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Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions

Xiao Wang Jian Gao Sheng He Zhiliang He Yan Zhou Ze Tao Jiankun Zhang Yi Wang

Xiao Wang, Jian Gao, Sheng He, Zhiliang He, Yan Zhou, Ze Tao, Jiankun Zhang, Yi Wang. Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions. Journal of Earth Science, 2017, 28(2): 315-332. doi: 10.1007/s12583-016-0921-7
Citation: Xiao Wang, Jian Gao, Sheng He, Zhiliang He, Yan Zhou, Ze Tao, Jiankun Zhang, Yi Wang. Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions. Journal of Earth Science, 2017, 28(2): 315-332. doi: 10.1007/s12583-016-0921-7

doi: 10.1007/s12583-016-0921-7

Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions

More Information
  • Figure 1.  (a) Structural sketch map of the Shizhu synclinorium in the western region of Mid-Yangtze. (b) Simplified geological map showing the position of the sampling profile. (c) Schematic lithostratigraphic column of the Shizhu synclinorium in the western region of Mid-Yangtze.

    Figure 2.  Outcrop images of calcite veins hosted in marine carbonate rocks in the Shizhu synclinorium. (a) Outcrop image of bed-parallel veins and cross-fold veins in the Lower Triassic limestone (sample SH2); (b) (c) Outcrop image of cross-fold calcite veins in the Lower Triassic (sample SH3) and Lower Ordovician (sample SH10) argillaceous limestone; (d) Outcrop image of strike veins in the Middle Ordovician limestone, two sets of closely spaced parallel calcite-sealed fractures were recognized with mutually crosscutting relationships. One set strikes at 96° and formed by successive events of crack and seal (sample SH8), the other strikes at 39°.

    Figure 3.  Photomicrographs of calcite veins from the Shizhu synclinorium. (a) Photomicrograph of calcite growing symmetrically and calcite-filled fracture having a sharp contact with the host lithology, from the Lower Permian Maokou Formation dark-grey micritic limestone (SH6). (b) Fracture with crack-seal texture, from the Lower Permian Qixia Formation dark-grey bioclastic limestone (SH7). (c) and (d) Optical and cathode luminescence photomicrograph of Stages 1 and 2 calcites from a crack-seal texture calcite vein, the early stage calcite (Stage 1) shows a moderate red luminescence, the later stage calcite (Stage 2) represents a younger cement phase and this calcite stage shows a dull-red luminescence, from the Middle Triassic Badong Formation grey argillaceous limestone (SH1). (e) and (f) Optical and cathode luminescence photomicrograph of Stages 3 calcites, The youngest Stage 3 calcite growing in the central parts of some thick calcite veins own an intensely zoned luminescence pattern, with bright red, dull red and non-luminescent zones, from the Upper Permian Changxing Formation grey argillaceous limestone (SH4).

    Figure 4.  Photomicrographs of fluid inclusions in calcite veins. (a) Aqueous inclusions in sample SH11 from the Upper Cambrian Maotian Formation grey limestone and (b) sample SH8 from the Middle Ordovician Shipuzi Formation limestone. Fluid-inclusion microthermometric results from sample SH11 show that homogenization temperatures of a fluid inclusion assemblage in stage 2 are 113.3-119.4℃, and those in stage 1 are 144.1-149.7 ℃.

    Figure 5.  Homogenization-temperature (Th) ranges for fluid inclusion assemblages (FIAs) in individual calcite veins. The numbers in the diagrams represent the number of fluid inclusions within each FIA. Stages indicate the relative timing of fracture opening and cementation stages based on textural interpretation of cathodoluminescence, crystal morphologies and microscopic interpenetration. The FIAs on the diagrams mirror from boundary to center their respective position within the calcite veins.

    Figure 6.  Binary plot of homogenization temperature (Th) and final melting temperature (Tm) for stage 1 and 2 calcite veins.

    Figure 7.  Plot of carbon and oxygen isotopic data for samples plotted against the stratigraphic age of the samples. Blue diamond represents host rocks, and purple squares show calcite veins. Calcite veins are depicted in the same stratigraphic formations as their host rocks to allow for an easy comparison, but they are younger than the stratigraphic ages.

    Figure 8.  Binary plots of stable isotope data showing (a) δ13C values for calcite veins and host rock matrix and (b) δ18O values for calcite veins and host rock matrix. Both carbon and oxygen data are relative to PDB. Overall, the carbon data are similar for both host rocks and calcite veins, but the oxygen data are different between host rocks and calcite veins.

    Figure 9.  Distribution of oxygen isotopic composition of the fluids precipitating the calcite veins, which were calculated from the homogenization temperatures obtained from aqueous fluid inclusions and the oxygen isotopic composition of the calcite veins.

    Figure 10.  Chondrite-normalized-REE patterns of representative calcite veins showing obvious enrichment in LREE relative to HREE, minor to no Ce anomalies, but obvious differences in the total REE concentrations and Eu anomalies.

    Figure 11.  Timing of fracture opening and calcite precipitation in each sampling stratum, which was determined by homogenization temperatures obtained from aqueous fluid inclusions in calcite veins and burial and thermal history of the study area. a, b, c and d represent the burial and thermal history of the Triassic, Permian, Ordovician and Cambrian, respectively.

    Table 1.  Fluid inclusion microthermometric data of calcite veins

    Formation Sample Stage FIA Number Th (℃) Tm (℃) Salinity (wt.%)
    NaCl Equivalent
    T SH1 2 FIA-1 2 78.6-82.4 -8.1 11.81
    FIA-2 1 105.3
    FIA-3 2 124.0-130.4
    1 FIA-4 2 117.4-115.2
    FIA-5 5 141.3-148.5 -1.2 2.07
    FIA-6 3 150.0-158.4
    FIA-7 5 163.8-183.6 -2.7--2.3 3.87-4.49
    SH3 2 FIA-1 1 89.0
    FIA-2 2 97.6-99.6
    FIA-3 3 115.2-117.4
    FIA-4 3 133.5-138.8
    1 FIA-5 2 101.3-104.1
    FIA-6 7 119.4-128.9 -11.3--9.8 13.72-15.27
    FIA-7 1 133.2 -11.5 15.47
    FIA-8 4 148.1-154.2 -8.8 12.62
    FIA-9 6 173.3-180.1 -10.8--8.1 11.81-14.77
    FIA-10 1 189.5
    P SH5 1 FIA-1 4 82.1-90.4 -17.6--11.0 14.97-20.67
    FIA-2 5 107.4-116.1 -18.8--14.2 17.96-21.54
    FIA-3 4 127.6-136.7 -19.5--14.8 18.47-22.03
    FIA-4 6 142.4-150.2 -20.6--15.8 19.28-22.78
    FIA-5 2 167.6-169.1 -14.7 18.38
    FIA-6 2 189.9-190.0 -17.8--11.1 19.29-22.78
    SH6 1 and 2 FIA-1 3 89.2-96.6 -10.2--9.5 13.40-14.15
    FIA-2 3 101.2-112.1
    FIA-3 1 149.9
    FIA-4 5 154.2-162.8
    FIA-5 5 170.8-178.6 -14.7--9.2 13.07--18.38
    FIA-6 4 185.6-194.2 -19.6--15.9 19.37-22.10
    FIA-7 3 205.7-215.5 -2.3 3.87
    SH7 2 FIA-1 3 82.5-89.1 -1.2--0.9 1.57-2.07
    FIA-2 7 90.4-98.8 -2.3--1.0 1.74-3.87
    FIA-3 4 101.7-106.1 -4.2--2.3 3.87-6.74
    FIA-4 5 111.9-119.1 -7.9--1.7 2.90-11.58
    FIA-5 1 139.7 -0.8 1.4
    O SH8 1 FIA-1 4 89.4-91.5 -16.7--8.9 12.73-19.99
    FIA-2 5 98.2-107.4 -20.7--16.5 19.84-22.85
    FIA-3 8 108.6-112.8 -21.1--13.5 17.34-23.11
    FIA-4 5 119.1-125.4 -20.6--15.2 18.80-22.78
    FIA-5 6 135.6-155.2 -18.5--11.6 15.57-21.33
    FIA-6 2 172.6-182.2 -19.6--16.9 21.15-22.10
    SH9 2 FIA-1 2 93.6-94.8 -3.2--2.0 3.39-5.26
    FIA-2 6 100.3-106.8 -6.2--7.1
    FIA-3 4 116.0-119.2 -1.6 2.74
    FIA-4 4 125.7-129.2 -15.5 19.05
    1 FIA-5 4 98.6-105.5 -4.3--2.3 3.87-10.61
    FIA-6 1 135.0
    FIA-7 4 150.9-159.4 -12.8--7 10.49-16.71
    FIA-8 3 170.9-186.2 -4.8--0.3 0.53-7.59
    O SH10 2 FIA-1 6 84.6-88.0
    FIA-2 6 95.6-105.3 -19.8--9.2 13.07-22.24
    FIA-3 4 112.7-124.9 -0.8--0.1 0.18-1.40
    FIA-4 5 128.2-138.3 -19.7--16.5 19.84-19.84
    FIA-5 1 146.6
    SH11 2 FIA-1 4 91.2-101.6 -18.7--15.5 19.05-21.47
    FIA-2 8 113.3-119.4 -18.1--17.3 20.45-21.04
    1 FIA-3 2 133.5-134.7
    FIA-4 4 134.4-139.6 -21.0--17.3 20.45-23.05
    FIA-5 5 144.1-149.7 -20.2--14.7 18.38-22.51
    FIA-6 7 156.1-160.7 -18.6--14.9 18.55-21.40
    SH12 1 FIA-1 2 91.5-97.7 -17.0--11.2 15.17-20.22
    FIA-2 7 105.7-115.6 -16.7--10.4 14.36-19.99
    FIA-3 4 119.2-122.7 -17.4--17.2 20.37-20.52
    FIA-4 3 129.6-134.7 -20.5--19.9 22.31-22.71
    FIA-5 5 147.3-154.6 -16.6--10.4 14.36-19.92
    FIA-6 2 169.3-176 -16.6--15.8 19.29-19.92
    FIA. fluid-inclusion assemblage; Number. number of inclusions in each FIA; Th. homogenization temperature (minimum fluid trapping temperature); Tm. final ice-melting temperature (freezing temperature); salinities calculated from ice-melting temperatures (Bodnar, 1993).
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    Table 2.  Carbon, oxygen and strontium isotopic compositions of calcite veins and host rocks

