Rare earth element characteristics of the carboniferous Huanglong Formation dolomites in eastern Sichuan Basin, southwest China: Implications for origins of dolomitizing and diagenetic fluids
Rare earth element characteristics of the carboniferous Huanglong Formation dolomites in eastern Sichuan Basin, southwest China: Implications for origins of dolomitizing and diagenetic fluids
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DOI：
10.1016/j.marpetgeo.2016.12.030
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发表时间：
2017-03
影响因子：
4.2
通讯作者：
Chaohui Liu;Q. Xie;Guiwen Wang;Wengang He;Yifan Song;Yong Tang;Yuhan Wang
中科院分区：
文献类型：
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作者：
Chaohui Liu;Q. Xie;Guiwen Wang;Wengang He;Yifan Song;Yong Tang;Yuhan Wang
石炭系黄龙组海相白云岩是川东地区的主要气藏。然而，关于白云石化和成岩流体来源的研究相对较少。本文在连续采集川东地区黄龙组二段(C2H2)长达47.33m的岩芯样品的基础上，利用海水标准化标准研究了白云岩的稀土元素特征，探讨了白云石化作用和成岩流体的成因。低的Th、Sc和Hf浓度(分别为0.791×10−6、4.751×10−6和0.214×10−6)、总稀土浓度(ΣREE)与Fe或Mn丰度的随机相关性以及海水样的Y/Ho比值(平均值为45.612)表明碳酸盐样品是进行稀土分析的有效样品。根据岩石学特征，划分出4种白云岩类型：泥晶白云岩(Dol-1型)、细粒-中型白云岩(Dol-2型)、中-粗粒白云岩(Dol-3型)和粗大鞍状白云岩(Dol-4型)。DOL-1白云岩具有正Ce异常(平均值6.398)、轻稀土(LREE)富集、重稀土(HREE)亏损(平均LREE/HREE比值为12.657)的特征，呈泥晶方解石状，指示其白云石化流体的海水来源。DOL-1白云石形成于萨布哈环境，而白云化流体因其微晶尺寸和致密岩性而起源于蒸发卤水。Dol-2白云岩，特别是Dol-2a亚型几乎不发育孔洞的白云岩，也具有泥晶方解石样稀土元素配分模式，表明其白云化流体为海水或海水来源的流体。铁、锰含量较低，分别为651mμg/g~1018mμg/g(平均值为863mμg/g)和65mμg/g至167mμg/g(平均值为105mμg/g)，而均一温度(Th，平均值为103m o C)表明Dol-2白云岩形成于埋藏环境。Dol-3白云石以Dol-2白云石胶结物形式存在，其稀土配分模式与寄主矿物(即Dol-2白云石)相似，表明其母质来源可能来自Dol-2白云岩。DOL-3白云岩的铁、锰含量较高，平均值分别为3346亿μg/g(2897万μg/g~3856亿μg/g)和236百万μg/g(178万μg/g至287万μg/g)，表明大气降水的参与。同时也证实了Dol-2白云岩的溶解是由大气降水淋溶作用引起的。Dol-4白云岩中Eu的正异常(平均值为1.406)，再加上较高的均一温度(平均值为314℃)，表明Dol-4白云岩是从热液中沉淀出来的。Dol-4白云岩中铁和锰的高含量(平均值分别为2521gμg/g和193gμg/g)可能是热液与深埋碎屑岩相互作用的结果。
Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks.