SPATIAL DISTRIBUTION OF ANKERITE AND DOLOMITE IN THE XISHA ISLANDS: EVIDENCE FROM GEOCHEMISTRY
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摘要:
成礁以来西沙群岛白云岩-铁白云岩大量发育，针对白云岩空间变异及成因演化的讨论具有重要意义。西科1井存在7层白云岩，对7层白云岩进行了划区处理，将7层白云岩层分为浅、中、深3类：<600 m为浅层（层一、层二、层三，合计厚度203 m），600～1 000 m为中层（层四、层五、层六，合计厚度79 m），>1 000 m为深层（层七，厚155 m）。通过对7层白云岩的主微量元素特征、成岩环境与成因模式及浅层、中层、深层白云岩地球化学数据之间的规律与差异的分析认为，西科1井白云岩受陆源物质影响小，高的SiO2可能来自西沙周缘火山地带。西沙地区白云岩成岩环境总的来讲属于氧化环境，但浅、中、深层成岩环境的氧化强度具有差异。西科1井白云岩的古盐度都较高，但层与层之间有着细微差异。西科1井白云岩锶含量总体偏低，受到淡水淋滤作用强烈，形成于古水深较浅的礁相台地环境，相比较而言，浅层受到大气淡水影响最强，中层次之，深层最弱。西沙白云岩成岩过程中受到高盐度海水、大气淡水和回流渗透作用等多种因素的影响。
Abstract:Dolomite and ankerite are widely distributed in the Xisha Islands. So far, seven layers of dolomite have been found in the well Xike 1. The seven layers of dolomite may be grouped into three zones, the shallow, the middle and the deep. The shallow zone is less than 600 meters in depth including the layer 1, layer 2 and layer 3 with a total thickness of 203 m; the middle zone includes the layer 4, layer 5 and layer 6 with a total thickness of 79 meters and occurs in a range of 600～1 000 m in depth; the deep zone includes the layer 7, 155 m in thickness buried underground more than 1000 meters in depth. The geochemical characteristics of major and trace elements of the seven layers of dolomite are studied in this paper, while diagenetic environment and genetic model discussed based on the newly acquired geochemical data combined with previous research results. Dolomite in the well Xike 1 was far beyond the affection of terrigenous materials, high content of SiO2 may come from the volcanic activities surrounding the islands. It is evidenced that the dolomitization in the Xisha area happened in an oxidation diagenetic environment, but the intensity of oxidation is different in the shallow, middle and deep zones. The paleosalinity of sea water was high when the dolomites formed, and there are slight differences between the three zones. The content of strontium in the dolomite of the well Xike 1 is generally low, as it was strongly leached by fresh water in an environment of reef platform with relatively shallow water. Comparatively, the influence of atmospheric fresh water was strongest in the shallow zone, followed by those in the middle and deep zones. The diagenesis of dolomite in the Xisha Islands is influenced by many factors, such as high palaeosalinity seawater, atmospheric fresh water and water reflux infiltration.
