7–1.3; (4) Y/Ho ratios of 26–38 (Fig. 9b). Samples 05WN73 and 05WN164 have higher REE concentrations of 141–717 ppm in wallrock carbonates than other samples, which are also higher than those of PAAS. Their REE + Y patterns are variable, from slight LREE depletion to no differentiation, with no obvious element anomaly (Fig. 9b).

4.3.2. The Lower Unit The veins have REE concentrations of 2.9–20.6 ppm and show similar REE + Y patterns, which are characterized by (Fig. 9c): (1) MREE enrichment with [Pr/Yb]PAAS of 0.6–2.3, [Pr/Tb]PAAS of 0.4–0.9 and [Tb/Yb]PAAS of 1.5–3.6; (2) positive La anomalies with [La/La*]PAAS of 1.2–5.5; (3) positive Ce, Er and Eu anomalies with [Ce/Ce*]PAAS of 1.1–1.6, [Er/Er*]PAAS of 2.0–25 and [Eu/Eu*]PAAS of

350

Table 2 LA-ICP-MS analyses of major and trace elements in vein and wallrock carbonates of the Upper and Lower Units from the Lantian Formation in southern Anhui (in ppm). Mg (%)

38.51 38.51 38.51 38.51

0.23 0.17 0.24 0.39

7.07 6.08 8.04 14.7

191 274 207 222

9.75 3.71 3.32 5.02

8.85 10.3 9.09 9.84

14.1 16.8 14.6 15.7

05WN73 1 W 2 W 3 W Vb 4 V 5 6 V W 7 8 V 9 V 11 V 12 V 13 V

39.20 39.20 39.20 39.20 39.20 39.20 39.20 39.20 39.20 39.20 39.20 39.20

3.54 2.14 1.08 0.08 0.11 0.10 1.79 0.05 0.07 0.05 0.09 0.07

853 563 278 11.5 16.2 15.8 494 4.56 5.01 3.24 10.5 7.44

367 371 309 306 378 363 301 199 281 299 322 221

562 473 202 5.36 7.44 7.42 210 0.76 1.33 1.03 4.34 1.67

62.4 35.6 32.4 10.4 9.73 10.8 53.9 5.81 7.07 6.82 9.38 5.79

99.0 69.2 52.8 16.3 16.6 18.3 112 9.21 10.9 11.4 15.6 9.28

05WN75 1 V 2 W 3 V 4 V V 5

38.98 35.98 38.98 38.98 38.98

0.03 0.39 0.02 0.03 0.03

0.01 14.2 0 0 0

398 198 325 348 346

0.00 7.83 0.04 0.01 0.00

20.9 9.51 21.3 17.8 25.0

05WN75 V 6 V 7 8 V V 9 V 10 11 V W 12 W 13 W 14 15 W 16 V 17 W 18 W 19 W W 20 V 22 23 V V 24

38.98 38.98 38.98 38.98 38.98 35.98 35.98 35.98 35.98 35.98 38.98 35.98 35.98 35.98 35.98 38.98 38.98 38.98

0.02 0.03 0.03 0.02 0.03 0.02 0.56 0.70 0.47 0.29 0.04 0.29 0.43 0.37 0.33 0.02 0.02 0.03

0 0 0 0 0 0.06 17.7 31.2 14.1 13.9 0) 11.9 14.4 15.4 12.5 0 0.01 0

296 359 351 414 385 310 201 219 192 184 385 264 189 190 195 378 323 375

0.01 0 0 0 0.01 0.07 5.69 6.65 5.62 5.12 0 4.66 6.88 9.15 5.72 0.02 0 0.02

05WN164 1 V 2 V V 3 V 4 V 5

34.11 39.02 39.02 38.41 38.41

0.14 0.17 0.16 0.16 0.15

0 0 0 0.01 0

1432 1577 1380 1442 1564

0 0 0 0 0.01

05WN69 1 Wa W 2 3 W W 4

Rb

Sr

Zr

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Y

1.73 2.03 1.84 1.87

7.61 7.94 8.35 8.67

1.72 2.28 2.26 2.30

0.44 0.60 0.44 0.43

2.36 2.80 2.40 2.47

0.39 0.47 0.37 0.44

2.51 2.89 2.34 2.57

0.46 0.55 0.43 0.55

1.26 1.19 1.07 1.27

0.16 0.18 0.15 0.16

1.10 0.88 1.05 1.08

14.6 17.0 14.3 15.1

9.83 9.85 5.53 2.10 1.91 2.34 13.5 1.13 1.26 1.29 1.89 1.02

42.5 51.6 23.2 8.07 9.87 9.41 46.4 4.18 7.49 6.50 9.68 5.31

8.94 14.0 5.51 2.74 3.01 3.48 7.19 1.45 2.62 1.86 2.99 1.27

2.07 2.63 0.82 0.54 0.55 0.55 2.25 0.15 0.46 0.40 0.58 0.24

10.76 10.92 4.92 3.42 3.96 4.25 8.39 2.06 3.23 2.78 4.10 1.49

1.65 1.79 0.73 0.57 0.73 0.74 1.28 0.30 0.54 0.49 0.73 0.35

11.3 11.8 5.72 3.11 3.81 4.15 8.97 2.27 3.09 2.57 3.67 1.98

2.61 2.51 1.27 0.58 0.68 0.77 1.98 0.35 0.49 0.56 0.76 0.42

7.83 6.52 3.31 1.46 1.50 1.66 4.41 0.79 1.15 1.20 1.38 0.89

1.24 1.15 0.51 0.18 0.21 0.22 0.77 0.11 0.12 0.15 0.16 0.10

7.98 7.29 3.77 1.24 1.20 1.31 2.63 0.43 0.83 0.71 0.73 0.62

63.2 65.9 30.6 20.9 23.3 23.7 46.9 10.7 17.5 17.9 22.4 13.6

32.9 15.0 32.9 27.8 37.9

3.81 1.95 4.06 3.29 4.30

14.3 8.18 16.6 14.2 16.8

2.94 1.96 3.75 3.12 3.43

0.56 0.37 0.59 0.57 0.70

2.98 2.44 4.65 3.19 3.60

0.51 0.41 0.74 0.49 0.53

2.79 2.52 4.17 2.79 3.03

0.48 0.50 0.73 0.52 0.55

1.19 1.22 1.61 1.14 1.38

0.14 0.18 0.18 0.15 0.17

0.97 1.05 1.04 0.86 1.02

17.9 16.8 27.5 18.0 19.2

15.3 13.2 14.8 16.1 16.6 20.6 10.1 10.1 7.17 8.77 20.5 11.1 7.56 7.64 8.44 16.1 13.0 19.2

23.0 20.6 23.1 25.9 26.1 31.3 15.9 16.5 12.2 13.4 32.5 18.5 12.9 13.1 14.6 26.0 20.2 29.3

2.62 2.53 2.82 3.17 3.19 4.08 1.99 2.17 1.51 1.64 3.79 2.28 1.64 1.65 1.92 3.08 2.47 3.76

10.5 10.4 11.0 12.0 12.1 16.1 8.22 10.1 6.64 6.92 14.6 9.35 7.00 7.24 8.59 12.5 9.78 15.4

1.97 2.30 2.41 2.61 2.57 4.46 2.75 2.30 1.67 1.45 2.93 2.16 1.83 1.93 2.28 2.65 2.15 3.46

0.34 0.45 0.44 0.46 0.52 0.71 0.42 0.62 0.37 0.36 0.56 0.47 0.40 0.39 0.48 0.44 0.42 0.64

