Genesis And Geological Significance Of Dolomite Ring Belts In Fengcheng Formation Of Permian In Mahu Sag,the Junggar Basin

The Fengcheng Formation of Mahu sag,Junggar Basin is an important source rock and reservoir in northwest China.A large set of dolomite with dolomite as the main mineral is developed.Previous studies have found that a large number of medium-sized dolomite minerals show multi-layer zones with alternating light and dark and different morphological characteristics under cathodoluminescence（CL）,while fine-grained dolomite develops 1-3zone,and larger-sized dolomite often develops 1-3 zones or shows uneven luminescence characteristics.Most of the previous studies believe that these dolomites belong to sedimentary genesis or hydrothermal genesis.Single sedimentary genesis cannot explain the large difference in dolomite particle size,and sedimentary genesis is difficult to form larger dolomite minerals.The hydrothermal genesis cannot explain the reason why some dolomite minerals show zoning under cathodoluminescence.Therefore,these genetic theories cannot well explain the reason why the medium-sized dolomite has a ring belts structure under cathodoluminescence,while the other two types of dolomite show different cathodoluminescence characteristics.In order to solve these scientific problems,on the basis of predecessors,this paper takes electron probe microanalysis（EPMA）and cathodoluminescence as the main research methods,and dolomite minerals in dolomite of Fengcheng Formation in Mahu Depression as the main research object.Through the methods of lithofacies classification,sedimentary facies analysis and paleoenvironmental restoration,the genesis of dolomite in Fengcheng Formation of Mahu Depression in Junggar Basin was systematically studied,and the following understandings were obtained:According to the sedimentary records from the first member to the third member of the Fengcheng Formation,the Fengcheng Formation experienced frequent evolution of paleo-sedimentary environment,and its paleo-climate,paleo-salinity,paleo oxidation reduction and paleo-water depth changed rapidly.This is an important evidence to study the climatic background of different types of dolomitization in the strata.Based on the mineral occurrence,distribution type and cathodoluminescence characteristics,the dolomite minerals in the dolomite of Fengcheng Formation are divided into three categories:lamellar dolomite,snowflake dolomite and agglomerate dolomite.1.Layered dolomite particles are small and uniform,close to each other,often mud crystal or mud microcrystalline,visible particle crystal nucleus,particle size is often below 50μm;the cathode luminescence intensity of most laminated dolomite is very strong,some of them are strong,and 1-3 layers of ring belts are often developed.Stable sedimentary components such as mud,quartz and feldspar are common,and alkaline minerals produced in alkaline lake sediments are also found.Based on the characteristics of cathodoluminescence,associated alkaline minerals and the petrographic characteristics of lamellar dolomite,it is inferred that lamellar dolomite should be formed in a semi-deep-deep lake environment with deep water and strong reducibility,and formed by dolomitization or metasomatism.2.The particle size of snowflake dolomite is relatively uniform,between 100-200μm,there is a certain interval between the particles,isolated output.The number of cathodoluminescence bands is at least 3 layers,and most of them are 5-6 layers.The light and dark phases are clear,and the outermost band can be seen as a straight edge,or a scalloped and serrated edge.The cathode luminescence intensity is between lamellar dolomite and agglomerate dolomite,weaker than lamellar dolomite and stronger than agglomerate dolomite.The element content of each zone shows that the depth and salinity of sedimentary water change regularly during the deposition of snowflake dolomite.Combined with the analysis of sedimentary facies and sedimentary environment,it is speculated that the genesis of snowflake dolomite should be:in the salt-freshwater mixed alternating zone,under the influence of rapidly changing water depth,redox and salinity,the pre-carbonate mineral is formed by mixed water dolomitization.3.The particle size of agglomerated dolomite is very large.Above 200μm,the particles are adjacent to each other to form a cluster,which is irregularly plaque-like,and cracks caused by pressure are often developed between the particles.The cathode luminescence intensity of agglomerated dolomite is the weakest,and the cathode luminescence intensity of dolomite particles is not uniform.The ring belt is not developed or very vague.The fuzzy ring belt is often missing.The development layer is mostly in the first section of the wind.Volcanic debris,feldspar,quartz and tuffaceous components are common.The weak and uneven cathode luminescence intensity indicates that the Mn/Fe ratio is low,and the larger particles indicate that the duration of dolomitization is longer.Fractures,volcanic debris and tuffaceous components indicate that the sedimentary environment should be under high pressure and participate in volcanic activity.The development of fuzzy zones in some agglomerated dolomite indicates that the water depth and salinity of the sedimentary environment changed at the end of dolomitization.Therefore,the massive dolomite should be the product of hydrothermal dolomitization,and some of the massive dolomite should be transformed by later fluid.On the basis of cathodoluminescence analysis and electron probe element analysis,combined with the evolution of sedimentary facies and sedimentary environment of Fengcheng Formation,the genesis of dolomite ring belt in Fengcheng Formation is the change of Mn²⁺/Fe^2+/3+ratio.It is mainly due to the frequent changes of water depth and salinity during the dolomitization of pre-carbonate rocks.The concentration of Mn²⁺and Fe^2+/3+in the water body changes.In the same sedimentary area,the water conditions in the low water level area change more frequently than the high water level sedimentary area,and the number of ring belts formed is more.The high-water-level sedimentary area has formed a wider but fewer ring belt structure.