INSIGHTS INTO THE ORIGIN OF SHALLOW MARINE DOLOSTONES, LONGWANGMIAO FORMATION (LOWER CAMBRIAN), NORTHEAST SICHUAN BASIN, CHINA: PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS

IF 0.9 4区 环境科学与生态学 Q4 ENVIRONMENTAL SCIENCES
Jin Wang, Lu Zhou, Baozhen Zhang, C. Gao
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引用次数: 0

Abstract

For a better understanding of the origin of Early Cambrian dolomites in the Sichuan Basin, which serve as valuable oil and gas reservoirs, we present and interpret complex multiparameter petrographic and geochemical analyses of dolomites in the Cambrian Longwangmiao Formation at the Yangsiqiao section. Our data show that these dolomites mainly occur in the middle and upper parts of the Longwangmiao Formation, and we identify two types of dolomites: micrite and fine-medium crystalline dolomites. The micritic dolomites, occasionally coupled with quartz and mica minerals, show dirty crystal surfaces, nonautogenous crystal shapes, and dark cathodoluminescence and mainly occur in pure micrite and granular dolostones, which retain the original sedimentary texture. Contrarily the fine-medium crystalline dolomites, mainly developed in crystalline and grain dolostones and generally coupled with residual calcites and altered sedimentary textures, have a dark heart-bright boundary texture under a polarized microscope and occasional zonal texture under cathode luminescence. Furthermore, the micritic dolomites have CaO and MgO contents approximately similar to those of the ideal stoichiometric dolomite, relatively high Sr concentration, Σ rare-earth element (REE) +Y concentrations, and δ13C and δ18O values located in the corresponding range of dolomite deposited from Early Cambrian marine water. However, light-REE enrichment (LREE), heavy-REE (HREE) depletion patterns, and high Al2O3 and SiO2 contents distinguish these micritic dolomites from typical marine carbonate deposits. Notably, the fine-medium crystalline dolomites have higher CaO content and lower MgO, Al2O3 and SiO2 contents, lower Sr and higher Fe and Mn concentrations than those of micritic dolomites. The ΣREE+Y concentrations and REE patterns of these fine-medium crystalline dolomites belong to the typical marine carbonate deposits. These features indicate considerable differences in origins between micrite and fine-medium crystalline dolomites. The micritic dolomites show a significant genetic response to the precipitation of seawater in shallow marine with the injection of terrestrial water during the syngenetic period, while the fine-medium crystalline dolomites have a typical response to the secondary metasomatism products of original calcites at the shallow burial stage.
川东北龙王庙组浅海白云岩成因的岩石学和地球化学约束
为了更好地了解四川盆地早寒武纪白云岩的成因,对杨泗桥剖面寒武纪龙王庙组白云岩进行了复杂的多参数岩石学和地球化学分析。我们的数据表明,这些白云岩主要出现在龙王庙组的中上部,我们确定了两种类型的白云岩:泥晶白云岩和中细晶白云岩。泥晶白云岩偶尔与石英和云母矿物结合,结晶表面肮脏,晶体形状非自生,阴极发光较暗,主要产于纯泥晶和粒状白云岩中,保留了原始沉积结构。相反,细-中结晶白云石,主要发育在结晶和颗粒白云岩中,通常与残余方解石和蚀变沉积结构相结合,在偏光显微镜下具有暗心亮边界结构,在阴极发光下偶尔具有带状结构。此外,微晶白云岩的CaO和MgO含量与理想化学计量白云岩的含量大致相似,Sr浓度相对较高,∑稀土元素(REE)+Y浓度较高,δ13C和δ18O值位于早寒武纪海水沉积白云岩的相应范围内。然而,轻REE富集(LREE)、重REE贫化模式以及高Al2O3和SiO2含量将这些泥晶白云岩与典型的海相碳酸盐矿床区分开来。值得注意的是,细-中等结晶白云石比微晶白云石具有更高的CaO含量和更低的MgO、Al2O3和SiO2含量,更低的Sr和更高的Fe和Mn浓度。这些细-中结晶白云岩的∑REE+Y浓度和REE模式属于典型的海相碳酸盐岩矿床。这些特征表明泥晶白云岩和中细晶白云岩在成因上存在相当大的差异。在同生期,泥晶白云岩对海水在浅海中的沉积和地下水的注入表现出显著的成因响应,而细-中晶白云岩在浅埋阶段对原始方解石的次生交代产物表现出典型的响应。
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来源期刊
CiteScore
2.30
自引率
25.00%
发文量
42
审稿时长
12-24 weeks
期刊介绍: The publishing of CARPATHIAN JOURNAL of EARTH and ENVIRONMENTAL SCIENCES has started in 2006. The regularity of this magazine is biannual. The magazine will publish scientific works, in international purposes, in different areas of research, such as : geology, geography, environmental sciences, the environmental pollution and protection, environmental chemistry and physic, environmental biodegradation, climatic exchanges, fighting against natural disasters, protected areas, soil degradation, water quality, water supplies, sustainable development.
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