1.88 2.20 2.42 2.99 2.50 5.30 2.73 2.65 2.09 1.98 3.03 2.48 1.91 1.88 2.26 2.75 2.50 3.63

0.31 0.35 0.40 0.47 0.41 0.81 0.38 0.42 0.31 0.36 0.51 0.36 0.32 0.32 0.39 0.45 0.36 0.63

1.64 2.11 2.16 2.43 2.19 4.83 2.71 2.54 1.94 1.91 2.82 2.32 2.03 1.99 2.10 2.51 2.01 3.32

0.32 0.38 0.41 0.44 0.44 0.88 0.53 0.54 0.40 0.41 0.49 0.41 0.39 0.40 0.43 0.46 0.34 0.63

0.75 0.90 0.92 0.94 0.95 1.94 1.33 1.29 0.98 0.92 1.31 1.05 1.00 1.06 1.12 1.04 0.84 1.45

0.10 0.10 0.11 0.11 0.10 0.23 0.13 0.16 0.15 0.15 0.16 0.13 0.15 0.14 0.15 0.13 0.08 0.15

0.56 0.57 0.78 0.65 0.65 1.19 1.22 0.96 0.96 0.98 0.96 0.89 0.96 0.94 0.98 0.78 0.57 0.87

11.2 12.4 14.2 15.4 15.0 31.6 16.6 14.0 11.4 11.4 16.5 13.5 11.8 11.8 13.1 16.0 12.6 20.8

11.8 13.8 23.9 13.1 3.66

20.1 23.1 29.5 17.6 6.31

2.87 3.34 3.64 2.29 1.01

12.8 13.7 15.1 8.96 4.20

3.07 4.06 3.65 2.44 1.04

1.04 1.09 1.38 0.71 0.40

4.30 4.84 5.39 3.23 1.68

0.74 0.87 0.96 0.65 0.30

4.11 5.29 6.61 3.97 1.77

0.72 0.88 1.26 0.73 0.30

1.38 1.88 2.81 1.57 0.61

0.16 0.22 0.31 0.18 0.07

1.00 1.16 1.50 0.97 0.38

24.3 30. 43.7 25.7 12.7

Y.-Y. Zhao, Y.-F. Zheng / Precambrian Research 224 (2013) 341–363

Ca (%)

Table 2 (Continued) Mg (%)

Rb

38.41 38.41 38.41 38.41 38.41 38.41 34.11 22.69 34.11 22.69 34.11 20.16

0.16 0.16 0.16 0.16 0.46 15.25 19.43 14.13 12.68 13.15 14.89 11.30

0.01 0 0 0.03 8.70 389 414 349 262 349 321 383

05WN167 1 V 2 V 3 V 4 V 5 W 6 W 7 V V 8 V 9 W 10 11 V

39.27 39.27 39.27 39.27 38.24 38.24 39.27 39.27 38.24 38.24 39.27

0.08 0.07 0.07 0.07 0.34 0.33 0.07 0.03 0.04 0.27 0.07

0 0.02 0.11 0 23.2 25.1 0.01 0.03 0.08 12.6 0.18

05WN167 12 V V 13

39.27 39.27

0.08 0.07

0 0

Sr

Zr

La

Ce

Pr

Nd

Sm

1530 1464 1359 1538 947 656 411 243 548 244 488 247

0 0 0 0 5.84 300 807 275 307 235 271 290

6.96 3.36 20.5 3.32 11.5 86.1 170 125 32.9 61.1 41.9 68.1

12.3 4.23 25.5 6.34 19.4 141 295 206 60.9 99.1 68.5 111

1.78 0.58 3.02 0.99 2.56 17.1 32.6 22.7 8.04 12.3 9.48 13.3

8.05 2.85 12.5 4.70 10.4 67.0 128 82.6 37.5 55.4 44.6 53.2

2.23 0.67 2.71 1.17 2.59 12.37 21.1 12.9 11.1 10.6 10.8 10.4

713 576 523 747 211 194 507 355 407 173 867

0 0 0.02 0 6.41 11.27 0 0.04 0.01 5.18 0.03

3.34 2.97 2.63 3.20 6.31 6.59 3.23 11.4 6.15 5.67 2.98

5.45 4.24 3.65 4.22 11.8 13.4 5.50 19.6 9.70 11.3 4.76

0.76 0.55 0.51 0.52 1.74 1.90 0.73 2.31 1.32 1.58 0.64

2.79 2.73 2.24 2.45 7.18 9.24 3.67 9.37 5.29 7.22 2.51

2.90 4.15

4.60 5.60

0.59 0.69

2.56 3.26

404 426

0 0

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Y

0.66 0.22 0.91 0.41 0.77 2.98 3.96 2.66 2.65 2.53 2.52 2.62

2.80 1.07 3.74 1.84 3.26 13.73 17.87 11.50 12.66 10.93 12.49 11.64

0.49 0.19 0.72 0.31 0.61 2.42 2.75 1.75 2.18 1.78 2.11 1.90

2.98 1.24 4.68 1.89 3.95 15.7 17.5 10.8 14.7 10.9 14.0 11.4

0.53 0.25 0.82 0.34 0.78 3.15 3.73 2.35 3.14 2.34 2.81 2.43

1.12 0.53 1.83 0.69 1.78 8.93 10.1 6.26 8.57 5.96 7.59 6.61

0.11 0.05 0.19 0.08 0.23 1.13 1.48 0.87 1.13 0.88 1.10 0.95

0.69 0.23 1.11 0.45 1.24 8.41 10.37 5.72 8.05 5.50 7.43 6.44

19.3 9.74 28.6 13.3 24.6 88.4 101 61.2 83.2 59.8 81.6 67.1

0.72 0.58 0.49 0.59 1.65 2.15 0.67 1.25 1.13 2.01 0.52

0.22 0.14 0.17 0.22 0.41 0.48 0.25 0.33 0.34 0.41 0.19

0.72 0.58 0.50 0.49 1.37 2.13 0.68 1.00 1.43 1.83 0.53

0.13 0.08 0.08 0.12 0.23 0.32 0.14 0.16 0.19 0.22 0.08

0.58 0.39 0.51 0.44 1.26 1.75 0.73 0.75 1.24 1.48 0.47

0.12 0.09 0.09 0.11 0.25 0.31 0.16 0.16 0.22 0.28 0.07

0.34 0.24 0.23 0.21 0.69 0.78 0.37 0.34 0.58 0.65 0.24

0.03 0.02 0.02 0.02 0.08 0.09 0.05 0.05 0.07 0.10 0.02

0.15 0.12 0.13 0.09 0.52 0.58 0.22 0.28 0.38 0.59 0.09

5.08 3.98 3.63 3.49 8.43 10.7 5.71 5.79 8.70 8.82 3.15

0.55 0.76

0.19 0.22

0.65 0.75

0.08 0.10

0.58 0.60

0.13 0.13

0.26 0.32

0.03 0.04

0.19 0.20

4.36 5.62

Y.-Y. Zhao, Y.-F. Zheng / Precambrian Research 224 (2013) 341–363

Ca (%) 05WN164 V 6 V 7 V 8 9 V 10 V 11 W 12 W 13 W W 14 W 15 16 W 17 W

351

352

Table 2 (Continued) Mg (%)

21.39 21.39 21.39 37.89 38.66 38.66 38.66 21.39 21.39 38.66 21.39 38.66 21.39 38.66 38.66

7.41 5.40 3.29 0.40 0.28 6.26 0.34 10.41 7.19 0.31 6.60 0.34 4.13 3.51 0.35

05WN155 1 W W 2 W 3 4 V V 5

20.78 20.78 22.07 38.30 38.30

9.92 3.31 0.35 0.20 0.27

0.09 0.07 0.01 0 0

05WN155 W 6 7 W 8 V V 10

20.78 20.78 38.30 38.30

0.26 11.65 0.37 0.35

0.01 0.18 0 0

05WN157 1 W W 2 W 3 4 W 5 W W 6 W 7

20.50 20.50 20.50 20.50 20.50 20.50 20.50

59.44 56.85 61.75 58.91 49.07 37.94 47.62

a b

W: wallrock. V: vein.

