Origin Of Dolomite Of Lower Cambrian Longwangmiao Formation,Eastern Sichuan Basin

By collecting samples from outcrops,we are able to observe thin sections,and analysis the major elments,trace elements,carbon and oxygen isotopic characteristics of dolomite of the Lower Cambrian Longwangmiao Formation at Bangdenggou profile,eastern Sichuan Basin.Base on the petrological and geochemical characteristics,we finally systematically recognize the origin of dolomite in the study area.According to the result of thin sections observation,cathode luminescence analysis,X-ray analysis,we recognize three types of dolomite of Longwangmiao Formation in the study area.1)Dolomicrite,which is mainly distributed in the lower and middle part of the target stratum,is mainly composed of micrite dolomite and contain associated mineral which can indicate the supratidal sedimentary environment such as anhydrite as well as high content of terrigenous clast.The degree order of dolomicrite with high-level development is relatively high.It has dull red to nonluminescence CL.2)Doloarenite is mostly located in the top of stratum.The degree order and CL charateristics of doloarenite is similar to dolomicrite.Doloarenite is mainly composed of arenite dolomicrite and sparry calcite cement,it is of the lower-level development in study area.3)Limy dolomite is characterized by redual texture of limestone,which has lower content of terrigenous clast and lower degree of order.Limy dolomite manily occurs in the upper part of stratum with relatively lower-level development,CL photomicrograph for limy dolomite shows lighter but also dull red luminescence.According to the characteristics of major and trace elements,carbon and oxygen isotope of dolomite,the properities of dolomitization fluid can be inferred.1)According to the correlation of MgO and CaO in dolomite,we divide the dolomite in study area into A-type and B-type.Dolomicrite and doloarenite belong to A-type dolomite which has the linear positive correlation between MgO and CaO.Limy dolomite belongs to B-type dolomite with the linear negative correlation between MgO and CaO.2)The A-type dolomite has relative lower content of Sr and Mn but relative higher content of Na and Fe.Content of?REE+Y in A-type dolomite is relatively high with evident negative anomaly of?Ce and?Eu.The value of?¹³C of A-type dolomite is close to contemporaneous marine brachiopod fossils.Compared to contemporaneous seawater,the?¹⁸O value of dolomization fluid of A-type dolomite is obvious higher but ⁸⁷Sr/⁸⁶Sr value is very close.3)B-type dolomite has relative higher content of Sr and Mn but lower content of Na and Fe.The value of?REE+Y in B-type dolomite is relatively lower with negative anomaly of?Ce and?Eu.?¹³C value of B-type is close to contemporaneous marine barachiopod fossils.The?¹⁸O value of dolomization fluid of B-type is slightly higher and the ⁸⁷Sr/⁸⁶Sr value is greatly higher than contemporaneous seawater.Combined with the petrology and geochemical characteristics analysed above,this paper indicates that dolomicrite and doloarenite mainly developed in the penecontemporaneous stage with evidently influenced by the infuse of terrigenous detrital,evaporite seawater with high salinity is the dolomitization fluid which means the origin of A-type dolomite is sabha dolomitization.The origin of limy dolomite is seepage reflux dolomitization,which gradually generated at shallow burial stage.The fluid of this type is reflux brine of high salinity.