    Sample Age Matrix
    δ13C (PDB, ‰)
    Matrix
    δ18O (PDB, ‰)
    Vein
    δ13C (PDB, ‰)
    Vein δ 18O
    (PDB, ‰)
    Matrix
    87Sr/ 86Sr
    Vein
    87Sr/ 86Sr
    SH1-1 T 2b 5.28 -4.38 3.50 -7.53
    SH1-2 T 2b 1.72 -5.74 1.34 -8.47
    SH2 T 1j -1.51 -7.05 -1.61 -6.83 0.708 14 0.708 098
    SH3 T 1j 1.61 -7.20 -0.41 -6.77 0.708 183 0.707 699
    SH5 P 2w 4.27 -5.22 4.26 -8.00 0.707 887 0.707 535
    SH6 P 1m 4.06 -5.67 0.36 -12.94 0.707 785 0.707 237
    SH7 P 1q 4.34 -6.69 3.71 -7.16 0.707 785 0.707 245
    SH8 O 2s -0.06 -9.58 -0.68 -9.82 0.709 300 0.711 502
    SH9 O 1h -2.14 -9.52 -2.14 -10.04 0.709 263 0.709 915
    SH10 O 1n -2.56 -9.42 -2.50 -10.76
    SH11 3m -1.59 -10.17 -2.23 -8.80 0.709 343 0.709 252
    SH12 1sp 0.41 -9.76 -0.07 -9.81 0.709 090 0.709 884
    SZ7-1 T 2b -1.621 -6.954 -1.735 -7.969
    SZ8-2 T 1j 1.947 -7.841 0.625 -8.91
    SZ9-2 P 2 3.576 -7.509 4.253 -11.30
    SZ9-1 P 2 5.031 -7.226 3.701 -9.746
    SZ28-1 O 2+3 0.142 -9.905 -0.715 -10.004
    SZ26-1 O 1 -0.444 -7.686 -1.431 -9.905
    SZ23-1 1 -0.43 -10.542 -0.36 -10.978
    LF14-1 1 3.045 -7.269 1.834 -9.8
    SZ1-17 1 0.497 -10.807 0.03 -10.257
    SZ1-20 1 1.23 -10.526 0.295 -10.749
    下载: 导出CSV

    Table 3.  Rare earth element analyses of calcite veins (ppm)

    Sample SH2 SH3 SH5 SH6 SH8 SH9 SH11 SH12
    Age T 1j T 1j P 2w P 1m O 2s O 1h 3m 1sp
    La 11.972 3 0.290 5 0.290 3 0.163 3 10.731 4 8.175 1 2.305 1 7.996 4
    Ce 24.513 8 0.564 6 0.618 6 0.270 6 25.026 5 12.902 0 5.287 1 18.687 1
    Pr 2.886 5 0.068 1 0.087 2 0.039 0 3.147 0 1.281 8 0.638 2 2.166 6
    Nd 11.023 3 0.294 6 0.384 1 0.126 9 12.366 0 4.233 3 2.507 5 7.960 1
    Sm 1.909 7 0.060 9 0.114 9 0.027 9 2.507 4 0.685 9 0.501 3 1.489 5
    Eu 0.433 2 0.012 5 0.029 3 0.006 6 1.043 3 0.325 5 0.106 8 0.545 2
    Gd 1.386 5 0.076 2 0.117 7 0.022 6 2.430 5 0.551 8 0.439 7 1.179 8
    Tb 0.150 2 0.009 5 0.018 9 0.006 0 0.393 7 0.077 9 0.072 1 0.185 2
    Dy 0.605 1 0.061 4 0.153 6 0.024 1 2.471 6 0.395 6 0.434 3 1.123 5
    Ho 0.084 9 0.012 9 0.033 0 0.006 4 0.474 7 0.067 9 0.088 2 0.225 5
    Er 0.177 7 0.037 2 0.089 7 0.019 5 1.382 1 0.185 0 0.252 1 0.629 1
    Tm 0.019 6 0.007 2 0.011 2 0.003 0 0.193 3 0.020 4 0.037 3 0.089 8
    Yb 0.125 9 0.032 9 0.095 5 0.021 9 1.327 5 0.116 5 0.247 7 0.560 2
    Lu 0.013 4 0.006 1 0.015 0 0.003 0 0.192 9 0.017 3 0.036 0 0.075 1
    ∑REE 55.302 2 1.534 6 2.059 0 0.741 0 63.687 9 29.036 2 12.953 5 42.913 1
    ∑LREE 52.738 8 1.291 2 1.524 4 0.634 3 54.821 6 27.603 7 11.346 1 38.845 0
    ∑HREE 2.563 4 0.243 4 0.534 6 0.106 7 8.866 3 1.432 5 1.607 4 4.068 1
    ∑LREE/∑HREE 20.573 8 5.305 0 2.851 5 5.945 2 6.183 2 19.269 5 7.058 5 9.548 7
    (La/Yb) CN 62.792 0 5.829 1 2.006 8 4.916 9 5.338 0 46.332 7 6.145 7 9.426 1
    (Gd/Yb) CN 8.844 4 1.860 8 0.989 7 0.829 2 1.470 4 3.803 5 1.425 8 1.691 4
    δCe 0.976 2 0.939 5 0.910 3 0.793 3 1.008 2 0.933 1 1.020 5 1.051 0
    δEu 0.822 2 0.564 7 0.777 3 0.815 9 1.305 3 1.634 1 0.702 6 1.270 3
    下载: 导出CSV
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Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions

doi: 10.1007/s12583-016-0921-7

English Abstract

Xiao Wang, Jian Gao, Sheng He, Zhiliang He, Yan Zhou, Ze Tao, Jiankun Zhang, Yi Wang. Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions. Journal of Earth Science, 2017, 28(2): 315-332. doi: 10.1007/s12583-016-0921-7
Citation: Xiao Wang, Jian Gao, Sheng He, Zhiliang He, Yan Zhou, Ze Tao, Jiankun Zhang, Yi Wang. Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions. Journal of Earth Science, 2017, 28(2): 315-332. doi: 10.1007/s12583-016-0921-7
    • The Shizhu synclinorium lies in the western Hubei and eastern Sichuan regions of the western part of Mid-Yangtze, delimited by the Qiyueshan anticlinorium and the Fangdoushan anticlinorium covering an area of about 4 300 km 2 (Fig. 1a). The study area has been considered to be prospective for oil/gas exploration due to potential oil/gas reservoir forming conditions in the Lower and Upper Paleozoic strata. The area represents the front edge of the thrust nappe of the Jiangnan-Xuefeng orogenic belt, and belongs to the western Hunan-Hubei and eastern Sichuan tectonic belt (Fig. 1a). The regional tectonic direction is NE-NNE. Since the Sinian, the area has experienced multiphase tectonic movements (Guo et al., 1996). During the Indosinian, the collision and subduction of the North China plate and the Yangtze plate occurred, leading to the closure of the Paleo-Qinling Ocean and ending the deposition of marine strata (Zhao et al., 2003). During the Late Yanshanian Orogeny, the study area experienced strong tectonic deformation, and a large scale thrust nappe developed in response to the continuous compression in a SE-NW direction initiated by the uplift of the Jiangnan-Xuefeng orogenic belt. The deformation styles present edejective folds in a plane which laid the foundation for the present structure framework (Fig. 1a; He et al., 2011). In the Himalayan, the long-range effects of the orogeny caused the early low angle thrust faults to reactivate, while the effects were much smaller than those in Late Yanshanian. Marine strata were deposited in the study area from the Early Sinian Period of the Late Proterozoic to the Middle Triassic of the Mesozoic Era. The Indosinian movement during the Late Triassic ended the marine deposition of sediments and initiated the continental deposition in the foreland until the Jurassic (Fig. 1c). The Late Yanshanian Orogeny in the Cretaceous caused the study area to experience strong folding, uplift and erosion, and exposing Palaeozoic and Mesozoic strata.