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Key words:
- Xisha Islands /
- well Xike 1 /
- dolomite /
- trace elements /
- diagenetic environments /
- genesis models
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表 1 西科1井白云岩层位及含量
Table 1. Position and content of dolomite in the well Xike 1
编号 井深/m 厚度/m 地层年代 白云石含量/% 层一 288.64～303.58 15 上新统 90.90 层二 375.37～411.88 36 上中新统 91.50 层三 423.78～575.9 152 上中新统 92.40 层四 620～635.06 15 中中新统梅山组一段 75.85 层五 759.1～775.05 16 中中新统梅山组二段 93.90 层六 972～1019.8 48 中中新统梅山组二段 61.70 层七 1 032.5～1 187.5 155 下中新统 82.50 
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表 2 西科1井白云岩层主量元素测试结果
Table 2. Test results of major elements in layers of dolostone in the well Xike 1
/Wt% 样品 CaO Fe2O3 K2O Na2O MgO P2O5 SiO2 层一（N=21） 最小值 36.7 0.016 0.018 0.256 12.559 0.045 4.930 最大值 45.8 0.079 0.048 0.945 21.357 0.081 13.870 平均值 39.2 0.037 0.030 0.455 19.711 0.060 9.589 层二（N=23） 最小值 32.5 0.012 0.009 0.137 17.932 0.044 9.710 最大值 37.9 0.210 0.046 1.486 21.252 0.095 24.600 平均值 36.7 0.069 0.027 0.420 20.130 0.060 14.234 层三（N=64） 最小值 34.3 0.008 0.006 0.053 1.514 0.013 5.100 最大值 52.2 0.232 0.061 1.008 22.374 0.075 19.390 平均值 37.8 0.037 0.018 0.295 20.140 0.038 12.094 层四（N=5） 最小值 36.5 0.007 0.008 0.056 5.830 0.016 0.000 最大值 52.5 0.085 0.052 0.694 20.785 0.051 15.160 平均值 41.8 0.029 0.024 0.311 16.490 0.039 8.576 层五（N=9） 最小值 34.9 0.012 0.011 0.100 18.842 0.030 8.900 最大值 38.5 0.081 0.036 0.887 22.088 0.057 17.670 平均值 36.9 0.040 0.020 0.334 20.452 0.043 13.299 层六（N=18） 最小值 30.7 0.002 0.008 0.145 3.042 0.025 3.180 最大值 51.4 0.261 0.075 1.054 18.619 0.083 28.680 平均值 38.1 0.041 0.021 0.443 13.416 0.053 17.966 层七（N=69） 最小值 29.5 0.006 0.003 0.069 1.168 0.013 6.960 最大值 51.3 0.196 0.074 0.472 19.950 0.086 30.160 平均值 33.4 0.039 0.015 0.153 17.938 0.053 22.154 总样品（N=209） 平均值 36.4 0.041 0.020 0.293 18.715 0.049 15.810
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表 3 各层白云岩CaO与MgO、SiO2与MgO相关系数及显著性值P
Table 3. Correlation coefficient and significance value P between CaO and MgO，SiO2 and MgO in layers of dolostone
层一 层二 层三 层四 层五 层六 层七 CaO与MgO相关系数 −0.870 7 0.833 0 −0.789 2 −0.974 9 0.265 7 −0.961 7 −0.886 7 CaO与MgO的显著性值P 1.37×10−7 2.38×10−7 5.77×10−15 4.76×10−3 0.49 2.03×10−10 3.91×10−24 SiO2与MgO相关系数 0.536 5 −0.909 3 0.164 8 0.869 5 −0.580 0 0.879 8 0.569 6 SiO2与MgO的显著性值P 0.01 3.12×10−10 0.19 0.06 0.10 1.50×10−6 3.25×10−7 注：<0.05为存在显著性
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表 4 浅、中、深层白云岩的CaO与MgO、SiO2与MgO相关系数及显著性值P
Table 4. Correlation coefficient and significance value P between CaO and MgO，SiO2 and MgO in the dolostone of shallow，middle and deep zones
浅层 中层 深层 CaO与MgO相关系数 −0.656 5 −0.788 1 −0.886 7 CaO与MgO的显著性值P 3.88×10−15 8.60×10−8 3.91×10−24 SiO2与MgO相关系数 0.069 9 0.365 5 0.567 0 SiO2与MgO的显著性值P 0.46 0.04 3.25×10−7 注：<0.05为存在显著性
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表 5 浅、中、深层白云岩主量元素平均含量
Table 5. Average content of major elements in the dolostone of shallow，middle and deep zones
/ Wt% CaO MgO Fe2O3 P2O5 K2O Na2O SiO2 浅层 37.84 20.05 0.044 0.047 0.022 0.353 12.079 中层 38.34 15.88 0.039 0.048 0.021 0.392 15.186 深层 33.40 17.94 0.039 0.053 0.015 0.153 22.154
