Geochemical characteristics and tectonic setting of Carboniferous sandstone in the eastern part of north Qiangtang depression
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摘要:
为了探讨羌塘盆地石炭纪的演化特征，于羌塘盆地北坳陷东部刻莫地区采集了一批早石炭世早期杂多群砂岩样品，对其进行了全岩地球化学测试。地球化学研究结果表明，岩石矿物成分主要在方解石-石英到含镁黑云母组分端元之间，部分样品接近钾长石组分端元；化学风化作用指标（CIW）、化学蚀变作用指标（CIA）和A-CN-K图解，反映该组砂岩的碎屑成分受到了中等的风化作用，并在风化过程中发生微弱钾交代，长石发生伊利石化。化学组分指标（ICV）表明岩石碎屑为第一次旋回沉积物，受沉积分选和再循环作用影响不大；A-CN-K图解还反映出砂岩碎屑源岩中斜长石含量高于钾长石含量，主要在花岗岩和花岗闪长岩之间变化。主量、微量和稀土元素特征指示，该组砂岩的沉积构造背景为活动大陆边缘和大陆岛弧的性质，说明在早石炭世早期古特提斯大洋已经开始进入俯冲消亡和萎缩阶段。
Abstract:In order to explore the evolution history of Carboniferous Qiangtang basin, samples were collected from Early Carboniferous Zaduo Group sandstone in the Kemo area, the east of north depression of Qiangtang basin, to carry out the geochemical studies.The results indicate that the sandstones mainly consist of K-feldspar, calcite, magnesium biotitea and quartz etc.The chemical indexes of weathering (CIW), alteration (CIA) parameters and the A-CN-K plot of the sandstone suggest that the clastic constituents were moderately weathered in the area, and the clastic constituents underwent K-metasomatisim and replacement of feldspar by illite during weathering processes.The chemical composition index (ICV) indicates that some clastics represent the first cycle sediment, which is not affected by sedimentation sorting and recycling.The A-CN-K diagram also shows that the content of plagioclase in sandstone source rocks is higher than that of potash feldspar, and the variation is mainly between granite and granodiorite.The characteristics of major, trace and rare earth elements indicate that the sedimentary tectonic background of sandstone represents active continental margin and continental island arc, which suggests that the ancient Tethys Ocean began to enter the stage of subduction extinction and shrinkage in the Early Carboniferous.
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Keywords:
- Carboniferous /
- sandstone /
- element geochemistry /
- tectonic setting /
- eastern Qiangtang depression
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致谢: 在从事青藏高原地质调查研究工作中，潘桂棠先生给予了悉心的指导和热情帮助，谨以此文表达衷心的感谢。
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图 1 羌塘盆地构造纲要图(a)和刻莫地区地质图(b)
Figure 1. Simplified tectonic map of Qiangtang Basin (a) and the geological map of the Kemo area (b)
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图 3 刻莫地区杂多群砂岩Al2O3-SiO2图解
Figure 3. Al2O3-SiO2 diagram of the Zaduo Group sandstone in the Kemo area
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图 4 刻莫地区杂多群砂岩Al2O3-Fe2O3图解
Figure 4. Al2O3-Fe2O3 diagram of the Zaduo Group sandstone in the Kemo area
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图 5 刻莫地区杂多群砂岩A-CN-K判别图解
T—英云闪长岩; Gd—花岗闪长岩; G—花岗岩；A=Al2O3; CN = CaO+Na2O; K=K2O
Figure 5. A-CN-K triangular diagram of the Zaduo Group sandstone in the Kemo area
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图 6 刻莫地区杂多群砂岩Zr/Sc-Th/Sc图解
Figure 6. Zr/Sc-Th/Sc diagram of theZaduo Group sandstone in the Kemo area
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图 7 刻莫地区杂多群砂岩主量元素构造背景判别图解
A—大洋岛弧; B—大陆岛弧; C—活动大陆边缘; D—被动大陆边缘
Figure 7. Major element compositions of the Zaduo Group sandstones in the Kemo area for tectonic setting discrimination
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图 8 刻莫地区杂多群砂岩La-Th-Sc、Co-Th-Zr/10和Sc-Th-Zr/10构造背景判别图解