      Figure 1.  (a) Structural sketch map of the Shizhu synclinorium in the western region of Mid-Yangtze. (b) Simplified geological map showing the position of the sampling profile. (c) Schematic lithostratigraphic column of the Shizhu synclinorium in the western region of Mid-Yangtze.

    • For this study, fresh outcrop samples of calcite veins and adjacent host rocks were collected from Cambrian, Ordovician, Permian and Triassic carbonate rocks in the Shizhu synclinorium. The veins represent open fractures filled with calcite. Therefore, geochemical analyses of the veins provide a record of the nature and properties of the subsurface fluids that have migrated through the rock at various times. Sample locations are given in Fig. 1b. Fluid-inclusion, stable (oxygen, carbon) isotope, strontium isotope and rare earth element studies were carried out on the samples.

    • Fluid inclusion analyses were conducted on doubly polished sections. Petrographic observations were performed using a NIKON-LV100 microscope. A NIKON-LV100 microscope-mounted Linkam THM600 heating-cooling stage was used for the microthermometry of two-phase aqueous inclusions, and synthetic CO 2 fluid inclusions and synthetic pure H 2O fluid inclusions were used to calibrate the freezing-heating stage. The stage has a precision of ± 0.1 ℃. Using the cycling technique, the reproducibility of Th values was often within 1 ℃, however, due to the small size of the inclusions and the poor transparency of the crystals, the reproducibility of the measurements could be as much as 3 ℃. The heating-freezing rate was 0.2 to 5.0 ℃ /min in general, but it was reduced to less than 0.2℃/min near the phase transformation. Final ice melting temperatures were converted to weight percentage NaCl equivalent using the equation of Bodnar (1993). As calcite is a soft and cleavable mineral, fluid inclusions may change during subsequent diagenetic events and even during the microthermometric measurements themselves (Barker and Goldstein, 1990; Prezbindowski and Larese, 1987). To limit the possibility of measuring deformed aqueous inclusions, only primary inclusions from the same field of view were measured during a single heating or freezing run. By restricting measurements to inclusions within the same field of view, any sudden changes in liquid/vapor ratios due to inclusion deformation could be observed, and removed from consideration (Morad et al., 2010; Suchy et al., 2000). Heating runs were conducted before freezing runs to reduce the possibility of inclusion stretching by freezing (Meunier, 1989; Lawler and Crawford, 1983). All the fluid inclusion data were obtained at KeyLaboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan.

    • After petrographic characterization of hand specimens, the finest-grained portion of each calcite vein was collected from the hand specimen using a dental drill, then all drilled samples were ground using an agate pestle and mortar to smaller than 200 mesh. Powder samples and adjacent rock matrix were analyzed in parallel for oxygen, carbon and strontium isotopes, as well as for trace elements.

      For oxygen and carbon isotope measurements, samples were analyzed using a Thermo KIEL VI coupled to a MAT253 gas mass spectrometer at State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Carbon dioxide gas was recovered using standard extraction techniques and was analyzed with a MAT253 stable isotope mass spectrometer. The measurement accuracy of an internal standard was ±0.02‰ for δ18O and ±0.01‰ for δ13C. Isotopic values are given in the δ-notation and reported relative to the VPDB standard. When δ18O values are reported relative to standard mean ocean water (SMOW), they have been calculated from PDB values using the equation of δ18O SMOW=1.03091× δ18O PDB+30.91 (Coplen et al., 1983).

      87Sr/ 86Sr isotope measurements of calcite powder were made by isotope dilution using a Trition thermalionization mass spectrometer (TIMS) at GPMR Isotope Laboratory, China University of Geosciences, Wuhan. About 100 mg samples were digested in a Teflon bomb with a mixture of concentrated HNO 3 and HF, the sealed bombs were kept in an oven at 190 ℃ for 48 h. The decomposed samples were then dried on a hot plate and converted into chlorides by adding 1ml of 6N HCl, followed by a final evaporation. The samples were dissolved again in 1ml of 2.5N HCl and then centrifuged. Correction for isotopic fractionation during the analyses was made by normalization to 86Sr/ 88Sr=0.119 4. The mean standard error of the mass spectrometer performance was ±0.000 03 for the standard NBS-987 (Wang et al. 2013).

      Trace element compositions were determined by inductively coupled plasma mass spectrometry (ICPMS) (Agilent 7700x) after acid digestion of the samples in Teflon bombs at GPMR, China University of Geosciences in Wuhan. Measured results for standard rocks of AGV-2, BHVO-2, BCR-2 and RGM-1 indicate that the precision and accuracy were higher than 95% for most elements and 90% for some transitional elements. The detailed analytical procedure is shown in Liu et al. (2008).

    • Calcite veins occur widely in Lower Cambrian to Middle Triassic limestones of the Shizhu synclinorium. At each sampling point, vein and joint orientations along with other structural data (e.g., strata occurrence, bedding, fold axes) were measured. The majority of the fractures strike predominantly W-E and NNW-SSE ( Fig. 2d), and the fold axes have an average trend of 30° to 40°. Multiple calcite veins in strata fractures and associated unmineralized joints have been classified into three groups based on their relationship to bedding. Vein set terminologies (bed-parallel veins, cross-fold veins, strike veins) follow that of Evans (1994) for the Appalachian Plateau province. (1) Bed-parallel veins occurring along bedding planes and slip zones (Fig. 2a). (2) Cross-fold veins that cut across fold limb strike (the average fold-axis trend) at a large angle and are usually subvertical, irrespective of bedding orientation (Figs. 2a, 2b, 2c). (3) Strike veins are subparallel to fold limb strike and are usually normal to bedding (Fig. 2d). In some subvertical veins, mineralization is continuous from a subvertical vein into a bedding-parallel fracture, indicating coeval opening and mineralization of both fractures (Fig. 2a).

      Figure 2.  Outcrop images of calcite veins hosted in marine carbonate rocks in the Shizhu synclinorium. (a) Outcrop image of bed-parallel veins and cross-fold veins in the Lower Triassic limestone (sample SH2); (b) (c) Outcrop image of cross-fold calcite veins in the Lower Triassic (sample SH3) and Lower Ordovician (sample SH10) argillaceous limestone; (d) Outcrop image of strike veins in the Middle Ordovician limestone, two sets of closely spaced parallel calcite-sealed fractures were recognized with mutually crosscutting relationships. One set strikes at 96° and formed by successive events of crack and seal (sample SH8), the other strikes at 39°.

      The maximum principal stress direction of the fractures is SE-NW according to rose diagram analyses of the fracture strike (He et al. 2014). This is perpendicular to the anticlinal axis (Fig. 1b), indicating fractures are related to tectonic stresses. The majority of the samples studied were collected from subvertical veins and locally high-angle (70°-80°) veins with respect to bedding. Their vertical persistence varies from few centimeters to several meters, whereas the vein width ranges from several millimeters to a maximum of 20 centimeters. Narrower fractures often occur in swarms or clusters, but the spacing of the veins is variable.

    • Calcite has diverse crystal shapes (equant/blocky, or fibrous) and color (milky to white-yellow), and grows symmetrically from each wall into the interior of the fractures (Fig. 3a). Many veins appear as composite veins containing crack-seal textures in which the first stage of calcite cement was fractured and pulled apart followed by the emplacement of a late stage calcite cement (Figs. 2d, 3b and 3c). These observations indicate multiple phases of fracture opening and cement precipitation. Veining and subsequent crystallization of vein minerals are complex multistage processes rather than a single event (Parnell et al., 2000; Suchy et al., 2000).

