摘要： 准噶尔盆地东部中二叠统平地泉组(相当于芦草沟组)发育一套陆内裂谷背景下的湖泊相砂岩、泥岩、灰岩、白云岩、凝灰岩互层以及它们的过渡岩石,是区内最主要的烃源岩和储集层。近年来,在帐北断褶带、石树沟凹陷和吉木萨尔凹陷中二叠统平地泉组暗色泥岩、泥晶白云岩中发现了一类特殊的沉积岩,该类岩石发育类似火山岩的斑状结构。“斑晶”主要为粗晶白云石、方解石、黄铁矿及方沸石等,在岩心标本上呈“树枝状”或“雪花状”散布于基质中;基质则主要由泥晶白云石或泥质沉积物构成,富含有机质,发育水平层理及小型变形层理等。岩石学和矿物学分析表明,“斑晶”往往为粗晶方解石或白云石的集合体,也常见黄铁矿、方沸石与碳酸盐矿物共生,“斑晶”方解石发育环带,而白云石未见明显环带。电子探针分析表明“斑晶”方解石具有低镁、低铁及锶分布不均匀的特点,而“斑晶”白云石(FeO含量介于7.272%~11.086%之间)与“基质”泥晶白云石(FeO含量为1.027%)相比具有明显富铁的特点。流体包裹体分析表明“斑晶”方解石均一温度平均为180.68,℃,“斑晶”白云石均一温度平均为320.95,℃。这种特殊 “斑状”白云岩和“斑状”泥质岩很可能是湖底热液喷流作用的结果。当湖水沿深大断裂下渗至地下深处,与围岩发生物质交换并被加热后再沿断裂返回地表喷涌而出,热液流体携带的离子达到过饱和后就会析出方解石、白云石及黄铁矿等集合体,随热液的喷涌作用上升并破碎散落于湖底细粒沉积物内。“斑状”白云岩的发现对新疆北部中二叠统热水喷流沉积作用及该区油气的成因研究具有重要的意义。

关键词: “, 斑晶”, 白云石, 方沸石, 热水喷流作用, 中二叠统, 平地泉组 , 准噶尔盆地

Abstract: The Middle Permian Pingdiquan Formation which contains the most important source rocks and hydrocarbon reservoirs in eastern Junggar Basin,a continental rift basin in late Paleozoic, is characterized by interbeded layers of lacustrine sandstones,mudstones,limestones,dolostones,tuffs,and their mixtures with various proportions. One type of special sedimentary rocks was found in Pingdiquan black mudstones and dark grey microcrystalline dolostones in Zhangbei fault-fold zone,Shishugou sag,and Jimsar sag. They are composed of “phenocryst”and “matrix”,which is similar to volcanic “porphyritic”structures. The “phenocrysts”are usually coarse calcite,dolomite,pyrite,and analcime,while the “matrix”is mainly consisted of argillaceous sediments or microcrystalline dolomite,rich in organic matter,with micro-horizontal laminae and small deformation beddings. The “phenocrysts”can be single carbonate mineral and its aggregations or multiple mineral assemblages. “Phenocryst”calcite shows zonal texture under cathode luminescence and back scatter scanning,but “phenocryst”dolomite does not have zonal texture. Analysis of EMPA reveals that “porphyritic”calcite has features of low content of magnesium and iron,and heterogeneous distribution of strontium. Compared with “matrix” microcrystalline dolomite(FeO 1.027%),“porphyritic”dolomite contains higher iron with FeO content ranging from 7.272% to 11.086%. The average homogenization temperature value of fluid inclusions in “porphyritic”calcite is 180.68,℃,lower than that of “porphyritic”dolomite with average value of 320.95,℃. The structures,textures of “phenocrysts”and their geochemical features indicate that the “porphyritic”dolostones and “porphyritic”mudstones were formed by sedimentary hydrothermal exhalation in deep lake environment in the Middle Permian Pingdiquan stage. Lake water penetrated deeply to subsurface along the fractures,heated by underlying strata or magma chamber,exchanged the soluble ions from host rocks,and then circulated upward with the ions. Once the rising hydrothermal fluid was oversaturated,the minerals such as dolomite,calcite,pyrite or analcime would crystallize. They were uplifted by the hydrothermal fluids and finally were broken and exhaled into the fine grained sediments of lake floor. The “porphyritic”dolostones and mudstones in Pingdiquan Formation play an important role to explore the origins of hydrocarbons in research area and also are helpful to better understand the hydrothermal exhalative activities widely distributed in the Middle Permian in North Xingjiang.

Key words: “porphyritic”, dolomite, analcime, hydrothermal exhalation, Middle Permian, Pingdiquan Formation, Junggar Basin