乌尔禾-风城地区二叠系白云质岩类岩石学特征及成因分析

为了探讨准噶尔盆地西北缘乌尔禾-风城地区二叠系风城组白云质岩类的成因, 利用岩矿鉴定技术，地球化学分析技术等手段, 系统分析该套云质岩类的地球化学特征。研究表明，该区云质岩类有别于海相碳酸盐岩，受外物源的影响较大，成份复杂，多为过渡性岩类。以泥质白云岩和白云质泥岩和白云质粉砂岩为主，含有少量的凝灰质白云岩和白云质凝灰岩。岩石矿物学特征和地球化学分析数据均表明，研究区的云质岩类主要形成于水体安静、深度较大、盐度偏高的半封闭陆源近海湖湾环境。二叠系时期气候炎热，蒸发作用强烈与火山活动频繁，加之淡水补给相对匮乏，为该区的云质岩类形成提供了良好的镁离子来源和水动力条件。这时湖湾中沉积的富含文石或高镁方解石的泥岩或凝灰岩，在早成岩阶段经过渗透回流白云石化作用形成了大量泥质白云岩、白云质泥岩和白云质粉砂岩。在中晚成岩阶段，经过埋藏作用生成自形或半自形的细晶-中晶白云石。

Petrological Characteristics and Origin of Permian Fengcheng Formation Dolomitic Rocks in Wuerhe-Fengcheng Area,Junggar Basin

In order to analyze the dolomite origin of Permian(Fengcheng Formation) in the northwest edge of Junggar basin.This article take the rock-mineral analysis,thin-section analysis and geochemical analysis,and systematically analyzed this set of dolomite geochemistry characteristics and their origin.The research indicated that the dolomite mainly could divide into muddy dolostone,dolomitic tuff and dolomitic mudstones.The crystal size of dolomite greatly varies,mainly consisting of powder crystal,fine or micrite dolomites,which crystals are euhedral or subhedral.Dolomite fills in bedding or cracks or distribute in rocks with patterns of strip belts,laminated structure or gobbets.Dolomitization is not incompletely,residual calcite still be found in rocks.Dolostone commonly show ribbon,crumb,sausage or lens shapes in rocks,partly laminate and satellite.Sr content varies greatly between 95.9 ～ 783.6 μg / g with an average of 447 μg / g because of replenishment of freshwater and intense evaporation.Conclusion can be reached that sedimentary environment is mainly salt water with little amount of freshwater supplement and related to evaporative lake.V / Ni ratio varies greatly,and largely distribute in an range of 1.2 ～ 2.9.It refers to dolomitic rocks form in brackish lacustrine environment with complex setting and various salinity.Mn content is relatively low,varies between 133 ～ 985 μg / g with an average of 579 μg / g.From Mn data,we can conclude that aragonite forms in relatively deep water.In studying area,δ 13 C is low and distributes in an range of-1.6‰ ～ 5.2‰(PDB) with an average of 2.9‰(PDB).δ 18 O data still be low,and varies between 14.8‰ ～ 3.2 with average of 3.01.The characteristics of petromineralogy and the data of geochemical analyzation in Fengcheng Formation show that the dolomite in this area mainly formed in a peaceful,deep and high salinity semi-closed bay environment.Permian periods climate is hot,speedy evaporation and frequent volcanic,besides,short of fresh water supply,those factors provide favourable source of magnesium ions and hydrodynamic condition to form dolomite.In this period,the bays deposit tuff or mudstones which are rich in aragonite or high magnesian calcite,during the early diagenesis,the rocks convert to plenty of dolomitic tuff and dolomitic mudstones infiltration and circumfluence effect.In the middle and late diagenesis,compaction and mesothermal and micro thermal processes,in the study areas mainly consist of fine or medium dolomites,in which crystals are euhedral or subhedral.