      Figure 3.  Photomicrographs of calcite veins from the Shizhu synclinorium. (a) Photomicrograph of calcite growing symmetrically and calcite-filled fracture having a sharp contact with the host lithology, from the Lower Permian Maokou Formation dark-grey micritic limestone (SH6). (b) Fracture with crack-seal texture, from the Lower Permian Qixia Formation dark-grey bioclastic limestone (SH7). (c) and (d) Optical and cathode luminescence photomicrograph of Stages 1 and 2 calcites from a crack-seal texture calcite vein, the early stage calcite (Stage 1) shows a moderate red luminescence, the later stage calcite (Stage 2) represents a younger cement phase and this calcite stage shows a dull-red luminescence, from the Middle Triassic Badong Formation grey argillaceous limestone (SH1). (e) and (f) Optical and cathode luminescence photomicrograph of Stages 3 calcites, The youngest Stage 3 calcite growing in the central parts of some thick calcite veins own an intensely zoned luminescence pattern, with bright red, dull red and non-luminescent zones, from the Upper Permian Changxing Formation grey argillaceous limestone (SH4).

      On the basis of microscopy and cathodoluminescence (CL) petrography, the calcite in the study area has been classified into three groups. In veins displaying crack-seal textures calcite vein, the early stage calcite (Stage 1) shows a moderate red luminescence (Figs. 3c, 3d). Two crystal morphologies were identified in Stage 1 calcite veins, which are filled with coarse-crystalline equant and radiating crystals, with cloudy transparency (Figs. 3b, 3c). The later stage calcite (Stage 2) represents a younger cement phase and shows a dull-red luminescence (Figs. 3c, 3d). Stage 2 calcite crystals have isometric textures and are more transparent (Figs. 3b, 3c). The youngest (Stage 3) calcite growing in the central parts of some thick calcite veins shows an intensely zoned luminescence pattern, with bright red, dull red and non-luminescent zones (Figs. 3e, 3f). Stage 3 calcite is generally non-deformed. It represents well-crystallized calcite aggregates that grew in open spaces. The intensely zoned pattern is often interpreted as meteoric cements, which are formed at shallow depth and low temperatures (Suchy et al., 2000; Meyers, 1974). The bright red luminescent zones would then reflect precipitation in a suboxic environment and the non-luminescent zones could have formed under oxic conditions.

    • The primary fluid inclusions in Stage 1 and 2 calcite were investigated to determine the nature and temperature of the fluids that circulated along the fractures.The temperature analyses combined with burial and thermal history modeling are used to constrain the timing of calcite precipitation.

      In this study, aqueous inclusions that occur as isolated or randomly distributed within the calcite crystals, are interpreted to be primary, possibly having been trapped during crystal growth of the calcite. Primary inclusions provide data on the fluid composition and environmental conditions during crystal growth. However, unequivocal evidence for a primary origin of fluid inclusions is rare, as there are no examples of fluid inclusions along crystal growth structures.Compared with radiating calcite crystals; coarse-crystalline equant calcite crystals are inclusion-rich. The inclusion morphologies range from ellipsoidal to irregular shapes and their sizes are commonly less than 6 μm in the longest dimension, with the majority ranging from 2 μm to 4μm. Vapor-liquid aqueous inclusions are colorless and transparent, with vapor/liquid ratios measured from screen images ranging from 5% to 20% ( Fig. 4).

      Figure 4.  Photomicrographs of fluid inclusions in calcite veins. (a) Aqueous inclusions in sample SH11 from the Upper Cambrian Maotian Formation grey limestone and (b) sample SH8 from the Middle Ordovician Shipuzi Formation limestone. Fluid-inclusion microthermometric results from sample SH11 show that homogenization temperatures of a fluid inclusion assemblage in stage 2 are 113.3-119.4℃, and those in stage 1 are 144.1-149.7 ℃.

      Using cathodoluminescence, crystal morphologies and microscopic interpenetration, we analyzed the formation stages of the calcite veins and combined the results with homogenization temperatures (Th) in order to reestablish the processes of fluid activity. In this study, we selected ten veins from Cambrian, Ordovician, Permian and Triassic units, and several individual fluid inclusions of each fluid inclusion assemblage (FIA) were measured. Measured homogenization temperatures range from approximately 80 to 215 ℃throughout the sampled calcite veins (Table 1; Fig. 5). Th values show clearly decreasing trends in the FIAs on the diagrams mirror from boundary to center their respective position within the calcite veins. For aqueous inclusions, the trends in Th, which represent minimum estimates of the trapping temperatures, are interpreted to represent variations in fluid temperature during fracture opening and cementation. When Th data are correlated with the formation stages of the calcite veins(Fig. 5), we observe a decreasing Th trend of FIAs. For example, the range in Th for crack-seal fluid inclusions in sample SH1 is from 183.6 to 78.6 ℃. Four sets of Stage 1 FIAs record a temperature decrease from 183.6 to 115.2 ℃, whereas three sets of Stage 2 FIAs show a decrease from 130.4 to 78.6 ℃. The othercalcite veins that entirely span the fractures show similar ranges and trends of decreasing homogenization temperatures, summarized in Fig. 5.

      Table 1.  Fluid inclusion microthermometric data of calcite veins

      Formation Sample Stage FIA Number Th (℃) Tm (℃) Salinity (wt.%)
      NaCl Equivalent
      T SH1 2 FIA-1 2 78.6-82.4 -8.1 11.81
      FIA-2 1 105.3
      FIA-3 2 124.0-130.4
      1 FIA-4 2 117.4-115.2
      FIA-5 5 141.3-148.5 -1.2 2.07
      FIA-6 3 150.0-158.4
      FIA-7 5 163.8-183.6 -2.7--2.3 3.87-4.49
      SH3 2 FIA-1 1 89.0
      FIA-2 2 97.6-99.6
      FIA-3 3 115.2-117.4
      FIA-4 3 133.5-138.8
      1 FIA-5 2 101.3-104.1
      FIA-6 7 119.4-128.9 -11.3--9.8 13.72-15.27
      FIA-7 1 133.2 -11.5 15.47
      FIA-8 4 148.1-154.2 -8.8 12.62
      FIA-9 6 173.3-180.1 -10.8--8.1 11.81-14.77
      FIA-10 1 189.5
      P SH5 1 FIA-1 4 82.1-90.4 -17.6--11.0 14.97-20.67
      FIA-2 5 107.4-116.1 -18.8--14.2 17.96-21.54
      FIA-3 4 127.6-136.7 -19.5--14.8 18.47-22.03
      FIA-4 6 142.4-150.2 -20.6--15.8 19.28-22.78
      FIA-5 2 167.6-169.1 -14.7 18.38
      FIA-6 2 189.9-190.0 -17.8--11.1 19.29-22.78
      SH6 1 and 2 FIA-1 3 89.2-96.6 -10.2--9.5 13.40-14.15
      FIA-2 3 101.2-112.1
      FIA-3 1 149.9
      FIA-4 5 154.2-162.8
      FIA-5 5 170.8-178.6 -14.7--9.2 13.07--18.38
      FIA-6 4 185.6-194.2 -19.6--15.9 19.37-22.10
      FIA-7 3 205.7-215.5 -2.3 3.87
      SH7 2 FIA-1 3 82.5-89.1 -1.2--0.9 1.57-2.07
      FIA-2 7 90.4-98.8 -2.3--1.0 1.74-3.87
      FIA-3 4 101.7-106.1 -4.2--2.3 3.87-6.74
      FIA-4 5 111.9-119.1 -7.9--1.7 2.90-11.58
      FIA-5 1 139.7 -0.8 1.4
      O SH8 1 FIA-1 4 89.4-91.5 -16.7--8.9 12.73-19.99
      FIA-2 5 98.2-107.4 -20.7--16.5 19.84-22.85
      FIA-3 8 108.6-112.8 -21.1--13.5 17.34-23.11
      FIA-4 5 119.1-125.4 -20.6--15.2 18.80-22.78
      FIA-5 6 135.6-155.2 -18.5--11.6 15.57-21.33
      FIA-6 2 172.6-182.2 -19.6--16.9 21.15-22.10
      SH9 2 FIA-1 2 93.6-94.8 -3.2--2.0 3.39-5.26
      FIA-2 6 100.3-106.8 -6.2--7.1
      FIA-3 4 116.0-119.2 -1.6 2.74
      FIA-4 4 125.7-129.2 -15.5 19.05
      1 FIA-5 4 98.6-105.5 -4.3--2.3 3.87-10.61
      FIA-6 1 135.0
      FIA-7 4 150.9-159.4 -12.8--7 10.49-16.71
      FIA-8 3 170.9-186.2 -4.8--0.3 0.53-7.59
      O SH10 2 FIA-1 6 84.6-88.0
      FIA-2 6 95.6-105.3 -19.8--9.2 13.07-22.24
      FIA-3 4 112.7-124.9 -0.8--0.1 0.18-1.40
      FIA-4 5 128.2-138.3 -19.7--16.5 19.84-19.84
      FIA-5 1 146.6
      SH11 2 FIA-1 4 91.2-101.6 -18.7--15.5 19.05-21.47
      FIA-2 8 113.3-119.4 -18.1--17.3 20.45-21.04
      1 FIA-3 2 133.5-134.7
      FIA-4 4 134.4-139.6 -21.0--17.3 20.45-23.05
      FIA-5 5 144.1-149.7 -20.2--14.7 18.38-22.51
      FIA-6 7 156.1-160.7 -18.6--14.9 18.55-21.40
      SH12 1 FIA-1 2 91.5-97.7 -17.0--11.2 15.17-20.22
      FIA-2 7 105.7-115.6 -16.7--10.4 14.36-19.99
      FIA-3 4 119.2-122.7 -17.4--17.2 20.37-20.52
      FIA-4 3 129.6-134.7 -20.5--19.9 22.31-22.71
      FIA-5 5 147.3-154.6 -16.6--10.4 14.36-19.92
      FIA-6 2 169.3-176 -16.6--15.8 19.29-19.92
      FIA. fluid-inclusion assemblage; Number. number of inclusions in each FIA; Th. homogenization temperature (minimum fluid trapping temperature); Tm. final ice-melting temperature (freezing temperature); salinities calculated from ice-melting temperatures (Bodnar, 1993).

