沁水盆地西缘寒武系-奥陶系三山子组白云岩碳氧同位素特征及其意义

Carbon and oxygen isotopes characteristics of dolomite from Cambrian-Ordovician Sanshanzi formation in the western margin of the Qinshui basin and its significance

为了约束山西沁水盆地西缘地区寒武系-奥陶系三山子组白云岩成岩环境以及成因模式,对沁水盆地西缘王家湾剖面三山子组白云岩进行岩石学和碳氧同位素的研究。结果表明:三山子组白云岩晶体形态变化大,包括微粉晶—中晶白云岩,局部粗晶;δ^(13)C和δ^(18)O变化幅度大,δ^(13)C由下到上呈降低趋势,反映1次高级别海退过程。根据岩石学和碳氧同位素的特征可将三山子组白云岩分为3类:A类集中于三山子组下部,岩性为粉—细晶白云岩,δ^(13)C均值为0.05%,δ^(18)O均值为-0.63%,形成于海退早期的近地表海底环境,属准同生交代模式,后期叠加了埋藏白云石化改造;B类集中在三山子组中部,岩性为细—中晶白云岩,局部粗晶,δ^(13)C均值为-0.1%,δ^(18)O均值为-0.68%,形成于埋藏环境,属埋藏白云石化模式;C类集中在三山子组上部,岩性为微粉—细晶白云岩,δ^(13)C均值为-0.34%,δ^(18)O均值为-0.61%,形成于海退晚期的大气淡水环境,属混合水白云岩化模式,后期叠加埋藏白云石化。结合前人研究,认为研究区三山子组白云岩的形成受海平面变化控制,白云岩化过程是多期次的,其成因机制具多样性与复杂性。

In order to constrain the diagenetic environment and genetic model of dolostone of Cambrian Ordovician Sanshanzi formation in western margin of Qinshui basin,Shanxi Province,petrology and carbon and oxygen isotopes of dolomite of the Sanshanzi formation in Wangjiawan section of the Western Qinshui basin were studied.It was found that the crystal morphology of Sanshanzi formation dolomite changed greatly,including micro-fine-medium crystal dolomite and local coarse crystal.δ^(13)C andδ^(18)O varied greatly,andδ^(13)C decreased from bottom to top,reflecting a high-level regression process.According to the characteristics of petrology and carbon and oxygen isotopes,the dolomites of Sanshanzi formation could be divided into three types.Type A was concentrated in the lower part of Sanshanzi formation,and its lithology was silty fine crystalline dolomite.The average value ofδ^(13)C andδ^(18)O was 0.05%and-0.63%respectively.It was formed in the near surface submarine environment in the early regression period,belonging to the paracontemporaneous metasomatic model,and superimposed with buried dolomitization in the later stage.Type B was concentrated in the middle of Sanshanzi formation.Its lithology was fine medium crystal dolomite and local coarse crystal.The average value ofδ^(13)C andδ^(18)O was-0.1%and-0.68%,respectively.It was formed in burial environment and belonged to buried dolomitization mode.Type C was concentrated in the upper part of the Sanshanzi formation.Its lithology was micro powder fine crystal dolomite,with the average value ofδ^(13)C being-0.34%andδ^(18)O being-0.61%.It was formed in the atmospheric fresh water environment of the late regression period,belonging to mixed water dolomitization mode and superimposed burial dolomitization in the later stage.Based on previous studies,it was considered that the formation of dolostone of the Sanshanzi formation in the study area was controlled by sea level fluctuations and the dolomitization process was multi-stage,and its genetic mechanism was diverse and complex.