Rb

Sr

2.58 128 110 1.28 1.99 128 693 0.02 0 965 306 1.68 0 976 123 1.04 1.47 141 0 1076 1.25 131 0.01 1023 1.34 122 1.10 388 0 835

444 332 411 367 299 261 236

Zr

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Y

4.35 2.98 6.72 0.24 0.04 6.45 0.12 24.7 2.03 0.10 11.5 0.01 23.9 0.78 0.01

3.15 3.43 3.19 19.6 2.81 5.08 4.85 3.42 3.02 3.25 2.80 2.09 2.73 4.93 0.70

6.86 8.87 9.11 39.1 6.04 13.7 8.52 8.47 6.74 6.52 6.71 4.49 7.46 12.1 1.42

0.81 1.12 1.36 4.50 0.75 1.93 0.82 1.17 0.88 0.73 0.88 0.56 0.99 1.47 0.15

3.84 6.21 7.47 20.4 3.39 9.32 3.59 5.95 4.62 3.62 4.25 2.48 5.27 8.07 0.60

0.96 1.47 1.81 3.78 0.64 2.58 0.59 1.30 1.23 0.82 1.24 0.64 1.50 1.87 0.13

0.33 0.49 0.57 1.80 0.60 0.88 0.71 0.49 0.47 0.73 0.37 0.90 0.51 0.84 0.45

0.84 1.50 1.82 3.70 0.67 2.21 0.58 1.43 1.24 0.66 1.05 0.57 1.75 2.22 0.13

0.16 0.22 0.28 0.57 0.10 0.36 0.08 0.20 0.16 0.10 0.19 0.09 0.26 0.32 0.02

0.80 1.37 1.53 3.07 0.56 2.26 0.41 1.25 1.03 0.52 1.14 0.51 1.48 1.86 0.08

0.18 0.27 0.30 0.58 0.10 0.44 0.08 0.24 0.18 0.08 0.21 0.09 0.28 0.39 0.02

0.55 0.73 0.74 1.36 0.29 1.17 0.16 0.73 0.53 0.24 0.55 0.25 0.91 0.98 0.07

0.08 0.09 0.10 0.17 0.04 0.14 0.02 0.08 0.07 0.02 0.09 0.03 0.11 0.13 0.00

0.46 0.55 0.68 0.75 0.10 0.87 0.13 0.65 0.49 0.22 0.60 0.14 0.78 0.74 0.05

5.72 8.99 10.8 20.7 4.37 15.0 3.17 7.24 6.11 3.54 7.39 4.18 10.61 13.0 1.07

143 181 247 379 310

9.76 5.17 0.30 0 0

4.16 16.6 5.30 4.70 2.34

5.57 31.6 7.11 7.77 2.94

0.51 4.29 0.66 0.84 0.29

1.67 17.2 1.79 2.52 0.99

0.29 3.76 0.38 0.69 0.15

0.13 2.33 0.13 0.28 0.21

0.42 3.61 0.37 0.69 0.16

0.07 0.37 0.05 0.10 0.02

0.44 1.86 0.37 0.66 0.15

0.08 0.23 0.07 0.10 0.03

0.26 0.51 0.18 0.26 0.06

0.04 0.05 0.02 0.04 0.01

0.26 0.26 0.18 0.28 0.04

2.95 8.41 2.66 4.43 1.16

231 67 370 395

0.01 4.51 0 0.02

0.91 1.94 0.76 3.30

1.63 3.70 1.04 4.65

0.16 0.39 0.11 0.39

0.54 1.43 0.40 1.04

0.12 0.30 0.09 0.20

0.09 0.08 0.15 0.16

0.13 0.28 0.10 0.23

0.02 0.04 0.02 0.04

0.11 0.29 0.10 0.24

0.01 0.06 0.02 0.05

0.04 0.20 0.04 0.14

0.01 0.03 0.01 0.02

0.04 0.19 0.04 0.12

0.82 2.19 0.93 2.19

3.88 3.45 3.71 3.17 3.06 6.13 2.95

10.95 8.01 8.85 8.05 8.01 16.64 7.98

1.40 1.73 1.26 1.09 1.03 2.00 1.07

9.81 9.70 10.30 8.73 6.98 14.93 8.79

311 303 359 328 373 1748 364

239 154 165 109 140 33.6 87.1

32.4 31.6 23.1 22.7 23.2 46.0 22.4

83.0 81.4 66.2 74.4 64.0 104 60.9

11.8 10.8 9.77 12.1 7.80 14.4 7.96

56.1 56.9 58.9 50.7 50.4 77.0 39.3

16.8 16.6 15.8 13.5 10.9 25.8 11.0

4.99 3.97 4.07 3.58 3.64 16.5 3.46

19.4 19.0 17.0 18.0 13.9 40.7 14.4

3.51 2.52 2.63 2.85 2.44 5.26 2.38

18.5 17.3 18.3 17.1 14.9 38.7 15.8

115 96.8 104 92.0 93.6 191 83.4

Y.-Y. Zhao, Y.-F. Zheng / Precambrian Research 224 (2013) 341–363

Ca (%) 05WN58 1 W 2 W W 3 4 V 5 V 6 W 7 V 8 W 9 W V 10 11 W V 12 13 W 14 W 15 V

Y.-Y. Zhao, Y.-F. Zheng / Precambrian Research 224 (2013) 341–363

353

4.4. Carbon and oxygen isotopes The bulk carbonate rocks in UU have ı13 C values of −10.9‰ to −9.3‰ and ı18 O of −23.6‰ to −20.6‰, consistent with our previous measurements (ı13 C values of −11.7‰ to −7.8‰ and ı18 O of −25.6‰ to −18.6‰; Zhao and Zheng, 2010). Seven samples were selected for the microdrilling analysis. The microcrystalline wallrock has ı13 C values of −11.8‰ to −8.9‰ and ı18 O of −24.9‰ to −19.2‰, and the calcite veins have ı13 C values of −11.1‰ to −8.6‰ and ı18 O of −27.6‰ to −22.0‰. For the LU, the bulk rocks have ı13 C values of −5.1‰ to −3.0‰ and ı18 O values of −8.0 to −15.2‰, also consistent with our previous measurements (ı13 C values of −5.2‰ to −2.1‰ and ı18 O of −14.8‰ to −3.9‰; Zhao and Zheng, 2010). Two samples were selected for the microdrilling analysis. The microcrystalline wallrock has ı13 C values of −4.4‰ to −3.8‰ and ı18 O values of −16.5‰ to −9.3‰. The veins have ı13 C values of −6.0‰ to −5.6‰ and ı18 O values of −28.6‰ to −23.6‰.

