| 川西地区中二叠统栖霞组白云岩成因分析 |
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| 引用本文： | 梁茹,苏中堂,马慧,方继瑶,林良彪.川西地区中二叠统栖霞组白云岩成因分析[J].沉积学报,2021,39(6):1506-1516. |
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| 作者姓名： | 梁茹 苏中堂 马慧 方继瑶 林良彪 |
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| 作者单位： | 1.成都理工大学沉积地质研究院，成都 610059 |
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| 基金项目： | 国家自然科学基金41872115 |
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| 摘 要： | 川西地区栖霞组白云岩成因及其与峨眉山玄武岩喷发热事件关系颇有争论。在野外剖面、岩心观察基础上，通过岩石学、全岩及矿物微区地球化学分析方法对其研究，结果表明川西地区栖霞组白云岩（石）可分为残余砂屑白云岩、晶粒白云岩以及鞍状白云石、脉体充填白云石四类。残余砂屑白云岩与晶粒白云岩均呈块状，阴极发光较暗，稀土配分模式与泥微晶灰岩相似，δ13C、87Sr/86Sr多处于同期海水范围内，δ18O略偏负，Ce负异常，残余砂屑白云岩Eu呈负异常，而晶粒白云岩Eu多为正异常。微区δ13C、δ18O分析显示，残余砂屑白云岩的白云石雾心与亮边内δ13C均处于同期海水范围内，δ18O亮边较雾心偏负；晶粒白云岩中不同大小的晶粒白云石微区δ13C、δ18O显示相近特征，表明这两类白云岩均为海水浅埋藏成因，但受到后期热流体叠加改造。曲面他形粗晶鞍状白云石充填于裂隙与溶蚀孔洞内，镜下具波状消光特征，发亮红色光，δ13C与二叠纪海水相近，δ18O明显负偏，Eu呈正异常，指示鞍状白云石为热液成因。脉体白云石岩石学与地球化学特征表明其为晚期埋藏成因。川西地区栖霞组白云岩主体为浅埋藏成因，峨眉山玄武岩喷发期热液对其叠加改造，仅西南地区充填裂隙与溶洞的鞍形白云石为热液成因。
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| 关 键 词： | 岩石学特征 地球化学特征 白云岩成因 栖霞组 川西地区 |
| 收稿时间： | 2021-01-14 |
Analysis of the Origin of Dolomites in the Middle Permian Qixia Formation,Western Sichuan |
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| LIANG Ru,SU ZhongTang,MA Hui,FANG JiYao,LIN LiangBiao.Analysis of the Origin of Dolomites in the Middle Permian Qixia Formation,Western Sichuan[J].Acta Sedimentologica Sinica,2021,39(6):1506-1516. |
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| Authors: | LIANG Ru SU ZhongTang MA Hui FANG JiYao LIN LiangBiao |
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| Institution: | 1.Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China2.Institute of Petroleum Exploration and Development in Zhongyuan Oilfield, Puyang，Henan 457001, China3.State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), Chengdu 610059, China4.Technical Innovation Center for Shale Gas Exploration and Development in Complex Structural Areas, Ministry of Natural Resources, Chengdu 610091, China |
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| Abstract: | The origin of the dolomite in the Qixia Formation in western Sichuan and its relationship to the Emeishan basalt eruption is controversial. The dolomites were divided into four types on the basis of field section and core observations， petrological studies， whole rock REE， C， O and Sr isotope and mineral micro-area C and O isotope analysis: residual granular dolomite， crystalline dolomite， saddle dolomite and vein-filling dolomite. The residual granular dolomite is thick and massive， mainly with flat planar and subhedral crystals， with dark cathodoluminescence （CD）. Its REE distribution pattern is similar to micritic limestone， with negative Ce and Eu anomalies and slightly negative O isotopic values. C and Sr isotopic values are mostly contemporaneous with seawater， and micro-area C and O isotopic analysis showed that the C isotopic values of dolomite with a foggy center and bright edge are contemporaneous with seawater. The bright edge of the O isotope is more negative than the foggy center， which indicates that this kind of dolomite was initially formed by shallow seawater burial， then reworked by subsequent thermal fluid superimposition. The dolomite grains are dense and massive， with dark CD. The REE distribution pattern is also similar to that of micritic limestone， but with negative Ce and slightly positive Eu anomalies. The C and Sr isotopes are the same as in seawater of the same period； the O isotope is slightly more negative than the seawater. The C and O isotopes of dolomite grain micro-areas of different size show similar properties， indicating that it was mainly distributed in the same period， formed by shallow burial in seawater， but recrystallized by later thermal fluid， forming anisotropic dolomite. The curved-surface allomorphic coarse-grained saddle dolomite occurs as fracture and dissolution cavity infill. Under the microscope， it has wavy extinctions and bright red color. Its C isotope is similar to that of Permian seawater， its O isotope is distinctly negative and Eu is positive， indicating that saddle dolomite is hydrothermal in origin. Because it has passed through silicate strata， its Sr isotope is obviously high. The petrological and geochemical characteristics of this dolomite indicate a late burial origin. In summary， the dolomite in the Qixia Formation in western Sichuan is mainly of shallow burial origin， then superimposed and re-formed by hydrothermal solution during the Emeishan basalt eruption. Only saddle dolomite occurring as fracture and cavity infill is of hydrothermal origin. |
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