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表 6 西科1井白云岩层微量元素测试结果
Table 6. Test results of trace elements in layers of dolostone in the well Xike 1
/mg/kg 微量元素 层一 层二 层三 层四 层五 层六 层七 全球第四系碳酸盐岩 Ti 最小值 12.236 5.550 4.430 8.805 34.713 55.516 41.184 400 最大值 63.870 108.121 131.175 72.230 89.901 299.868 381.004 平均值 30.882 42.984 22.249 32.616 53.371 121.133 95.857 Al 最小值 93.726 89.125 45.191 65.701 88.556 55.303 84.469 4 200 最大值 991.137 3 191.563 5 299.111 891.471 587.954 2 633.081 1 765.158 平均值 332.346 1 307.560 457.073 384.526 308.530 561.604 442.430 Zr 最小值 0.257 0.147 0.137 0.346 0.261 0.285 0.347 最大值 1.846 1.915 1.281 1.579 1.379 5.466 5.171 平均值 0.652 0.895 0.577 0.804 0.708 1.352 1.208 19 Sc 最小值 0.331 0.226 0.212 0.322 0.203 0.346 0.253 1 最大值 0.549 1.107 0.694 0.500 0.630 0.903 0.848 平均值 0.453 0.440 0.374 0.439 0.360 0.559 0.453 Th 最小值 0.043 0.030 0.019 0.038 0.044 0.048 0.047 1.7 最大值 0.186 0.349 0.274 0.195 0.227 0.429 0.963 平均值 0.103 0.102 0.071 0.113 0.101 0.141 0.142 V 最小值 1.237 0.339 0.350 4.109 2.004 0.370 0.592 20 最大值 10.707 10.261 25.966 11.967 22.362 7.720 31.826 平均值 2.377 3.139 4.197 8.234 8.196 1.909 9.554 Mo 最小值 0.044 0.078 0.031 0.058 0.047 0.042 0.028 0.4 最大值 1.682 2.542 1.376 0.350 1.042 1.545 1.559 平均值 0.217 0.389 0.176 0.140 0.307 0.230 0.206 U 最小值 0.735 0.461 0.510 0.770 0.487 0.317 0.384 2.2 最大值 2.317 2.956 3.004 3.452 16.491 0.635 13.511 平均值 1.169 1.070 1.159 1.705 4.344 0.501 2.489 Sr 最小值 240.700 175.300 166.800 192.000 169.300 176.900 187.700 610 最大值 376.800 264.000 537.700 271.700 202.800 425.800 526.500 平均值 285.776 210.874 209.189 217.720 184.944 250.889 231.986 V/Sc 最小值 2.814 1.455 1.212 10.795 4.372 0.737 1.901 最大值 12.128 30.539 72.107 23.928 63.168 8.550 72.846 平均值 5.409 6.608 11.539 18.067 24.354 3.169 23.313 V/Cr 最小值 0.121 0.043 0.038 0.271 0.110 0.024 0.091 最大值 1.254 1.486 2.918 0.679 2.072 0.486 3.002 平均值 0.468 0.540 0.414 0.445 0.569 0.119 0.796 B/Ga 最小值 19.274 3.178 4.335 5.435 14.816 4.967 8.008 最大值 73.912 163.711 147.777 57.495 105.234 69.986 86.939 平均值 43.941 36.786 60.035 33.087 64.408 29.659 35.411 Sr/Ba 最小值 33.421 40.444 18.781 50.086 13.490 12.329 11.674 最大值 112.020 106.453 175.044 177.605 64.644 172.538 133.263 平均值 77.080 69.347 88.117 98.117 46.605 77.721 65.274 注：全球第四系碳酸盐岩平均值来自参考文献[26]
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表 7 浅、中、深层白云岩微量量元素平均含量
Table 7. Average content of trace elements in the dolostone of shallow，middle and deep zones
/(mg/kg) 微量元素 浅层 中层 深层 V 3.618 4.666 9.554 Mo 0.229 0.237 0.206 U 1.142 1.770 2.489 Sr 224.440 227.159 231.986 V/Sc 9.297 11.455 23.313 V/Cr 0.451 0.296 0.796 B/Ga 51.955 39.968 35.411 Sr/Ba 81.973 72.156 65.274
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表 8 浅、中、深层白云岩成岩环境对比
Table 8. Comparison of diagenetic environment of dolomites in the shallow，middle and deep zones
浅层 中层 深层 氧化强度 较强 一般 较弱 古盐度 高 中 低 锶含量 较低 中 较高 古水深 较浅 中 较深 浓缩海水白云岩化作用影响 较强 一般 较弱 大气淡水淋滤溶蚀作用影响 较强 一般 较弱 回流渗透白云岩化作用影响 较弱 一般 较强
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