5. Preservation of primary records

Fig. 8. The content variations of oxide of different spots in the studied samples. The sample locations are marked in Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1.8–15.9; (4) no obvious Gd anomaly with [Gd/Gd*]PAAS of 0.7–1.1; (6) a large range of Y/Ho ratios from 33 to 53 (Fig. 9c). The wallrock has variable REE concentrations from 8.9 to 411 ppm and REE + Y patterns. The REE + Y patterns for samples 05WN58 and 05WN157 are similar and characterized by (Fig. 9d): (1) minor LREE depletions with [Pr/Yb]PAAS of 0.3–0.7, [Pr/Tb]PAAS of 0.3–0.9 and [Tb/Yb]PAAS of 1.0–1.5; (2) positive La anomalies with [La/La*]PAAS of 1.5–6.5; (3) positive Ce, Er and Eu anomalies with [Ce/Ce*]PAAS of 1.1–1.6, [Er/Er*]PAAS of 1.6–2.4 and [Eu/Eu*]PAAS of 1.5–2.0; (4) no obvious Gd anomaly with [Gd/Gd*]PAAS of 0.8–1.4; (5) Y/Ho ratios of 31–38. The REE + Y patterns for different localities of the wallrock are similar to those of calcite as derived from leaching of the acetic acid (Fig. 9d). Wallrock carbonate of 05WN155 has lower REE concentrations than those of other samples (Fig. 9d). Its REE + Y patterns are different from those of other samples and characterized by: (1) no obvious fractionation relative to PAAS with ([Pr/Yb]PAAS of 0.6– 5.3, Pr/Tb]PAAS of 0.6–1.1 and [Tb/Yb]PAAS of 0.8–5.2); (2) positive La anomalies with [La/La*]PAAS of 1.0–1.5; (3) no obvious Ce anomaly with [Ce/Ce*]PAAS of 0.9–1.1; (4) positive Er and Eu anomalies with [Er/Er*]PAAS of 1.1–10.8 and [Eu/Eu*]PAAS of 1.3–3.5; (4) no obvious Gd anomaly with [Gd/Gd*]PAAS of 0.9–1.5; (6) a large range of Y/Ho ratios from 35 to 54 (Fig. 9d).

The post-depositional fluids would cause the carbonate alteration in two pathways, often termed the meteoric and burial diagenesis (Hoefs, 2009). Carbonates deposited in shallow marine and continental shelf environments are susceptible to exposing to surface and suffering the influence of meteoric water. The burial diagenesis often occurs in marine environments during the compaction of sedimentary carbonates, with porewater in isotope equilibrium with the assemblage of carbonate minerals. In contrast to the meteoric pathway, the water flow is confined to squeezing of porewaters upwards into the overlying or adjacent sedimentary columns. If the diagenetic fluids were derived the same compositions of depositional water, no change would be expected in the geochemical compositions of carbonates. On the other hand, significant changes in carbonate geochemical compositions could occur if the composition of post-depositional fluids is different from that of depositional fluids (Derry, 2010). Theoretically, the O isotope composition of carbonate would not change appreciably with the burial diagenesis because the porewater ı18 O values are also of depositional origin. With increasing burial depth, nevertheless, the sediments and often the porewater Exhibit 18 O depletions by several per mill. The carbonate ı18 O shift in the mineral phases is mostly due to an increase in temperature with increasing the burial depth. Comparison analyses of different microfacies can test whether the wallrock carbonates undergone post-depositional alteration. If there are significant differences in geochemical compositions between vein/cement and wallrock carbonates, the wallrock may have experienced the post-depositional alteration and the vein/cement-forming fluid is externally derived (Jacobsen and Kaufman, 1999; Knauth and Kennedy, 2009; Zhao and Zheng, 2010). In contrast, if there is the similarity in geochemical records between vein/cement and wallrock carbonates, that the post-depositional fluid has the same geochemical features as the depositional fluid. In this case, the vein/cement-forming fluid was internally derived and the both vein/cement and wallrock carbonates have preserved their primary compositions. An extreme case is that the wallrock carbonates were thermodynamically reequilibrated with the porewater at elevated temperatures, resulting in internally buffered alteration like self-metasomatism (e.g., Hausegger et al., 2009). Besides the temperature effect, however, the porewater of internal origin would not make the significant influence on the altered carbonates. Therefore, the present study of stable isotopes and trace elements is capable of providing constraints on the temporal and spatial relationships with respect to the post-depositional effects.

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Fig. 9. PAAS-normalized REE + Y patterns for (a) vein and (b) wallrock of the UU, and for (c) vein and (b) wallrock of the LU. 05WN74-R and 05WN58-R denotes the pattern of bulk rock for 05WN74 and 05WN58, respectively.

5.1. The Upper Unit The wallrock carbonates are usually enriched in the elements such as Al, Ba, Rb, Pb, Na and Th, which are dominated by the siliciclastics (Fig. 7a–d). The wallrock carbonates in the UU exhibit variable REE + Y patterns (Table 2 and Fig. 9b), which can be categorized into two groups. The first group includes 05WN73 and 05WN164 (Fig. 9b), and exhibits high concentrations of SiO2 , Al2 O3 , Th, Zr, Ti and Sc (Tables 1 and 2), which are several orders of magnitude higher than those in seawater and hydrothermal fluids (Bau, 1993; Slack et al., 2007). It also exhibits variable REE + Y patterns from slight LREE depletion to no differentiation (Fig. 9b), and a positive correlation between Zr and REE concentrations (Fig. 10a). This suggests that the REE + Y concentrations of carbonate were contaminated by the terrigenous siliciclastics (Nothdurft et al., 2004; Bolhar and Van Kranendonk, 2007; Zhao et al., 2009; Frimmel, 2010). This is particularly true for sample 05WN164, whose REE + Y patterns are more like those of PAAS (Fig. 9b), a tendency to flatten REE + Y distribution with the increasing REE + Y concentrations (Nothdurft et al., 2004). The second group including samples 05WN69, 05WN75 and 05WN167 (Fig. 9b) has and exhibits lower concentrations of SiO2 , Al2 O3 , Th, Zr, Ti and Sc than the first group, with CaO contents higher than 53% (Table 1). This is consistent with the main mineral of calcite and little terrigenous silicate detritus. It is known that even small amounts of detritus contamination (e.g., 1–2%) could

change the REE + Y patterns significantly to the PAAS-like patterns with reduced La and Ce anomalies and greatly decreased degrees of LREE depletion (Nothdurft et al., 2004). Thus, the contamination by the non-carbonate materials should be excluded in our analysis based on the element concentrations and REE + Y patterns. The Al, Th, Zr, Ti and Sc concentrations can be used as a further proxy for small amounts of contamination by terrigenous silicate detritus (Bolhar et al., 2004; Kamber et al., 2004; Nothdurft et al., 2004). If the contamination existed, it would systematically increase REE concentrations with Al, Th, Zr, Ti or Sc concentrations. An upper threshold value of 4 ppm Zr can be applied, which corresponds to about 2% terrigenous detrital contamination (Frimmel, 2009). The Zr concentrations of wallrock in the second group are variable, even as high as 11.3 ppm. Nevertheless, the REE + Y patterns in the wallrock with Zr concentrations of >4 ppm exhibit similar features to those with Zr concentrations of <4 ppm (Figs. 9a and 11c). This indicates that the REE + Y patterns in the second group are nearly free from non-carbonate contamination and are likely to reflect the primary geochemical records. It is interesting to note that the LA-ICP-MS results for the wallrock carbonates are almost similar to those obtained by dissolution in 0.5 M acetic acid (Fig. 9b). The slight difference in HREE possibly is due to the different components by the two different analytical methods. The 0.5 M acetic acid can only dissolve the calcite and little dolomite, in which process the REE in the silicate contaminant cannot be dissolved completely. However, the

