Sedimentary Geochemistry Of Neoproterozoic Carbonates In The Lantian Formation From Southern Anhui In South China

The Late Neoproterozoic Eon was accompanied by drastic climate perturbations with serious glaciations. At least two glacial events (Sturtian and Marinan iceages) are well recorded in sediment rocks, which are usually covered by cap carbonate with no gap between them. In particular, the carbonate subsequent to the Marinoan glaciation exhibits unique sedimentary structures, major and trace elemental compositions and stable isotopes ratios, and has become a focus of much studies. The rocks of Sinian system (Ediacaran system) in the late Neoproterozic outcrop at many localities across the Yangtze Platform and provide good opportunities for the investigations of paleoenvironments, paleoclimate and depositional waters and biological evolution after the "snowball Earth event".The Leigongwu diamictite in southern Anhui belongs to deposition during the Marinoan glaciation, corresponding to the Nantuo diamictite on other profiles in South China. The Lantian Formation sharply and disconformably overlies the Leigongwu diamictite. Biostratigraphic, chemostratigraphic and geochronological studies have demonstrated that the Lantian Formation is equivalent to the Doushantuo Formation elsewhere in South China. Two distinct carbonate units occur in the Lantian Formation in southern Anhui. The carbonate in the lower unit directly follows the Leigongwu Formation and corresponds to the Marinoan cap carbonate. The carbonate in the upper unit of the Lantian Formation, conformably overlying the black shale, compared well with the top carbonate of the Doushantuo Formation elsewhere in South China. Because of the similarities and differences between the carbonates in the Lantian Formation and in equivalent sequences in other sections, the study of the Lantian Formation can provide a good opportunity to decipher paleoenvironmental and paleoclimatic evolution.In this thesis, a systematic study of major and trace elements as well as C-O-Sr isotopes in the carbonate of the Lantian Fornation was carried out. Veinlets are found in the carbonate, so that their elemental and isotopic compositions were analyzed together with their wallrock. The results provide geochemical constraints on the relationship between the depositional environments and the snowball Earth event.The upper and lower carbonate units in the Lantian Formation are impure and composed of various components. These components include main carbonate minerals, such as aragonite, calcite and dolomite, as well as non-carbonate materials such as siliciclastics, oxides and sulfides. Because these components have a variety of physical and chemical characteristics, they suggest possible interaction during sedimentary and diagenetic processes as well as laboratory analyses. Thus, we attempted to separate the different components by stepwise chemical leaching for the element concentrations and Sr isotopic compositions. The results show that the carbonate component dissolved in 0.5 M acetic acid (Dissolution I) is principally composed of calcite and almost free from non-carbonate contamination. Thus its elemental and isotopic compositions can be used to unravel the geochemical feature of the water from which the carbonate precipitated. The Sr concentrations and isotope ratios, and REE+Y concentrations and patterns of Dissolution I are similar to each other in the two units. They exhibit high Sr concentrations and high ⁸⁷Sr/⁸⁶Sr ratios, depletion of LREE relative to MREE and HREE, weak positive La anomalies, subtle negative Ce, Gd and Er anomalies, and near-chondritic Y/Ho ratios. There are obvious positive Eu anomalies in the lower unit of carbonate. These indicate that the two carbonate units were precipitated from the similar nature of basin water, which lack of trace element composition typical of normal marine carbonate but similar to those in freshwater carbonate. The negative Ce anomalies in the carbonate of the two units suggest oxygenation of the precipitation water.The selective acid extraction method was used to liberate CO₂ from calcite and dolomite in the Lantian carbonates. The results enable us to distinguish between primary and secondary carbonates for the purpose of constraining their origin. In view of trace element compositions and water-rock interaction modeling for coexisting dolomite and calcite, the primaryδ¹³C andδ¹⁸O values for the upper unit at Shiyu are -25.6 to -18.6‰and -11.8 to -7.9‰in PDB, respectively; and those at Piyuancun are higher than -11.9‰and -10.5 to -9.5‰, respectively. In this regard, the primary sedimentaryδ¹³C andδ¹⁸O values for the lower unit at Shiyu are -5.3 to -3.5‰and -12.8 to -10.9‰(in PDB), respectively. The very lowδ¹⁸O values of-25.6 to -18.6‰(PDB) for the upper carbonate unit at Shiyu are comparable to those for modern Arctic carbonates, corresponding to waterδ¹⁸O values as negative to -30 to -18‰in SMOW. This is interpreted as precipitation of carbonate from freshwater-derived fluid in a continental marginal basin. In contrast, the carbonate with less lowδ¹⁸O values for the upper unit at Piyuancun and lower unit at Shiyu would precipitate from seawater-like fluid. Our interpretations