准噶尔盆地吉木萨尔凹陷中二叠统芦草沟组云质岩沉积环境及白云石成因探讨*

准噶尔盆地吉木萨尔凹陷中二叠统芦草沟组发育一套陆内裂谷背景下,由粉砂级-泥级陆源碎屑、凝灰碎屑、碳酸盐及热液矿物组成的湖相细粒云质岩。本研究以云质岩为研究对象,通过岩石学、矿物学及地球化学研究探讨其形成环境和白云石形成机理。云质岩岩矿特征和主、微量元素特征表明,在中二叠世为处于干旱气候的浅水咸化湖盆,区域火山活动强烈,湖盆内幔源热液活动发育。云质岩中的白云石主要呈自形-半自形的微粉晶颗粒,形成细纹层或与其他组分混合,局部可见自形-半自形的细晶白云石,微粉晶白云石为低有序度白云石(0.46),成分均匀,被纹层包绕,细晶白云石交代残留结构发育,为成岩交代的产物;两类白云石均富锶贫镁,细晶白云石较微粉晶白云石更富铁和锰。微粉晶白云石具有较低的δ¹⁸O_PDB(平均为-4.72‰),基于白云石的形成流体由湖水和幔源热液混合的假设,所计算得到的形成温度介于54.61~186.46 ℃之间;δ¹³C_PDB偏高(平均为8.79‰),推测其与芦草沟组高的有机质埋藏量及产甲烷古菌的代谢活动有关。微粉晶白云石为同生期-准同生期沉淀或交代的原白云石,其Mg²⁺来自于咸化湖水、凝灰碎屑和幔源热液,热液带来的高温和产甲烷古菌的代谢活动打破了白云石形成的动力学屏障。

Sedimentary environment and formation mechanisim of dolomitic rocks in the Middle Permian Lucaogou Formation,Jimusar Depression,Junggar Basin

The Middle Permian Lucaogou Formation of Jimusar Depression,Junggar Basin is characterized by lacustrine fine-grained dolomitic sediments,which are composed of silt-sized to mud-sized terrigenous clasts,tuff clasts,carbonate and hydrothermal minerals. This paper is aimed to understand the sedimentary environment and formation mechanism of dolomitic rocks by studying their petrological,mineralogical and geochemical features. Petrological-mineralogical and major and trace element characteristics of the dolomitic rocks suggest the study area was a shallow salty lake formed under an arid climate during the Middle Permian,and was characterized by intense regional volcanism and active mantle-derived hydrothermal in the basin. The dolomites in the dolomitic rocks mainly present as euhedral-subhedral micritic crystals,forming fine lamination or mixing with other compositions,while fine-crystalline dolomites can only be found occasionally. The micritic dolomites,with low order degrees(0.46)and homogeneous compositions,are usually surrounded by laminations. The fine-crystalline dolomites are featured by well-developed metasomatic relict texture and thereby should be the product of diagenesis. Both two kinds of dolomites have high strontium and low magnesium contents,while fine-crystalline dolomites have higher contents of iron and manganese than that of micritic dolomites. The micritic dolomites have a low δ¹⁸O_PDB(av.-4.72‰). The forming temperature,54.61℃ to 186.46℃,is calculated from the δ¹³C_PDB(av. 8.79‰)of micritic dolomites might be resulted from the gigantic buried organic matter of Lucaogou Formation and metabolic activities of methanogens. In conclusion,the micritic dolomites are contemporaneous-penecontemporaneous protodolomite. Magnesium ion was supplied by the salty lake water,tuff materials as well as mantle-dervied hydrothermal fluid. The dynamic barrier of dolomite formation was broken by the high temperature brought from hydrothermal fluid and the metabolic activities of methanogens.