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Fig. 11. (a) Normalized REE + Y patterns for the cross section of vein in sample 05WN75 from the Upper Unit of the Lantian Formation. The sample locations are marked in Fig. 4b. (b) The correlation between Mn/Sr ratios and Sr concentrations of the vein and wallrock in the Upper Unit. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. The correlations between Zr and REE concentrations in vein and wallrock carbonates from the Lantian Formation in southern Anhui. 05WN73-V and 05WN73W denotes the concentrations of vein and wallrock in 05WN73, respectively.

all components were sampled by the LA-ICP-MS analysis. In this regard, the REE + Y patterns obtained by LA-ICP-MS represent the bulk features derived from Dissolution I (dissolved by 0.5 M acetic acid), Dissolution II (dissolved by 3.4 M acetic acid) and residues (dissolved by HF + HNO3 ) in the stepwise analyses of Zhao et al. (2009). The REE + Y patterns for the veins in the UU are similar to those of wallrocks in the samples which exhibit low Zr concentrations, MREE enrichment relative to LREE and HREE, minor or obvious positive La anomalies, no Ce, Gd, Er and Eu anomalies, and lower Y/Ho ratios (Table 2 and Figs. 9 and 11). This implies that the trace elements of vein and wallrock carbonates are broadly similar to each other and the precipitation fluids for the both vein and wallrock carbonates were primarily controlled by the same source of terrigenous detritus with the same geochemical signals (Zhao et al., 2009). Specifically, the negative Ce anomaly, typical

of water and carbonate of the oxidative environment, is evident in both vein and wallrock carbonates (Fig. 9). Moreover, the center and margin of vein in sample 05WN75 have similar REE + Y patterns (Figs. 3 and 11a), suggesting that the supplies of the terrigenous detritus are consistent for post-depositional fluids. The REE + Y patterns of vein and wallrock with low Zr concentrations are different from those of normal marine carbonates (Bolhar et al., 2004; Shields and Webb, 2004; Van Kranendonk et al., 2003; Webb and Kamber, 2000), but similar to the freshwater carbonate influenced by terrestrial detritus (Bolhar and Van Kranendonk, 2007; Zhao et al., 2009). Therefore, depositional fluids for the both vein and wallrock carbonates in the UU are probably related to freshwater. The ı13 C and ı18 O values for the wallrock are −11.8‰ to −8.9‰ and −24.9‰ to −19.2‰, respectively, and for the veins are −11.1‰ to −8.6‰ and −27.6‰ to −22.0‰, respectively (Fig. 12a). The ı18 O and ı13 C values for the both wallrock and vein carbonates are significantly lower than those for normal marine carbonates with ı18 O = −3‰ to −2‰ and ı13 C = 0‰ ± 1‰ (Veizer et al., 1997; Shields and Veizer, 2002; Zhao and Zheng, 2010). For the individual samples, nevertheless, the veins have slightly higher ı13 C values, but slightly lower ı18 O values, than the adjacent wallrock, resulting in a negative correlation in the individual samples (Fig. 13). The ı13 C and ı18 O values for the bulk whole-rock analyzed by the

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Y.-Y. Zhao, Y.-F. Zheng / Precambrian Research 224 (2013) 341–363 Table 3 Analyses of carbon and oxygen isotope ratios for bulk carbonate from the Lantian Formation in southern Anhui. Thickness (m)

ı13 CPDB (‰)

ı18 OPDB (‰)

Upper Unit 05WN61A 05WN61B 05WN62 05WN165 05WN63 05WN64A 05WN166 03WN84 05WN65 03WN85 05WN167 05WN66 05WN67 05WN168 05WN68Ab 05WN68Bb 05WN169 05WN170 05WN69 05WN70 05WN171 05WN71 05WN72 05WN73 05WN74 05WN172 05WN75 05WN76 05WN173

91.23a 91.63 91.73 92.03 92.24 92.25 92.33 92.41 92.5 92.79 92.92 93 93.63 93.78 93.88 94.08 94.28 94.4 94.52 94.66 94.84 95.02 95.19 95.38 95.58 95.89 96.02 96.17 96.42

−10.9 −11.0 −10.4 −10.9 −10.7 −11.0 −10.2 −10.9 −10.9 −11.0 −9.1 −11.2 −10.7 −9.6 −9.9 −10.1 −9.4 −10.4 −10.6 −9.6 −10.8 −10.5 −10.3 −9.7 −9.3 −10.8 −9.6 −9.6

−23.3 −23.2 −22.4 −21.9 −23.3 −22.7 −23.3 −24.1 −25.9 −24.6 −25.1 −23.9 −22.5 −22.1 −25.1 −23.9 −22.4 −22.5 −25.2 −24.7 −21.8 −24.4 −24.1 −24.7 −21.5 −22.1 −22.4 −21.9 −20.6

Lower Unit 05WN154 03WN79 03WN80 03WN81 05WN155 05WN52 05WN53 05WN54 05WN55 05WN56 05WN156 05WN157 05WN57 05WN58

0.35 0.45 0.8 1.05 1.45 1.8 1.85 1.88 1.9 1.95 2.25 2.55 2.7 3.45

−4.4 −5.1 −4.5 −3.9 −3.6 −3.5 −3.8 −4.0 −4.0 −4.3 −8.3 −5.3 −4.5 −5.4

−11.0 −14.5 −11.9 −10.0 −12.4 −10.1 −11.5 −9.1 −14.1 −11.0 −11.5 −11.3 −13.9 −12.5

Sample no.

Fig. 12. The correlation between ı13 C and ı18 O values for vein and wallrock carbonates from the Upper and Lower Unit. 05WN73-V and 05WN73-W are the vein and wallrock of sample 05WN73.

solution method are approximately the average of the microdrilling analytical results (Tables 3 and 4). The Mn/Sr ratio and ı18 O value are widely used as an indicator of diagenetic alteration. The carbonates with Mn/Sr ≤ 1 are considered to be primary. According to the distribution coefficient model (Jacobsen and Kaufman, 1999; Derry, 2010), an increase in Mn/Sr is approximately linearly correlated with a decrease in ı13 C for sedimentary carbonates. However, it is worth noting that the C and O elements occupy crystallographic positions in carbonate minerals, neither Sr nor Mn does (Derry et al., 1992). In this case, the Mn/Sr ratio may be not a proper indicator of potentially significant alteration on C and O isotopes. Thus, ı18 O values are considered as a

a Thickness was calculated from the interface between the Leigongwu diamictite and the Lower Unit of the Lantian Formation. b Samples 05WN68A and 05WN68Bare both from sample 05WN68 with slightly different colors.

more sensitive indicator of the diagenetic alteration and the lower ı18 O values than −10‰ are usually considered as the effect of postdepositional alteration by low ı18 O surface water (e.g., Jacobsen and Kaufman, 1999). The vein and wallrock carbonates would reach

Table 4 A summary of the microdrilling analyses of carbon and oxygen isotopes in vein and wallrock carbonates from the Lantian Formation in southern Anhui. ı13 C (‰)PDB

ı18 O (‰)PDB

Section

Sample no.

