Geochemical Characteristics Of Carbonate Rocks In The Mesoproterozoic Yangzhuang Formation In Jixian,Tianjin

The Mesoproterozoic is an important period in geological history,between the two Great Oxygenation Events,and is a critical period for the origin of eukaryotes.To further understand the palaeo-ocean environmental evolution and palaeo-climate characteristics at that time,samples were collected from the Yangzhuang Formation in the Jixian Section and were run for C-O isotope and elemental geochemistry.The δ¹³C values of the samples are relatively negative with little variation and some cyclicity.The curve of theδ¹³C values shows that there is a large cycle from the first member to the third member.In the third member of the key section,three cycles with small but meaningful changes were identified.Theδ¹⁸O values show a large variation and show some oscillation.The Z values are all greater than 120 and are well correlated with theδ¹⁸O andδ¹³C values,indicating that they could be used as an indicator of salinity.In addition,intensive testing was conducted on the 5m red-gray dolomite in the third member.It was found that theδ¹³C cyclicity is related with the changes in lithology.On the whole,the C-O isotopic characteristics of the Yangzhuang Formation in studied sections are consistent with the stable isotope compositions of the Mesoproterozoic.The indicators of paleo-climate,paleo-depth,paleo-salinity and redox conditions show that the Yangzhuang Formation was deposited in an oxidized to sub-oxidized shallow water tidal flat environment,with a high-salinity seawater in an arid climate.With the frequent changes of sea level,the sedimentary environment of the Yangzhuang Formation fluctuated between the intertidal zone and the supratidal zone,accompanied by changes in water depth,redox condition and input of terrigenous clastic.The microfacies of the first member were mainly deposited in an intertidal environment,with relative deep water depth,low degree of oxidation,minor input of terrigenous clastic.However,the microfacies of the second member were mainly formed in the supratidal zone,which shows relatively shallow water depth,high degree of oxidation,large input of terrigenous debris.The microfacies of the third member were interpreted to have been deposited in the intertidal to supratidal zones with periodic changes in water depth,oxidation level and input of terrestrial clastic,indicating that the oxygen content of the ocean surface was fluctuating,and the weaker Ce negative anomaly indicated that the oxygen content of the ocean surface at that time was relatively low and the redox interface was relatively shallow.In addition,the formation of the red beds of the Yangzhuang Formation was studied,and it was concluded that the formation of the rhythmic deposits was related to the change of sedimentary facies from the intertidal zone to the supratidal zone that caused by the rises and falls of sea level.A preliminary detrital provenance analysis was also carried out on samples with high rare earth elements content,indicating that terrigenous clastic of carbonate rocks in the Yangzhuang Formation may have originated from upper-crust felsic granodiorites or granites.