Characteristics Of Dolomitic Rocks And Formation Mechanisim Of Dolomites In The Middle Permian Lucaogou Formation,Jimusar Depression

The Middle Permian Lucaogou Formation of Jimusar Depression,Junggar Basin is characterized by fine lacustrine dolomitic sediments,which are composed of silt-pelitic terrigenous clasts,tuff clasts,carbonate and hydrothermal minerals.This paper focuses on the dolomitic rocks and research for their forming environment and mechanism on the basis of petrology,mineralogy and geochemistry.It can be indicated by the petrological-mineralogical and major and trace element characteristics of the dolomitic rocks that the study area was a shallow salty lake with arid climate during the Middle Permian.The surrounding of the lake was characterized by intense volcanism while the mantle-derived hydrothermal activities were dramatic as well.The dolomites in the dolomitic rocks can be divided into three types:?1?Euhedral-subhedral micritic-powder dolomites?EMDs?which formed fine lamination or mix with other compositions;?2?Euhedral-subhedral fine-crystalline dolomites?EFDs?which can only be found in the dolomitic rocks occasionally;?3?Anhedral micritic-powder dolomites?AMDs?which formed analcime-albite dolostone and silicic dolostone.The EMDs are speculated to form in depositional or penecontemporaneous stage for their low degree of order,0.46,uniform composition and their being surrounded by microscopic lamina,while the fine-crystalline dolomites formed in diagenesis stage since they developed metasomatic relict texture.The AMDs present mosaic texture with hydrothermal minerals,analcime,buddingtonite,albite,chalcedonite,and thus likely formed by crystallizing.The AMDs show as spherical,botryoidal or saddle crystal and possess curved crystal face related to high temperature.The analcime-albite dolostone is speculated to form in depositional sta ge as well since the lamina of the sedimentary rock under the dolostone occurred plastic deformation.It can be revealed by the Sr isotope of EMDs and AMDs that their metallogenic fluid is the mixture of lake water and mantle-derived hydrothermal fluid,and the metallogenic fluid of AMDs is more relevant to mantle.Both of EMD and AMD have low?¹⁸O_PDBDB and high?¹³C_PDB.The forming temperature can be calculated by the?¹⁸O_PDBDB based on the hypothesis that the fluid was the mixture of lake water and mantle-derived hydrothermal fluid.The results show that the forming temperature of EMDs ranges from 54.61?to186.46?while that of AMDs ranges from 77.69?to 197.27?.The gigantic buried organic matter of Lucaogou Formation and metabolic activities of methanogens are responsible for the high?¹³C_PDB.The EMDs are likely to form in the remote area from the hydrothermal vent where relative ly few mantle-derived hydrotherma l fluid mixed with lake water.Since the magnesium ion was supplied by the salty lake water,tuff materials and hydrothermal fluid and the dynamic barrier was broken by the high temperature and the metabolic activities of methanogens,EMDs formed by replacing carbonate particles.The AMDs are speculated to form near the vent where relatively more hydrothermal fluid mixed with few lake water.Due to the high temperature and adequate Mg2+of the fluid,the AMDs are likely to crystallize from the fluid directly.