      Figure 5.  Homogenization-temperature (Th) ranges for fluid inclusion assemblages (FIAs) in individual calcite veins. The numbers in the diagrams represent the number of fluid inclusions within each FIA. Stages indicate the relative timing of fracture opening and cementation stages based on textural interpretation of cathodoluminescence, crystal morphologies and microscopic interpenetration. The FIAs on the diagrams mirror from boundary to center their respective position within the calcite veins.

      Initial ice-melting temperatures were not measured because of the small size of the inclusions. Final ice melting/freezing temperature (freezing point) of aqueous inclusions was used to determine the salinity of the entrapped aqueous phase (Bodnar, 2003). The freezing temperatures of inclusions in calcites were recorded in the range of approximately -21.1 to -0.1 ℃ (Table 1), corresponding to salinities of 0 and 23.11 wt.% NaCl equivalent. Thesalinities of Stage 1calcite veinshave a dominant range at approximately 12.73 wt.%-21.61 wt.% NaCl equivalents, while thesalinities of Stage 2calcite veins distribute evenly throughout the salinities range (Table 1, Fig. 6).

      Figure 6.  Binary plot of homogenization temperature (Th) and final melting temperature (Tm) for stage 1 and 2 calcite veins.

    • Twelve samples were analyzed for carbon and oxygen isotopes. Strontium isotopes were analyzed in nine samples in order to compare the chemical signatures of the host rocks and the calcite veins supported by carbon and oxygen isotopic data of ten samples from the same study area published by Yang et al. (2011) (Fig. 1b).

      The results of the isotopic analyses are shown in Table 2 and Figs. 7- 9. The δ18O V-PDB and δ13C V-PDB values of calcite range from -12.94 ‰ to -6.77‰ and from -2.50 ‰ to +4.26‰, respectively. The composition of the host rocks ranges between -10.81‰ and -4.38‰ PDB for oxygen and between -3.56‰ and +5.80‰ PDB for carbon (Table 2, Fig. 7). The Sr isotopic compositions of calcite and the host carbonate rocks range from 0.707 24 to 0.711 50 (Table 2), most of these values are comparable to the contemporary normal seawater Sr isotopic composition. Interpretation of the isotopic signatures of carbonate cements can provide important clues to the origin of fluids and the cementation conditions of calcite veins.

      Table 2.  Carbon, oxygen and strontium isotopic compositions of calcite veins and host rocks

      Sample Age Matrix
      δ13C (PDB, ‰)
      Matrix
      δ18O (PDB, ‰)
      Vein
      δ13C (PDB, ‰)
      Vein δ 18O
      (PDB, ‰)
      Matrix
      87Sr/ 86Sr
      Vein
      87Sr/ 86Sr
      SH1-1 T 2b 5.28 -4.38 3.50 -7.53
      SH1-2 T 2b 1.72 -5.74 1.34 -8.47
      SH2 T 1j -1.51 -7.05 -1.61 -6.83 0.708 14 0.708 098
      SH3 T 1j 1.61 -7.20 -0.41 -6.77 0.708 183 0.707 699
      SH5 P 2w 4.27 -5.22 4.26 -8.00 0.707 887 0.707 535
      SH6 P 1m 4.06 -5.67 0.36 -12.94 0.707 785 0.707 237
      SH7 P 1q 4.34 -6.69 3.71 -7.16 0.707 785 0.707 245
      SH8 O 2s -0.06 -9.58 -0.68 -9.82 0.709 300 0.711 502
      SH9 O 1h -2.14 -9.52 -2.14 -10.04 0.709 263 0.709 915
      SH10 O 1n -2.56 -9.42 -2.50 -10.76
      SH11 3m -1.59 -10.17 -2.23 -8.80 0.709 343 0.709 252
      SH12 1sp 0.41 -9.76 -0.07 -9.81 0.709 090 0.709 884
      SZ7-1 T 2b -1.621 -6.954 -1.735 -7.969
      SZ8-2 T 1j 1.947 -7.841 0.625 -8.91
      SZ9-2 P 2 3.576 -7.509 4.253 -11.30
      SZ9-1 P 2 5.031 -7.226 3.701 -9.746
      SZ28-1 O 2+3 0.142 -9.905 -0.715 -10.004
      SZ26-1 O 1 -0.444 -7.686 -1.431 -9.905
      SZ23-1 1 -0.43 -10.542 -0.36 -10.978
      LF14-1 1 3.045 -7.269 1.834 -9.8
      SZ1-17 1 0.497 -10.807 0.03 -10.257
      SZ1-20 1 1.23 -10.526 0.295 -10.749

      Figure 7.  Plot of carbon and oxygen isotopic data for samples plotted against the stratigraphic age of the samples. Blue diamond represents host rocks, and purple squares show calcite veins. Calcite veins are depicted in the same stratigraphic formations as their host rocks to allow for an easy comparison, but they are younger than the stratigraphic ages.

      Figure 8.  Binary plots of stable isotope data showing (a) δ13C values for calcite veins and host rock matrix and (b) δ18O values for calcite veins and host rock matrix. Both carbon and oxygen data are relative to PDB. Overall, the carbon data are similar for both host rocks and calcite veins, but the oxygen data are different between host rocks and calcite veins.

      Figure 9.  Distribution of oxygen isotopic composition of the fluids precipitating the calcite veins, which were calculated from the homogenization temperatures obtained from aqueous fluid inclusions and the oxygen isotopic composition of the calcite veins.

    • Rare earth element data of the calcites are presented in Table 3 and Fig. 10. The REE concentrations were normalized to chondritic values (CN, Fig. 10). On the chondrite-normalized diagram, the calcite veins show enrichment of light REE (LREE) over heavy REE (HREE) (∑LREE/∑HREE varies from 2.85 to 20.57; (La/Yb) CN ratio varies from 2.01 to 62.79, Table 3), flat to slightly depleted patterns for the HREE, (Gd/Yb) CN ratio varies from 0.83 to 8.84, Table 3), and minor to no Ce anomalies (δCe ratio varies from 0.79 to 1.05, Table 3). However, there are obvious differences in the total REE contents and Eu anomalies among the calcite veins from different strata. The total REE content of samples SH3, SH5 and SH6 (∑REE varies from 0.74 to 2.06, Table 3) are lower than that of other samples (∑REE varies from 12.95 to 63.69, Table 3); Samples SH8, SH9 and SH12 are distinguished by a pronounced positive Eu anomalies (δEu ratio varies from 1.27 to 1.63, Table 3), and the others are characterized by the presence of moderate negative Eu anomalies (δEu ratio varies from 0.56 to 0.82, Table 3).

      Figure 10.  Chondrite-normalized-REE patterns of representative calcite veins showing obvious enrichment in LREE relative to HREE, minor to no Ce anomalies, but obvious differences in the total REE concentrations and Eu anomalies.