A—大洋岛弧; B—大陆岛弧; C—活动大陆边缘; D—被动大陆边缘
Figure 8. Tectonic setting discrimination of La-Th-Sc, Co-Th-Zr/10 and Sc-Th-Zr/10 for the Zaduo Group sandstone in the Kemo area
表 1 刻莫地区杂多群砂岩地球化学元素测试结果
Table 1 Geochemical analytical results of the Zaduo Group sandstone in the Kemo area
样品号 SiO2 Al2O3 Fe2O3 FeO CaO MgO K2O Na2O TiO2 Co Sc Zr Th La CIA CIW ICV KM-1 76.46 9.26 0.78 1.94 1.70 1.38 1.38 1.86 0.78 8.44 9.63 350.00 19.40 73.80 65.21 72.23 0.85 KM-2 77.10 9.14 0.81 1.82 1.67 1.28 1.36 1.90 0.69 8.50 6.56 296.00 14.00 51.70 64.96 71.91 0.84 KM-3 74.99 9.79 0.85 1.92 2.18 1.33 1.50 1.88 0.56 8.27 7.33 198.00 12.00 40.70 63.78 70.69 0.85 KM-4 63.01 14.47 2.79 2.57 1.74 2.32 2.81 1.36 0.77 16.70 12.70 179.00 13.30 45.10 71.00 82.36 0.81 KM-5 78.51 8.96 0.76 1.70 1.44 1.11 1.28 1.91 0.60 7.28 6.90 238.00 11.90 43.40 65.93 72.79 0.79 KM-6 75.29 10.22 1.16 1.86 1.50 1.38 1.58 1.89 0.52 8.55 6.95 151.00 9.51 35.30 67.28 75.09 0.79 KM-7 74.56 9.69 1.53 1.57 2.12 1.46 1.47 1.88 0.68 8.32 7.13 284.00 13.40 49.40 63.92 70.78 0.94 KM-8 70.72 10.20 1.96 1.77 3.19 1.64 1.76 1.67 0.55 10.70 7.38 137.00 8.07 28.50 60.64 67.73 1.06 KM-9 73.84 10.58 1.74 1.79 1.58 1.33 2.00 1.63 0.48 11.00 6.85 129.00 8.49 32.00 67.00 76.72 0.83 KM-10 75.28 9.97 1.59 1.75 1.70 1.20 1.73 1.70 0.41 9.34 6.45 104.00 7.40 28.70 66.03 74.57 0.84 KM-11 77.54 9.52 0.94 1.49 1.56 1.02 1.54 1.87 0.47 6.88 5.91 196.00 9.16 32.20 65.70 73.51 0.78 KM-12 77.70 9.77 1.68 1.04 1.01 0.93 1.61 1.89 0.46 8.02 6.10 117.00 10.00 32.90 68.42 77.11 0.78 KM-13 78.74 9.32 1.86 0.80 0.90 0.85 1.50 1.85 0.54 7.90 6.36 189.00 14.10 45.50 68.68 77.22 0.80 KM-14 71.58 11.38 1.89 1.93 1.77 1.60 2.09 1.76 0.53 12.10 7.50 115.00 10.00 33.30 66.94 76.32 0.85 KM-15 78.25 9.12 1.25 1.42 1.35 0.98 1.66 1.77 0.38 7.15 5.75 97.50 8.68 29.50 65.61 74.51 0.81 KM-16 78.30 9.02 1.33 1.19 1.56 0.89 1.60 1.81 0.41 7.67 6.45 113.00 11.60 36.60 64.47 72.80 0.84 KM-17 78.75 8.75 1.55 1.19 1.38 0.88 1.59 1.74 0.48 6.05 5.11 154.00 10.80 36.90 65.01 73.72 0.87 KM-18 78.24 9.20 1.34 1.46 1.08 1.02 1.66 1.71 0.58 6.98 7.02 210.00 14.00 45.60 67.40 76.73 0.80 KM-19 78.05 9.13 1.36 1.42 1.29 0.97 1.67 1.70 0.55 7.11 5.63 179.00 12.30 39.90 66.21 75.33 0.83 KM-20 80.60 7.77 1.12 1.48 1.52 0.70 1.34 1.68 0.42 5.05 6.52 146.00 12.00 40.40 63.12 70.83 0.87 KM-21 75.86 10.38 1.68 1.72 0.98 1.14 1.96 1.69 0.42 9.30 6.38 102.00 8.67 28.60 69.15 79.54 0.76 KM-22 77.45 9.24 1.52 1.31 1.54 0.97 1.69 1.74 0.54 7.19 4.52 189.00 12.50 37.10 65.02 73.80 0.87 KM-23 77.85 8.96 1.41 1.48 1.60 1.02 1.63 1.66 0.41 7.01 5.47 120.00 8.05 29.40 64.69 73.32 0.86 KM-24 75.42 9.67 1.43 1.57 2.10 1.11 1.81 1.69 0.45 8.79 7.72 150.00 11.00 37.00 63.33 71.84 0.89 KM-25 78.04 9.04 1.42 1.50 1.35 1.03 1.64 1.72 0.45 7.25 6.25 136.00 10.50 35.50 65.75 74.65 0.84 KM-26 74.23 9.64 1.49 1.42 2.83 1.15 1.86 1.67 0.38 8.10 6.01 105.00 9.54 30.30 60.25 68.18 0.97 KM-27 78.55 8.87 1.08 1.50 1.56 0.90 1.62 1.74 0.42 7.62 4.70 138.00 8.62 30.20 64.32 72.88 0.83 KM-28 78.48 8.74 1.05 1.50 1.65 0.98 1.60 1.70 0.44 6.90 4.60 147.00 9.77 32.20 63.84 72.29 0.85 平均值 76.19 9.64 1.41 1.58 1.64 1.16 1.68 1.75 0.51 8.36 6.64 166.77 11.03 37.92 65.49 73.91 0.85 CIA(蚀变作用指标)=Al2O3/(Al2 O3+CaO+Na2O+K2O)×100；CIW(化学风化作用指标)= Al2O3/(Al2O3+CaO+Na2O)×100；ICV(化学组分指标)=(Fe2O3+K2O+Na2O+CaO+MgO+TiO2)/Al2O3；主量元素含量单位为%，微量元素含量单位为10-6 
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