Geochemistry and dolomitization mechanism of Majiagou dolomites in Ordovician,Ordos,China

The types and genetic mechanism of the Majiagou dolomite in Ordos basin are investigated using petrological analysis and geochemical methods.The results show that dolostones in Majigou Formation mainly consist of dolomicrite and crystalline dolostone.Dolomicrite often associates with gypsum or halite,with medium jacinth light in cathoduluinescence observation,lower degree of order,and high content of Sr and Na.It provides similar δ13C and REE distribution pattern with micrite.The value of Sr isotope is close to that of sea water in the same period,and fluid-inclusion homogenization tempetrature approximate to earth's surface.These suggest that dolomitization fluid is high salinity fluid that closely related with sea water,considered as penecontemporaneous dolomitization which can be explained by evaporation pump mode.Crystalline dolostone contains little associated minerals,only pyrite observed occasionally.This crystal structure is usually xenomorphic curve.Crystalline dolostone shows medium-weak jacinth light in cathoduluinescence,whith high degree of order and low content of Sr,while Na and Mn content is close to that of dolomicrite.REE distribution pattern is similar with dolomicrite and close to micrite.The distribution of δ13C is also corresponding to micrite,and fluid-inclusion homogenization tempetrature is hight.These indicate that dolomitization fluid of crystalline dolostone is similar with that of dolomicrite.Crystalline dolostone is buried dolomitisation,where continuous compaction from overlying deposits urges the sealed brine to flow laterally,thus supplies unfailing Mg2+ for dolomitisation.