      Table 3.  Rare earth element analyses of calcite veins (ppm)

      Sample SH2 SH3 SH5 SH6 SH8 SH9 SH11 SH12
      Age T 1j T 1j P 2w P 1m O 2s O 1h 3m 1sp
      La 11.972 3 0.290 5 0.290 3 0.163 3 10.731 4 8.175 1 2.305 1 7.996 4
      Ce 24.513 8 0.564 6 0.618 6 0.270 6 25.026 5 12.902 0 5.287 1 18.687 1
      Pr 2.886 5 0.068 1 0.087 2 0.039 0 3.147 0 1.281 8 0.638 2 2.166 6
      Nd 11.023 3 0.294 6 0.384 1 0.126 9 12.366 0 4.233 3 2.507 5 7.960 1
      Sm 1.909 7 0.060 9 0.114 9 0.027 9 2.507 4 0.685 9 0.501 3 1.489 5
      Eu 0.433 2 0.012 5 0.029 3 0.006 6 1.043 3 0.325 5 0.106 8 0.545 2
      Gd 1.386 5 0.076 2 0.117 7 0.022 6 2.430 5 0.551 8 0.439 7 1.179 8
      Tb 0.150 2 0.009 5 0.018 9 0.006 0 0.393 7 0.077 9 0.072 1 0.185 2
      Dy 0.605 1 0.061 4 0.153 6 0.024 1 2.471 6 0.395 6 0.434 3 1.123 5
      Ho 0.084 9 0.012 9 0.033 0 0.006 4 0.474 7 0.067 9 0.088 2 0.225 5
      Er 0.177 7 0.037 2 0.089 7 0.019 5 1.382 1 0.185 0 0.252 1 0.629 1
      Tm 0.019 6 0.007 2 0.011 2 0.003 0 0.193 3 0.020 4 0.037 3 0.089 8
      Yb 0.125 9 0.032 9 0.095 5 0.021 9 1.327 5 0.116 5 0.247 7 0.560 2
      Lu 0.013 4 0.006 1 0.015 0 0.003 0 0.192 9 0.017 3 0.036 0 0.075 1
      ∑REE 55.302 2 1.534 6 2.059 0 0.741 0 63.687 9 29.036 2 12.953 5 42.913 1
      ∑LREE 52.738 8 1.291 2 1.524 4 0.634 3 54.821 6 27.603 7 11.346 1 38.845 0
      ∑HREE 2.563 4 0.243 4 0.534 6 0.106 7 8.866 3 1.432 5 1.607 4 4.068 1
      ∑LREE/∑HREE 20.573 8 5.305 0 2.851 5 5.945 2 6.183 2 19.269 5 7.058 5 9.548 7
      (La/Yb) CN 62.792 0 5.829 1 2.006 8 4.916 9 5.338 0 46.332 7 6.145 7 9.426 1
      (Gd/Yb) CN 8.844 4 1.860 8 0.989 7 0.829 2 1.470 4 3.803 5 1.425 8 1.691 4
      δCe 0.976 2 0.939 5 0.910 3 0.793 3 1.008 2 0.933 1 1.020 5 1.051 0
      δEu 0.822 2 0.564 7 0.777 3 0.815 9 1.305 3 1.634 1 0.702 6 1.270 3
    • The petrographic and geochemical results obtained indicate that the calcite veins have been subjected to diagenesis by fluids of variable origins and compositions.

    • The δ13C values of the whole calcite veins and the majority of host rocks (20 out of 22) fall within -5‰ to 5‰, which suggests that marine carbonates are the principal carbon source (Hudson, 1977). Figure 7 shows the carbon and oxygen isotopic values for both the host rocks and the calcite veins in relation to the stratigraphic age of the host rocks (from Cambrian to Triassic). The calcite veins are definitely younger- formed during Early Cretaceous-Early Eocene confirmed by the following analysis. Most of the δ13C values (18 out of 22) of the vein calcite are close to those of the carbonate host rocks, suggesting that the dissolved carbon needed for calcite veins formation was derived from the host rocks. Furthermore the carbon isotopic compositions of the host limestones and the calcite veins exhibit a good correlation coefficient of 0.86 (Fig. 8a). This similarity is significant, considering the differences in the stratigraphic ages of the samples, as well as the fact that δ13C values are not affected by temperature changes. This similarity suggests that carbon in the fracture-filled calcite veins was derived from the same limestone formation. In other words, carbon extracted by fluids from the host limestone was not transported across formations but was rather precipitated in veins of the same formation (Sorkhabi, 2005). While four samples from the Lower Permian Maokou Formation (SH6), the Middle Permian Wujiaping Formation (SZ9-1) and the Lower Triassic Jialingjiang Formation (SH3 and SZ8-2) have obviously low δ13C values compared to the host rocks, these veins are also characterized by the presence of high salinity inclusions (13.07 wt.%-22.10 wt% NaCl and 12.62 wt.%-15.47 wt.% NaCl, respectively), precluding the effect of meteoric water. In general, low δ13C values can be the result of a higher input of 12C-rich CO 2, derived from sulfate-reducing bacterial degradation or thermogenic degradation of organic matter (Clayton, 1994; Irwin et al., 1977). Black carbonaceous shale and dark-grey micritic limestonewere deposited as part of the Maokou Formation, and the Jialingjiang Formation was deposited as anhydrite and dark-grey limestone with argillaceous intercalary strata (Fig. 1c); organic matter is abundant in both formations. However, the available homogenization temperatures of fluid inclusions trapped in those calcite veins reveal that the calcite crystals were precipitated at temperatures above 89 ℃(Table 1). Above this temperature, almost all sulfate-reducing microbes cease to metabolize (Machel, 2001). Thus, it is less likely that the low δ13C values result from degradation of organic matter by sulfate-reducing bacterial in the study area.

    • δ18O values are markedly different between the host rocks and the calcite veins (Table 2; Fig. 7). The fact that most of the δ18O values of the calcite veins fall below the 1 : 1 line in Figure 8b indicates that the water-precipitating calcite veins was either at higher temperature or more depleted in δ18O. The δ18O values of the calcite veins are more negative than those of host rocks, indicating that the calcite veins were more affected by isotope exchange with low δ18O water or elevated temperatures.

      Assuming calcite precipitated in equilibrium with the ambient fluids, the oxygen isotopic composition of the fluids precipitating calcite can be calculated from the precipitation temperature and the oxygen isotopic composition of the calcite cements. The relationship between temperature and the oxygen values of calcite and water is given by the following equation (O'Neill, 1969):

      $$\begin{array}{l} {\rm{1000}} \times {\rm{ln}}\left[ {\left({{\rm{1000 + }}{\delta ^{{\rm{18}}}}{{\rm{O}}_{{\rm{calcite}}}}} \right){\rm{/}}\left({{\rm{1000 + }}{\delta ^{{\rm{18}}}}{{\rm{O}}_{{\rm{water}}}}} \right)} \right]{\rm{ = }}\\ \left({{\rm{2}}{\rm{.78}} \times {\rm{1}}{{\rm{0}}^{\rm{6}}}{{\rm{T}}^{ - {\rm{2}}}}} \right) - {\rm{3}}{\rm{.39}} \end{array}$$ (1)

      where T is water temperature in Kelvin, and δ18O calcite and δ18O water are given relative to SMOW. The δ18O calcite values of calcite veins can be obtained from sample analyses (Table 2). Deciphering the origin of water from which calcite has precipitated in the vein is difficult. A significant complicating factor is temperature. If some constraints can be placed on the temperature of fluid migrating through the veins, we can obtain some idea about the composition of the fluid.

      The calculations were made using homogenization temperatures obtained from microthermometry of aqueous inclusions in calcite veins. Uncertainty arises from the fact that the fluid inclusion homogenization temperatures (in the absence of pressure data) are minimum fluid-trapping temperatures. Therefore, the calculated δ18O water value relative to the true δ18O water value is low. By calculating temperatures of 78.6 to 189.5 ℃ and δ18O calcite values of -8.47‰ and -6.77‰ PDB, we find that calcite from the Triassic precipitated from a fluid with δ18O V-SMOW values between 2.81‰ and 14.40‰. Analogously, δ18O water values of calcite from the Cambrian, Ordovician and Permian range vary from 3.02‰ to 10.23‰, 1.25‰ to 10.57 ‰and -0.41‰ to 12.87‰ (relative to SMOW), respectively (Fig. 9). These values indicate water δ18O compositions above normal ocean water (with 0 SMOW). The oxygen isotopic composition of the hydrothermal fluid in equilibrium with the initial mantle magma is between 6‰ and 8‰ (Zheng and Chen, 2000). The oxygen isotopic composition of sedi mentary rocks is between 10‰ and 44‰. Carbonate rocks have the highest δ18O value in sedimentary rocks varying from 22‰ to 44‰ (Wang, 2000). Considering the significantly positive δ18O water values (10.23‰-14.42‰ SMOW) and high homogenization temperatures of aqueous inclusions in calcite veins (78.6-215.5 ℃), the effect of meteoric water and magmatic hydrothermal fluids is limited, andit is therefore plausible that the range of these δ18O water values indicate precipitation from the marine basin fluid itself (such as diagenetic, pore-filled, or formation water), as water-rock interaction causes the δ18O values of basinal brines to shift toward more positive values with temperature and salinity increase (Sheppard, 1986). This interpretation is also consistent with high salinities (>11% NaCl equivalent) determined for the majority of the aqueous inclusions ( Table 1).

    • Former studies indicate that the residence time of strontium in seawater is considerably longer than seawater blending time. Therefore, the isotopic composition of marine strontium during any time is uniform globally, which accordingly makes the isotopic composition of strontium in seawater a geological function of time (McArthur, 1994). Therefore, after the 87Sr/ 86Sr of calcite (Table 2) is measured and compared with that of seawater through time, it could be determined that the fluid originated from re-dissolution of carbonates from identifiable strata of the same age or of different ages, or belongs to other fluid sources (Wang et al., 2011).