Vein

Wallrock

Vein

Wallrock

Upper Unit

05WN63 05WN73 05WN75 05WN164 05WN166 05WN167 05WN170

−10.77 to −9.86 −9.87 to −9.73 −9.87 to −8.82 −9.18 to −10.16

−26.66 to −22.05 −27.39 to −24.40 −25.55 to −27.45 −26.42 to −27.45

−11.11 to −8.62 −9.06 to −10.23

−11.81 to −10.13 −9.88 to −9.33 −10.30 to −9.53 −11.14 to −8.89 −11.04 to −9.22 −10.15 to −10.69 −9.43 to −11.12

−27.14 to −23.63 −26.07 to −23.99

−23.33 to −19.21 −26.44 to −20.79 −23.94 to −19.92 −27.55 to −21.34 −26.78 to −22.03 −22.32 to −24.94 −18.50 to −23.66

05WN58 05WN155

−6.02 to −5.57 −5.65 to −5.55

−4.68 to −3.77 −5.06 to −3.97

−27.49 to −23.57 −28.64 to −26.82

−15.51 to −9.33 −16.53 to −11.17

Lower Unit

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Fig. 13. The correlation between ı13 C and ı18 O values for vein and wallrock carbonates from individual samples in the Upper Unit of the Lantian Formation.

O isotope equilibrium at high temperatures, which would result in a regular or uniform ı18 O distribution in the wallrock. There are small differences in ı13 C values (from −3.19‰ to 2.22‰) but large differences in ı18 O values (from −2.87‰ to 8.39‰) between the wallrock and vein carbonates (Fig. 14). However, the ı18 O values vary slightly at different locations in the wallrock carbonates, with regardless to the locations either distal or proximal to the veins (Fig. 14). This indicates that the carbonates have not experienced high temperature post-depositional alteration and the ı18 O values of wallrock carbonates have not been significantly altered. The distinct differences in ı18 O value between the closely adjacent vein and wallrock carbonates in UU (Figs. 13 and 14) suggest that the post-depositional fluids are different from the depositional one in O isotope compositions or burial temperatures. Nevertheless, there are the similar REE + Y patterns between the wallrock and vein carbonates, suggesting the contribution from the sources of the same REE + Y patterns to the wallrock and vein depositional fluids. In other words, the primary geochemical records of wallrock carbonates would have been preserved. This may be ascribed to the low REE + Y concentrations of post-depositional fluid (Kamber et al., 2005) and to resistance to the post-depositional modification of carbonate REE + Y patterns (Banner et al., 1988). Negative correlation is observed between the ı18 O and ı13 C values for the both wallrock and vein carbonates in the UU carbonate (Fig. 13). This can be explained by a two-component mixing, where one end-member has high ı18 O but low ı13 C values and the other end-member has low ı18 O but high ı13 C values. In this regard, the “primary” isotope signatures are recorded by one of the following two associations: (1) the highest ı18 O (−19.2‰) and lowest ı13 C (−11.8‰) values of wallrock carbonate for one end-member, and

the lowest ı18 O (−27.4‰) and highest ı13 C (−8.6‰) of calcite veins for the other end-member; (2) the lowest ı18 O (−27.6‰) and highest ı13 C (−9.2‰) of wallrock carbonate for one end-member, and the highest ı18 O (−22.‰) and lowest ı13 C (−11.1‰) of calcite veins for the other end-member. 5.2. The Lower Unit The wallrock carbonates in the LU exhibit large variations in REE + Y concentrations, from 3.8 to 411 ppm. Nevertheless, they have similar REE + Y patterns with minor LREE depletion, slightly positive La anomalies, obviously positive Ce, Er, and Eu anomalies and low Y/Ho ratios (solution I, Zhao et al., 2009) (Fig. 9d). It is noted that the REE + Y patterns for sample 05WN58 obtained by the LA-ICP-MS analysis are similar to those obtained by the stepwise solution bulk analysis with the 0.5 M acetic acid (Fig. 9d). Both REE and Zr concentrations of sample 05WN157 are much higher than those of sample 05WN58. Moreover, there is a positive correlation between REE and Zr concentrations in sample 05WN157 (Fig. 10b), suggesting that wallrock REE concentrations were probably influenced by the terrigenous detritus (Nothdurft et al., 2004; Bolhar and Van Kranendonk, 2007). 05WN155 has a La/Nd ratio greater than 1.0 and a positive Eu anomaly, different from those of samples 05WN58 and 05WN157. These features are probably attributed to the presence of different silicate minerals in the carbonates. For example, there is chlorite in 05WN157, but no in 05WN155 (Fig. 5). The similar REE + Y patterns of veins in sample 05WN58 and cements in 05WN155 indicate that the post-depositional fluids for the LU carbonates were influenced by the same geochemical source of terrigeous materials. The cements in sample 05WN155

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Fig. 14. The correlation between ı13 C and ı18 O values for vein and wallrock carbonates from the Upper Unit of the Lantian Formation. The sample locations are marked in Fig. 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

have similar REE + Y patterns to the wallrock, but the veins in sample 05WN58 have different REE + Y patterns to the wallrock (Fig. 9c and d). This difference is probably caused by the presence of different silicate components in the wallrock carbonates (Fig. 5d and e). The wallrock and vein (or cement) carbonates in the LU have different ı18 O and ı13 C values (Table 4). The ı18 O and ı13 C values for the wallrock are markedly higher than those for the veins (or cements), resulting in positive correlations between the ı18 O and ı13 C values for the wallrock and vein (or cement) carbonates (Table 4, Fig. 12b). Such differences suggest that post-depostional fluids did not achieve the stable isotope equilibria with the wallrock. Moreover, the ı18 O values vary slightly at different locations in the wallrock carbonates, with regardless to the locations either proximal or distal to the veins (Table 4). In this case, the wallrock and vein carbonates may still have retained their primary signatures of stables isotopes to some extent.

6. Sources of depositional and post-depositional fluids Potential depositional fluid reservoirs for carbonates include seawater and meteoric water as well as formation water and deeply-derived metamorphic or magmatic fluids (Pili et al., 2002; Kopf et al., 2003; Lin et al., 2003; Schneider et al., 2008). Fluids originating from different reservoirs would be mixed at the supracrustal level and modified by water–rock interaction, leaving geochemical signatures that are substantially different from their original counterparts (Janssen et al., 1997; Kirschner et al., 1999; Budai et al., 2002). Nevertheless, the source of post-depositional fluids can be described by three distinct fluid-evolution mechanisms:

local in situ generation (internally derived water), mixing between external and internal fluids, and incursion of external fluids (Schneider et al., 2008).

