Characteristics And Origin Of Lacustrine Dolomite Of Eocene Niubao Formation In The Lunpola Basin,Tibetan Plateau

Dolomite genesis and its characteristics have been the research focus of long-standing scientific interest.Traditional studies have focused more on marine dolomite,and research on lacustrine dolomite has been weak.A set of lacustrine dolomite was developed in the Eocene Niubao Formation of the Lunpola basin of Tibetan plateau.The dolomite was formed at the earliest stage of the Himalayan movement with special tectonic background,the discovery of which also indicates a unique environmental significance.Presently,little research for the origins of the dolomite has been done and the research degree is relatively low.In this study,the sedimentary facies,dolomite characteristics,and dolomitization mechanism and its distribution were investigated using a combination of core observation,thin section identification,rock and mineral testing,and geochemical methods.The main conclusions are as follows:?1?The Niubao Formation in the Lunpola Basin is dominated by lacustrine deposits,with a depositional background consisting mainly of the northern area developing fan delta,the central area developing lacustrine gravity flow,and the braided river delta facies developed in the south.Under this background,argillaceous dolomite,dolomitic mud,dolomitic shale,tuffaceous dolomite,and other dolomite types are formed under the shallow to semi-deep lake sedimentary background,the output of which is controlled by the slope break zone,with characteristics dolomite distributing around the basin.?2?The sedimentary background of the lake basin during the formation of dolomite was studied.Comprehensive analysis of carbon and oxygen isotopes of dolomites,the presence of a large amount of gypsum minerals and the salinity index Z greater than 120,confirming that the ancient lake basin in the study area was a semi-open to semi-closed lake with relatively high salinity.During the Eocene-Oligocene period,the northwestern China was generally in arid and semi-arid climate with strong evaporating environments and thus formed widespread lacustrine dolomite.?3?The dolomite formation stage and fluid environment characteristics were revealed.The degree of ordering in dolomite in the study area is relatively low?average 0.58?,Sr/Ba ratio is greater than 1,the rock is dense and there is no obvious external fluid transformation phenomenon.It is speculated that the dolomites in the study area were formed in the penecontemporaneous to shallow burial period.Carbon and oxygen isotopes and methods proposed by Allen and Wiggins show that paleo-temperatures during the formation of dolomite were high,and the presence of a variety of hot minerals also confirms the hot fluids characteristics during its formation.The prevalence of hot fluids also indicates the characteristics of the general tectonic activities in the study area.?4?The sources of Mg²⁺and the mechanism of dolomitization were discussed.It is speculated that the dolomite in the study area is mainly formed by the evaporative pump model.The high-carbonates transport from the hot springs in the fault zone and the surrounding source areas,had laid a good foundation for the formation of lacustrine carbonate rocks in the basin.At the same time,the volcanic activities around the basin,the volcanic materials from the Jurassic stratum,and the mixed formation water from the fault might supply a rich source of Mg²⁺,plus the strong evaporation,provided a favorable condition for the formation of evaporative pumping dolomitization model.