Geochemistry Of Sedimentary Carbonates From The Late Ediacaran To The Early Cambrian In The Lower Yangtze Region Of South China

Marine sedimentary carbonate rocks record important historical information on the evolution of the Earth's surface,and the reconstruction of ancient ocean composition and carbon cycle by carbonate isotope changes is an important and hotspots topic in the study of paleo-environment and paleo-climatic change.The sedimentary environment and diagenetic history could be indicated by the trace elements in the carbonate rocks.The rare earth element patterns can identify the sedimentary environment,and the relationship of the Y/Ho ratio and(Sm/Nd)PAAS and the stable isotopes can be used to restore the historical sea level changes.The elements with high seawater-crustal partition coefficient(Kx)in the pure carbonate can used to indicate the composition of the open sea,and the water-insoluble elements with low Kx and shorter residence times such as REE,Al,Sc and Y could be used to identified the affects of the input of the terrestrial material on the composition of the seawater.The U-Pb age of the detrital zircon could be used as the index of the intensity and time of the magmatic activity during the geological history and the source of the sedimentary environment area.Carbonate and detrital sedimentary rocks from the late Ediacaran to early Cambrian are exposed at Chaohu,and providing an excellent natural laboratory for studying the depositional and diagenetic processes of these sedimentary rocks.The carbon and oxygen isotopic composition,the organic carbon isotope,the contents of the major and trace elements and non-carbonate materials in the late Ediacaran-early Cambrian sedimentary carbonate rocks in the Tangshan section of the Chaohu area in South China are carried out in this thesis.Four significantly positive ?13Ccarb anomalies(DPI,LP1,LP2 and BP1)and four negative anomalies(ECN,LN1,LN2 and BN1)were illustrated in this profile.However,the change of?18Ocarb is relatively simple,a significant decrease of about 2.5 ‰ from the late Ediacaran(average-3.5‰)to the early Cambrian(average-6.0 ‰)appeared at the ECB,and it isconsisted to the negative ?13 Ccarb anomaly at this time.Carbon isotopes of organic matter(?13Corg)in carbonate rocks show three distinct positive anomalies(DPI,LP1 and LP2)and two significant negative anomalies(ECN and LN1)in the Tangshan section.There was a significant positive correlation between ?13Corg and ?13Ccarb in the Dengying(DY)and the Lengquanwang(LQW)formation,while the ?13Cord and 13C,arb decoupled in the Bantang(BT)formation and the Shanaoding(SAD)group.Based on the comparison of the ?12Ccarb of the Tangshan section and the contemporary strata in other sections from the South China,it is concluded that the boundary of the Late Ediacaran and early Cambrian in the Tangshan section should be lined at between the DY and the LQW formation.The Mg isotopes of the dolomite in the late DY and the early LQW of Tangshan section are unchangeable indicate that the oceanic Mg cycle is in a steady state at this time.The results of the major elements,XRD and SEM indicate that the carbonates from late Ediacaran to early Cambrian strata in the Tangshan section are mainly comprised of proto-dolomite.The small size and clearly boundary of mineral crystal could indicate these dolomite are formed with quickly deposited rate.The carbonate in the middle DY Formation have marine like REE+Y patterns and high Y/Ho ratios,could reflect the normal marine sedimentary environment at this period,while the carbonate in the late Ediacaran and early Cambrian have smooth REE+Y patterns and low Y/Ho ratios,which indicate that they are deposited in the shallow water where affected by the terrestrial effects.The contents of the water-insoluble elements(e.g.,Sc,Al,Th and REE)were positively correlated with the content of non-carbonate materials,and there was a significant increase at the ECB.The results of decrease in ?18Ocarb and increase in the content of water-insoluble elements indicate increase of a large amount of fresh water with the terrigenous material input event at this period.The negative correlation between the ?13Ccarb anomalies and the contents of water-insoluble elements indicates that the negative anomalies of the carbon isotopes may be related to the increased terrigenous input.The cross plot of Y/Ho ratio and the(Sm/Nd)PAAS,(Gd/Nd)PAAS indicate the mixing of fresh water and seawater had occurred in the Tangshan section,and the partition of fresh water was changeable which should be related to the changes of sea level in this period.The change of the sea level may record the regression and transgression which may be related to the supercontinent activity of this period.The transgression occurred at the early BT result in the link of the lagoon and the open ocean could be indicated by the decoupled of the paired?13Corg and ?13Ccarb in the carbonate.Because of the buffer of the large organic pool,the?13Corg in the upper early Cambrian strata is unchangeable.It is questioned the time and space scale of large dissolved organic carbon reservoirs assumed by the precious researches.The U-Pb age and trace elements of the detrital zircons from the shale shows four distinct magmatic activities pink.The most U-Pb ages of?800Ma could be identified,which indicate the intense magmatic activity caused by the breakup of the Rodinia super continental in the Neoproterozoic.The U-Pb ages of detrital zircon illustrate that the main sedimentary material source of Chaohu area in the middle DY from the late Ediacaran is from the South China Craton.