6.1. The Upper Unit The vein carbonates in the UU occur regularly on the outcrop surface and across the different laminations (Fig. 3). Moreover, some calcite grains in the veins are coarse (Fig. 4), suggesting appropriate conditions for sufficient crystallization of calcite. The wallrock in the UU is underlain by the black shale and mudstone (Fig. 2), which contain no carbonate as indicated by the Gasbench C O isotope analysis (Zha et al., 2010). This suggests that the veinforming fluids could not be derived from the lower sedimentary successions. The wallrock carbonate in the UU is overlain by the compacted chert and slate (Fig. 2), which also contain no carbonate (Guo et al., 2007). Therefore, the post-depositional fluids would be most likely derived from the wallrock itself in the stage of carbonate lithification from the depositional basin or the adjacent basin. The negative ı13 C excursion of −11.8‰ to −9.2‰ for the wallrock carbonates in the UU can be correlated globally, namely the Shruam-Wonoka anomaly (Halverson et al., 2005; Le Guerroué and Cozzi, 2010; Le Guerroué et al., 2006b; Zhu et al., 2007). Knauth and Kennedy (2009) and Derry (2010) argued that the negative ı13 C values were a result of meteoric diagenesis rather than the record of primary seawater signature. According to Derry (2010), the positive correlations between carbonate ı18 O and ı13 C values for a number of the stratigraphic sections were possibly caused by the diagenetic alteration at high temperatures and high water/rock ratios. Based on the ı18 O and ı13 C values for the different microfacies in the UU,

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the wallrock carbonates did not achieve stable isotope equilibria with post-depositional fluids. The highest ı13 C value of −9.2‰ for the wallrock and its negative ı18 O–ı13 C correlation suggest that the primary ı13 C for the wallrock value would be lower than−9.2‰. These negative ı13 C values were related to the oxidation of terrigenous dissolved organic carbon (DOC) (Bristow and Kennedy, 2008; Jiang et al., 2007; McFadden et al., 2008; Zhao and Zheng, 2010; Tahata et al., 2012). The ı18 O values lower than −10‰ were unacceptably altered for normal marine carbonates (Jacobsen and Kaufman, 1999). Nevertheless, the extremely low ı18 O values as negative as −24.9‰ for the wallrock in the UU were considered to be ascribed to the continental deglacial meltwater (Zhao and Zheng, 2010). The REE + Y patterns for the wallrock carbonates are different from those for the normal marine carbonates, lending support to the origin of deglacial meltwater with the incorporation of abundant terrigenous weathering (Zhao et al., 2009). The ı13 C values of vein carbonates are primarily controlled by carbon sources. The 13 C-enrichment in veins is usually attributed to microbial methanogenesis of organic matters during carbonate precipitation (Irwin et al., 1977). In contrast, the 13 C-depletion commonly results from oxidization of organic matters (Irwin et al., 1977). The veins in the UU exhibit slightly higher ı13 C values than the adjacent wallrock of individual samples (Figs. 13 and 14). The small differences in ı13 C values between the vein and wallrock carbonates indicate that the carbon source in the post-depositional fluid may be similar to that in the wallrock-depositing fluid, but more 13 C-enriched materials were added to the vein-forming fluids. There are negative Ce anomalies in the veins (Fig. 9a), suggesting that the vein-forming fluids are oxidative. In this regard, the microbial methanogenesis of organic matter cannot happen in the vein-forming stage. Thus, the post-depositional fluid would contain more 13 C-enriched materials than the depositional fluid before the vein-forming fluid entered the wallrock carbonate. The ı18 O values of −27.6‰ to −22.0‰ for the vein carbonates are slightly lower than those of −24.9‰ to −19.2‰ for the wallrock carbonates (Figs.12a and 13). Carbonate veins of low ı18 O values were reported to precipitate at temperatures higher than 180 ◦ C (ı18 O = −22.4‰ to −21.3‰, Herbert and Compton, 2007), even to 270 ◦ C (Tritlla et al., 2001). Such high temperatures could have been reached if there were hydrothermal activities or deep burial depth. However, no Eu anomalies in the vein carbonates indicate that the vein-forming fluids are not related to hydrothermal activity. Moreover, more uniform oxygen and, in particular, carbon isotope trend is expected by the deep burial depth and regional metamorphic recrystallization. Low 47 values indicates that the depositional or diagenetic temperatures were lower than 200 ◦ C (Bristow et al., 2011). The fluid-inclusion study of Hood et al. (2003) demonstrated that the baroque dolomite, developing after the first-generation calcite, formed at temperatures of 65–80 ◦ C. Schneider et al. (2008) estimated that the first generation of calcite formed in the early stage of diagenesis, without the growth of barite, and at temperatures below 50 ◦ C based on the presence of all-liquid primary aqueous inclusions in the crystals. The lack of calcite recrystallization and the uniform C O isotope distribution in the UU veins (Fig. 12a) strongly suggest that the veins were formed at low temperatures, probably lower than 80 ◦ C (Hendry, 2002; Schneider et al., 2008). In addition, the precipitation temperature of vein carbonates could be similar to, or slightly higher than, that of wallrock carbonates (Wilson et al., 2000; Fayek et al., 2001; Hendry, 2002). The negative ı18 O value of −27.6‰ for veins (PDB) would require a high temperature up to 250 ◦ C assuming a modern seawater of ı18 O = 0‰ (relative to VSMOW) and the magmatic or metamorphic fluids normally have higher ı18 O values than 6‰ (Hoefs, 2009). Therefore, the low ı18 O values for the veins indicate that the veins were precipitated from waters that were isotopically distinct from the modern seawater or the magmatic

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fluid. The ı18 O values of meteoric water vary from −55‰ to −10‰ (VSMOW) in the geological history (Hoefs, 2009). The ı18 O value of modern meteoric water over Arctic Canada is −20‰ (VSMOW), and the value for precipitation over the center of the Greenland Ice Sheet is −35‰ (VSMOW) (Dansgaard and Tauber, 1969). The different REE + Y patterns of veins from those of various different rivers in the world (Chung et al., 2009; Singh, 2009, 2010; Viers et al., 2009) indicating that the post-depositional fluid of negative low ı18 O values is probably derived from the continental deglacial meltwater (Zhao and Zheng, 2010). Moreover, authigenic cements within the Reynella Member of the glacial Elatina Formation show a ı18 O value of −25‰, implying meltwater released from glacial ice (Kennedy et al., 2008). Therefore, the negative ı18 O values of −28.6‰ to −22.0‰ for the veins were probably related to the meltwater of continental deglaciation. The variation in ı18 O value for the vein and wallrock carbonates may be caused by the variation of temperatures or the different proportions of mixing between the deglacial meltwater and the seawater. The veins have slightly lower ı18 O values than the wallrocks, which could probably form at different temperatures if the depositional and post-depositional fluids have the same oxygen isotopic compositions. For example, a fluid with a ı18 O value of −21.6‰ (VSMOW) can precipitate a vein with a calcite ı18 O value of −26‰ (PDB) at 35 ◦ C and a wallrock with a calcite ı18 O value of −19.8‰ (PDB) at 22 ◦ C. Thus, taking the temperature into account, the depositional and post-depositional fluids could be similar to each other. The vein and wallrock carbonates could also form at similar temperatures if the depositional and post-depositional fluids have different isotope compositions by different proportions of mixing between the deglacial meltwater and the seawater. The vein and wallrock carbonates exhibit the similar REE + Y patterns (Fig. 11a), suggesting the same geochemical source of terrigenous materials in the precipitation fluids. The similar REE + Y patterns also occur in the both margin and center of vein in sample 05WN75 (Fig. 11a), indicating that the post-depositional fluids were homogenized during veining and had a similar geochemistry to that of the wallrock-depositing fluid. Therefore, the vein-forming fluid would have the same origin as the wallrock-depositing fluid, which was an internally-derived fluid and ultimately derived from the continental deglacial meltwater subsequent to the Gaskiers iceage (Zhao and Zheng, 2010).