      The representative sample from the Cambrian was taken from the Maotian Formation (∈ 3m) of the Upper Cambrian and the Shipai Formation (∈ 1sp) of the Lower Cambrian. The 87Sr/ 86Sr ratios of the calcite veins (0.709 252 and 0.709 884, respectively) and host rocks (0.709 343 and 0.709 090, respectively) of the Maotian Formation and the Shipai Formation are close to that of the contemporary normal seawater, which ranges from0.709 1 to 0.709 3 (Denison et al, 1998), and 0.708 4 to 0.710 0 (Shi et al, 2002), respectively. It shows that the fluids-precipitating the calcite veins came from the host strata.

      The 87Sr/ 86Sr ratios of calcarenite and calcite veins from the Middle Ordovician are 0.709 300 and 0.711 502, respectively. The 87Sr/ 86Sr ratio of limestones from the Lower Ordovician is 0.709 263, whereas that of the calcite veins infilling fissures remains at 0.709 915. Thus, it is obvious that these ratios are more radiogenic than Middle Ordovician seawater (0.708 8-0.709 0, Denison et al, 1998) and Late Ordovician seawater (0.708 8-0.709 2, McArthur, 1994). The 87Sr/ 86Sr ratios of felsic clastic rocks and mudstones are generally more radiogenic, because the two types of rocks contain a large amount of radiogenic 87Sr. For example, the 87Sr/ 86Sr ratios of the siliceous clasts collected from the late Cenozoic deposits at the Alpha ridge in the middle of Atlantic Ocean range from 0.7131 to 0.725 1 (Winter et al, 1997). Therefore when meteoric water leaches siliceous and argillaceous materials at the ground surface, the meteoric water will be enriched in 87Sr. Although the 87Sr/ 86Sr ratios can be relatively high if the calcite veins precipitated from such fluid, the presence of high salinity inclusions (12.73-23.11 wt.% NaCl) in calcite veins precludes the injection of terrigenous 87Sr introduced by meteoric water. The Upper Ordovician Wufeng groups and the Silurian were deposited as thick mudstone and siliciclastic rock. The most likely explanation of more radiogenic 87Sr/ 86Sr ratios lies in the water/rock interaction with those rocks.

      For samples from the Permian, the 87Sr/ 86Sr ratio of limestones from the Lower Permian is 0.707 785 and that of calcite veins infilling fissures vary from 0.707 237 to 0.707 245. The 87Sr/ 86Sr ratio of calcite vein of Middle Permian is 0.707 535. Earlier studies indicate that the 87Sr/ 86Sr ratios of normal seawater from the Early and Middle Permian are 0.707 6-0.708 2 and 0.706 7-0.707 6, respectively (Shi et al, 2002). The distinctive 87Sr/ 86Sr ratios of calcite veins from the Lower Permian are less radiogenic compared to those of its host strata and Early Permian seawater, while the 87Sr/ 86Sr ratios of those less radiogenic samples and the Middle Permian calcite are within the range of values from the Middle Permian seawater, indicating that the calcite veins could have formed from fluids that originated from Middle Permian seawater.

      Calcitesamples from the Lower Triassic Jialingjiang Group (T 1j) yield 87Sr/ 86Sr ratios from 0.707 699 to 0.708 098 and the matrix limestones yield 87Sr/ 86Sr ratios ranging from 0.708 140 to 0.708 183. These ratios fall in the range of Early Triassic seawater (0.707 6-0.708 2; Shields et al., 2003), suggesting that the fluid forming the secondary calcite came from the host strata, and achieved equilibrium with its host rock.

      As discussed above, 87Sr/ 86Sr ratios of calcite from different ages vary from seawater ratios to values that are more radiogenic. This implies that the precipitation of calcite from basinal fluids was involved in the host strata, and partly suffered the effect of fluid from the upper strata.

    • Rare earth elements (REE) have been extensively used in the last four decades as a proxy for various geologic processes and sources of fluid composition (Morad et al., 2010). Because of their similar valence, ionic radii, and high similarity in electronic structure to trivalent actinides, REE behave as a coherent group in natural environments (Lee et al., 2003; Webb and Kamber, 2000). For carbonate minerals in particular, REE features are used to gain information on the chemistry, temperature and redox conditions of the fluids precipitating calcite (Gao et al., 2014; Bau and Moller, 1992; Al-Aasm and Veizer, 1986; Sverjensky, 1984). Evidence for the role of hydrothermal fluids vs. marine basinal fluids on the distribution of REE in authigenic carbonates is unequivocal and may provide an important tool to constrain the origin offluids and their relation to the tectonic evolution of sedimentary basins (Morad et al., 2010).

      On achondrite-normalized diagram (Fig. 10), the aforementioned REE characteristics of calcite veins can make the following explanations.

      (Ⅰ) Enrichment in LREE relative to HREE. LREE/HREE fractionation can occur as a result of sorption and complex processes. Because the ionic radius between Ca 2+ and LREE 3+ is similar (Bau, 1991), the normalized-REE patterns of calcite veins are presumably controlled by complex processes rather than by adsorption to oxides and oxyhydroxides (Morad et al., 2010; Uysal et al., 2007).

      (Ⅱ) Flat to slightly depleted HREE patterns. These may be attributed to the low CO 32− concentrations compared with other carbonate anionic groups that form stronger complexes with HREE than LREE (Barker et al., 2006).

      (Ⅲ) Small negative Ce anomalies. These are typical for marine carbonate sediments, indicating that Ce 3+predominates (Murthy et al., 2004), suggesting the establishment of reducing conditions (Bau and Alexander, 2006).

      (Ⅳ) A depletion in total REE concentrations in samples SH3 and SH6.Thisis attributed to the influx of organic fluids that are characterized by low concentrations in REE (Wang et al., 2010). This conclusion is in agreement with our previous analyses on the origin of 13C in samples SH3 and SH6.

      (Ⅴ) The presence of pronounced positive Eu anomalies in samples SH8, SH9 and SH12. Thermodynamic calculations and theoretical considerations suggest that temperature is the most important parameter that controls the Eu 3+/Eu 2+ redox potential in hydrothermal environments (Bau and Moller, 1992; Sverjensky, 1984). At temperatures exceeding 250 ℃, Eu 2+ dominates over Eu 3+ and may substitute for Ca 2+ preferentially over trivalent REE, leading to positive Eu anomalies in mineral precipitates. Microthermometric measurements for samples SH8, SH9 and SH12 (the maximum homogenization temperature value of fluid inclusions is 186.2 ℃) indicate that the fluid precipitating these calcites never reached such high temperatures. Another mechanism for positive Eu anomalies is attributed to terrestrial input, especially the dissolution of plagioclase (Li et al., 2012; Cai et al., 2008; Lee et al., 2003). This conclusion is in agreement with the reason that causes the high 87Sr/ 86Sr ratios in those samples. Therefore, positive Eu anomalies in normalized-REE patterns a re attributed to terrestrial input. While the presence of moderately negative Eu anomalies in calcite veins is not only a manifestation of the inheritance of host rocks, it also may reflect the formation environment, suggesting low temperature and high pH conditions (Uysal et al., 2007).

    • Measured homogenization temperatures range from approximately 80 to 215 ℃ throughout the sampled calcite veins. The wide range in Th values is related to the ease with which calcite fractures and the trapping of fluids throughout the uplift history (Ulrich and Bodner, 1988). Because the homogenization temperatures of aqueous inclusions represent a minimum trapping temperature, the microthermometry of primary fluid inclusions on fracturing-sealing calcite veins indicates that the calcite crystals were precipitated at temperatures higher than 80 ℃. The freezing temperatures of inclusions in calcites were recorded in the range of approximately -21.1 to -0.1 ℃. This indicates that the entrapped fluids have a salinity range of 0 to 23.11 wt.% NaCl equivalent, indicating that salinities ranged from meteoric water through seawater to basinal brines. Analyzing the fluid inclusion data, low salinity fluid inclusions (<3.5 wt.% NaCl equivalent) are generally not common compared to high salinity fluid inclusions (>3.5 wt.% NaCl equivalent). The low salinity fluid inclusions are mainly from sample SH7. Petrographic observations of sample SH7 show that it belongs to the later stage 2 calcite veins. Thus, the low salinity fluids are interpreted as meteoric water involved in the precipitation of those calcites during the uplift to the surface ( Yang et al., 2013; Sorkhabi, 2005; Wang, 2000), whereas very high-salinity fluids (>11%) are considered to indicate either the migration of deep-burial basin brines or the interaction of meteoric water with evaporate beds along its flow path. Because evaporate deposits constitute a minor portion of the Paleozoic-Mesozoic strata in the region, highly saline fluids were probably deep-burial basin brines.