are strengthened by terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios for the both carbonate units of the Lantian Formation. Since the negative carbonateδ¹³C excursion from -11.7 to -7.8‰in the upper unit is also associated with the extreme ¹⁸O-depletion, the terrestrial dissolved organic carbon reservoirs is considered as a basic cause for the lowδ¹³C values. The C, O and Sr isotopic ratios and elemental concentrations of carbonate in the upper and lower units of the Lantian Formation indicate that the precipitating water are composed not only of the seawater but also of the meltwater from glaciation. As for the upper carbonate unit at Piyuancun, the REE+Y patterns and Sr isotopic compositions indicate that a significant amount terrigenous materials pouring into the depositional basin. The relatively high carbonateδ¹⁸O values imply that the O isotopic ratios of water are similar to those of Neoproterozoic seawater. This indicates that the precipitating water has the similar O isotopic compositions to the seawater, but terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios. As for the carbonate in the upper unit at Shiyu, the precipitating water is characterized as extremely lowδ¹⁸O values, terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios. Since the upper unit predates the appearance of Ediacaran biota but postdates the Gaskiers iceage, the freshwater would probably originate from continental deglacial meltwater in the post-Gaskiers phase. The large differences in the concentrations of the trace elements and stable isotopes between the carbonate in the upper unit and the equivalent Neoproterozoic carbonates suggest that the precipitation rate of the upper carbonate would be high without the homogeneity between glacial meltwater and seawater. Thus, the continental marginal basin precipitating the Lantian carbonates may be one of rift basins between the Cathaysia and Yangtze Blocks with limited channel to the open ocean. Piyuancun was located at distal section whereas Shiyu was located at proximal section and thus was not connected with the open ocean. As for the lower carbonate unit, the REE+Y patterns and Sr isotopic compositions are similar to those infreshwater carbonate although theirδ¹⁸O values are in the same range with those of Neoproterozoic marine carbonate in the equivalent carbonate elsewhere in the world. The carbonate in the lower unit deposited after the global Marinoan glaciation. The seawater-like fluid would be derived from global deglacial meltwater in the post-Marinoan phase and carry the terrigenous materials poured into the sedimentary basin. Thus the precipitating water has the similarδ¹⁸O values to the seawater and terrigenous REE+Y patterns and high ⁸⁷Sr/⁸⁶Sr ratios.The differences in texture, elemental and isotopic compositions between wallrock and veinlet in the both lower and upper carbonate units indicates that the wallrock did not experience significant modification by diagenetic fluid. Theδ¹³C values for the calcite veinlet are in the same range with the values for the micritic wallrock in both the upper and lower carbonate unit of the Lantian Formation, indicating that the carbon sources for the veinlets are probably the micrite carbonate in the wallrock. Theδ¹⁸O values and REE+Y patterns are different between the calcite veinlets and micritic wallrock in most carbonate, which imples that the diagenetic fluids are external, except that other veinlets of the upper carbonate at Piyuancun is internal fluid. The veinlets of one group in not only the upper unit at Shiyu and Piyuancun and the lower unit at Shiyu have lowerδ¹⁸O values of-28.6 to -18.1‰which were from the continental glacial meltwater, but different REE+Y patterns which were controlled by the different terregenous weathering materials. Such veinlets occur at both Shiyu and Piyuancun. The veinlets in the other group occur in the upper unit at Piyuancun, which have relatively higherδ¹⁸O values of-17.2 to -11.3‰similar to the wallrocks. This indicates that the diagenetic fluids are internal and have the similar source to the precipitation fluids. But the REE+Y patterns are different in veinlets and wallrock, which implies that they influenced by the different terregenous weathering matterials.Correlating the chemical stratigraphy in southern Anhui with that elsewhere in South China and in the world can help us to understand the palaeoenvironment, palaeoclimate, atmospheric oxygen content, organic productivity and fluid compositions. Moreover, the elemental and isotopic compositions of the sedimentary carbonate rocks are closely associated with the circulation of geochemistry and biochemistry and thus can indicate the environmental evolution. In particular, the study of in-situ microanalyses of C-0 isotopes and trace elements in veinlet and its wallrock provides a new insight into the geochemistry of diageneic fluid, enabling us to trace its source. As a consequence, we have constructed a model for carbonate formation based on the change of elemental and isotopic compositions. Thus, our study can help to understand the palaeoenvironmental evolution during the Ediacaran phase, which is important to understanding of the radiation of the Ediacaran biota.