6.2. The Lower Unit The veins or cements in the LU exhibit no spatial relationship to the laminations (Fig. 3c). However, the wallrock and cement are interpenetrated, with sharp edges and angles in sample 05WN155 (Fig. 8c–f). The wallrock breccias are dolomitized, which would be possible in the environment of hydrofracturing (Phillips et al., 2006). No matrix is present, but intense cementation by calcite encases the clasts or fills the fractures. This structure would probably form through reworking of consolidated to semi-consolidated thin-bedded calcareous deposits (Kullberg et al., 2001), suggesting that post-depositional fluids infiltrated soon after the wallrock deposition and before consolidation with a sudden, explosive genesis from migrating fluids (Breesch et al., 2010). The cap carbonate overlying the Marinoan glaciogenic succession in South China has been extensively studied in the last decade. This carbonate succession is interpreted as precipitation from the oxic surface meltwater overlying anoxic deep ocean (Shen et al., 2005, 2008; Shields, 2005), marine deglacial meltwater (Zhao and Zheng, 2010), or hydrothermal origin (Bristow et al., 2011; Huang et al., 2011). Clumped isotope temperatures of 86–156 ◦ C were obtained from carbonate C O isotope analyses (Bristow et al., 2011). There are positive correlations between the ı18 O and ı13 C

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values for the wallrock and vein/cement carbonates in the LU (Fig. 12b), indicating that the wallrock was pervasively altered by post-depositional fluids. In this case, the primary ı18 O and ı13 C values for the wallrocks can be approximated by the highest ı18 O value of −9.3‰ and the highest ı13 C value of −3.9‰. On the other hand, the post-depositional fluids were influenced by the local wallrock carbonates during their flow through and reaction with the wallrocks, resulting in increases in the both ı18 O and ı13 C values of the vein/or cement carbonates. In this regard, the ı18 O and ı13 C values for the post-depositional fluids are generally lower than those recorded by the veins. Therefore, the primary ı18 O and ı13 C values for the post-depositional fluids can be approximated by the lowest ı18 O and ı13 C values of −23.6‰ and −5.6‰, respectively. The difference in REE + Y patterns, ı18 O and ı13 C values between the depositional and post-depositional fluids is evident, indicating different origins. The wallrock and vein/cement carbonates in the LU exhibit positive Eu anomalies, which are prominent in carbonates precipitated from hydrothermal fluid (Bristow et al., 2011; Huang et al., 2011). However, some samples from the wallrock and vein/cement carbonates exhibit extremely high Ba concentrations. Thus, the positive Eu anomalies are likely artifacts due to the Ba interference during the LA-ICP-MS analysis. Nevertheless, the low Ba concentrations and the lack of Ba-Eu correlation in the other samples suggest that the Eu anomalies in these analyses are primary and can be used as a genetic indicator. The vein/cement in the LU is different from the wallrock in their REE + Y patterns, ı13 C and ı18 O values. This indicates that the post-depositional fluid is different from the depositional fluid and thus of external origin. The REE + Y patterns for all the veins and cements suggest that the terrigenous chemical weatherings were uptaken by the post-depositional fluids. The veins and cements in the LU exhibit different REE + Y patterns from those in the UU (Fig. 9c and d), indicating that the post-depositional fluids of the two units are controlled by different terrigenous sources. The ı13 C values for the vein and cement in the LU are markedly lower than those for the wallrock (Fig. 12b). This indicates that the post-depositional fluid is more 13 C-depleted than that of the wallrock-depositional fluid. This 13 C-depleted carbon may be derived from the oxidation of methane clathrate (Jiang et al., 2003a; Wang et al., 2008), the oxidation of organic matter (Scasso and Kiessling, 2001; Rothman et al., 2003; Zhu et al., 2007), or the oxidation of DOC (Zhao and Zheng, 2010). For the LU carbonate, there is no sedimentological evidence, like distinctive sedimentary structures, textures and extremely negative 13 C values, for the vein/cement to be derived from the destablization of methane hydrate. But such a kind of evidence has been found from the carbonate of the Doushantuo Formation in South China (Jiang et al., 2003b; Wang et al., 2008). One possible explanation for this difference is that the Lantian and Doushantuo carbonates were deposited in different types of marginal basins on the continental shelf (Zhao and Zheng, 2010). This may result in differential survival of hydrate pools in association with the Leigongwu and Nantuo diamictites, respectively, during the destabilization of gas hydrate at the onset of Marinoan deglaciation. Thus, the oxidation of DOC may be the better explanation for the lower ı13 C values for the vein/cement relative to the wallrock. The large ı18 O differences between the wallrock and vein/cement carbonates in the LU would require a temperature difference as high as 220 ◦ C if the depositional and post-depositional fluids have the same ı18 O value. At such a high temperature, the wallrock and vein/cement carbonates would result in more uniform O isotope compositions. Thus, the large differences in the ı18 O values between the wallrock and vein/cement carbonates are attributed to the different O isotope compositions of fluids rather than to the different temperatures. The vein and cement

carbonates with the low ı18 O values requires that the postdepositional fluids have the low ı18 O values, which were probably derived from the meteoric water subsequent to the marine Marinoan deglaciation. The ı18 O values for the vein/cement carbonates in the both UU and LU fall in the same range, suggesting that they probably have the similar sources of water. However, fine-grained shale and mudstone occur between the LU and UU, which generally have extremely low permeability and high hydraulic impedance, and there is no carbonate in the shale and mudstone according to the stable isotope analyses by the GasBench II technique (Zha et al., 2010). Moreover, the vein and cement carbonates in the UU and LU exhibit different REE + Y patterns. Thus, the post-depositional fluids in LU may not have the same geochemical source as those in the UU. The contribution of continental deglacial meltwater in the Gaskiers iceage to the deposition of the UU carbonate is prominent (Zhao and Zheng, 2010). For the LU carbonate, on the other hand, the meteoric water may be involved in the post-depositional alteration. This process may have taken place subsequent to the marine Marinoan iceage with the ice sheet covering the global Earth (Hoffman et al., 1998; Hoffman and Schrag, 2002). 7. Conclusions The Ediacaran carbonates of the Lantian Formation contain abundant calcite veins and cements. The petrographic arrangement and geochemical composition of vein/cement and wallrock carbonates indicate that post-depositional fluids infiltrated the wallrock during or after lithification. Their stable isotope and trace element compositions exhibit significant differences and similarities that can be used to distinguishing post-depositional fluid from the depositional fluid. The ı18 O values are slightly different between the vein and wallrock carbonates in the Upper Unit. The ı13 C values and REE + Y patterns for the vein and wallrock carbonates in this unit are also similar to each other. The REE + Y patterns suggest that the terrigenous materials were incorporated into the depositional basin and taken up by the depositional and post-depositional fluids. There is a coupled negative correlation between the ı13 C and ı18 O values for the both vein and wallrock in individual samples, indicating that the geochemical records of wallrock were slightly altered by the post-depositional fluid. The unusually low ı18 O values for the vein and wallrock indicate the dominant oxygen contribution from the continental deglacial meltwater subsequent to the Gaskiers iceage. The geochemical signatures are markedly different in wallrock and vein/cement carbonates of the Lower Unit, suggesting that the post-depositional fluid is different from the depositional fluid and thus has an external origin. The REE + Y patterns for the vein/cement and wallrock carbonates suggest that the terrigenous materials were differently taken up by the depositional and post-depositional fluids. The vein/cement carbonates exhibit much lower ı13 C and ı18 O values than the wallrock carbonates, and the wallrock shows a covaried trend between ı13 C and ı18 O values. This indicates that the geochemical records of wallrock were significantly altered by the post-depositional fluids. The unusually low ı18 O values for the vein and cement in this unit indicate the large contribution from meteoric water, which could be infiltrated subsequent to the marine Marinoan deglaciation. Acknowledgments This study was supported by funds from the Natural Science Foundation of China (41102062 and 40921002), the Fundamental Research Funds for the Central Universities (WK2080000015) and the Natural Science Foundation of Anhui Province. Thanks are due

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