    • The fluid inclusion data on fracturing-sealing calcite veins were used to estimate their timing with respect to the burial history of the basin, considering the precipitation of the fracture-sealing minerals to be in thermal equilibrium with the host rock.

      To determine the timing of fracture opening and cementation relative to the burial history, one-dimensional (1-D) basin modeling of the sampled localities was performed using BasinMod1-D software, and calibrated to the measured maximum burial temperatures based on the thermal Kaiser effect of the rock thermo-acoustic emission signals (Zhang, 2014; Zhang et al., 2014)

      The rocks at the surface in the sampling area are located 6-10 km from Well Sanxing1 sites and are assumed to have had a similar burial and tectonic history (Fig. 1b). In this study, the burial history reconstruction took into account measured thicknesses of formations drilled in the well Sanxing1, measured the geological sections and regional geological reports. Absolute ages of depositional and erosional events were defined using the chronostratigraphic framework of the Sichuan basin (Zhang, 2014a). Determination of the porosity is extremely important because it has significant influence on the thermal conductivity and heat capacity of sediments which are in close relation with the thermal properties of a basin. The equation for porosity reduction as a function of the initial porosity and burial depth are widely used in basin modelling. The porosity-depth relationship for decompaction correction of Falvey and Middleton (1981) was used in this paper. The initial porosity and compaction factor of a pure lithology were adopted from the default values in the BasinMod1-D software. Mixed lithologies were created by specifying percentages of the pure lithologies for one-dimensional modelling. The mixed lithological properties such as the initial porosity, compaction factor, density and other parameters are then calculated by using the relevant pure lithological properties. The initial time of folding and uplift-denudation of the study area in the Jurassic is 136 Ma referencing the research of apatite fission track dating in different tectonic units of the western region of Mid-Yangtze (Shi et al., 2012; Mei et al., 2010). The strata denudation thickness of the study area in Yanshanian is about 3 500 m reconstructed by vitrinite reflectance, apatite fission track and the measuring of geological sections (Shi et al., 2012).

      The temperature at any given point and time in a basin depends on both the heat flow and the thermal conductivity of the basin, which are functions of tectonic settings and lithologies (Abdalla et al., 1999). Because of the lack of borehole temperatures (BHT) in the sampling outcrops, present-day heat flow is acquired from the terrestrial heat flow chart of southern China, which assigns 44 mW/m 2 (Lu et al., 2007; Yuan et al., 2006). The model assumes a mean annual surface temperature of 18 ℃ (Shi et al., 2012). Based on the geological evolution of the Shizhu synclinorium, the modified Jarvis and Mckenzie (1980) algorithm for the rifting heat flow model was used to calculate the paleo-heat flows and assigned values that are typical for the evolution of superimposed basins (Lu et al., 2007). Previous research on the paleo-heat flow in the western Hubei and eastern Chongqing regions show that the study area was a craton depression basin from the Precambrian to the Silurian, and paleo-heat flow varied in a low heat flow background with an increasing trend from craton to passive continental margin (Lu et al., 2007). In the early Late Permian, the paleo-heat flow reached its maximum (68-78 mW/m 2) influenced by the regional extension and the eruption of basalt in eastern Sichuan, and then decreased gradually. From the Late Triassic to the Jurassic, the study area developed into a foreland basin, the heat flow decreased continuously until now and the average value of paleo-heat flow of Late Jurassic was 47.6 mW/m 2 (Lu et al., 2007). Excellent correlation between the modelled maximum burial temperatures and the measured maximum burial temperatures of the Hanjiadian Formation (S 2h) based on the thermal Kaiser effect of the rock thermo-acoustic emission signals in the study area (Zhang, 2014; Zhang et al., 2014) indicates that the thermal modeling is suitable for the study area.

      Field observations show that the maximum principal stress direction of the fracture is perpendicular to the anticlinal axis, indicating that the formation of fractures is contemporaneous with tectonic compression and uplift. The tectonic compression on the one hand is conducive to the forming of faults and fractures, which can provide migration systems for large scale fluid flow and, on the other hand, also provides the driving force for large scale fluid migration. Furthermore, based on petrography and fluid inclusion analyses of calcite veins, the median Th value of aqueous inclusion assemblages in later stage calcite veins are generally lower than those in early stage calcite, which further proves that the calcite veins were formed during tectonic uplift. Calcite veins in an adjacent study area are interpreted to have formed during the Yanshanian and Himalayan orogeny on the basis of Rb-Sr isochron age and fluid inclusions analysis (Yang et al., 2014; Li et al., 2013; Wang et al., 2009). Therefore, projecting the homogenization temperatures of the primary aqueous inclusions to the period of tectonic compression and uplift of the strata correspond with the geological facts.

      The projection of homogenization temperatures in the burial history diagram displays the timing of the fracture opening and fluid activity in the Cambrian rocks occurred at 93.39-50.65 Ma, and the formation depth ranges from 6 796.89 m to 3 670.92 m (Fig. 11d); Analogously, the timing of the fracture opening and fluid activity in the Ordovician, Permian and Triassic rocks occurred at 92.53-49.37 Ma, 125.6-57.49 Ma, 135.69-65.61 Ma, respectively; and the formation depth ranges from 6 444.22 to 3 430.46 m, 6 828.95 to 3 157.94 m, 5 546.50 to 2 757.17 m, respectively (Figs. 11a, 11b, 11c).

      Figure 11.  Timing of fracture opening and calcite precipitation in each sampling stratum, which was determined by homogenization temperatures obtained from aqueous fluid inclusions in calcite veins and burial and thermal history of the study area. a, b, c and d represent the burial and thermal history of the Triassic, Permian, Ordovician and Cambrian, respectively.

      The different types of calcite veins encountered in the carbonate strata of the Shizhu synclinorium can result from various episodes of structural deformation that affected these rocks spanning a time frame from 135 to 50 Ma (Early Cretaceous-Eocene), which most likely occurred as a result of undergoing continuous SE-NW compression initiated by the migration of the Jiangnan-Xuefeng orogenic belt in the Late Yanshanian. The long-range effect of the early Himalayan orogeny could also have some influence on the formation of fractures. Fluid flux that resulted in the precipitation of calcite veins could have been controlled by these deformation events during a longer geologic timeframe, and the geochemical characteristics of calcite veins can provide the basis for these deformation events. The fluid inclusions and geochemical characteristic of calcite veins suggest that fluid flux associated with tectonic compression and uplift in the Shizhu synclinorium were mainly fluid flow within the same layer, and cross layer flow and influence of surface water infiltration count for little, with good petroleum preservation conditions.

    • Petrographic, fluid inclusion microthermometric and geochemical (C-, O- and Sr-isotopes as well as REE) study of calcite veins combined with basin modeling in the Shizhu synclinorium allows us to conclude the following:

      (1) Fluid inclusion microthermometry analyses indicate that the fluid inclusions have a wide range of homogenization temperatures varying from 78.6 to 215.5 ℃, with variable salinities(0-23.11 wt.% NaCl equivalent). The low-salinity fluids (<3.5%) and the Stage 3 calcite with intensely zoned luminescence pattern are interpreted as meteoric water involved in the precipitation of those calcites, this will be demonstrated by further study.

      (2) All the calcite veins have negative Ce anomalies, which are the typical characteristic of marine carbonate sediments, it is therefore plausible that all the calcite veins were precipitated from the marine basin fluid. 87Sr/ 86Sr ratios, calculated δ18O water values and the distinct REE pattern simultaneously suggest that the fluids forming calcite veins in each stratigraphic unit could be due to the involvement of fluids that originated from coeval seawater and evolved through different degrees of water/rock interaction. Fluids with more radiogenic 87Sr/ 86Sr ratios than coeval seawater and the presence of pronounced positive Eu anomalies in Lower to Middle Ordovician calcites suggest terrestrial input from upper strata mudstones and siliciclastic rocks could be involved in the precipitation of calcite.

      (3) Dissolved carbon needed for the formation of calcite veins is derived from multiple sources including marine carbonates hosting the calcite veins and the degradation of organic matter. Most of the δ13C values are remarkably similar between calcite veins and host limestone rocks, suggesting that the derivation of carbon in the veins were from host limestone rocks, while some calcites the from Lower Permian and Lower Triassic with obviously low δ13C values and depletion in total REE content are attributed to the influx of organic fluids.

      (4) Fluid inclusion analyses combined with burial and thermal history modeling indicate that the flow of various generations of brine through the carbonate formation fractures spanning a time frame from the Early Cretaceous to Eocene (135-50 Ma), and this time span indicates that the precipitation of calcites were related to the continuous SE-NW compression initiated by the migration of the Jiangnan-Xuefeng orogenic belt in Late Yanshanian and the long-range effect of the Early Himalayan orogeny.

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