Geochemical Characteristics And Implications Of The Nantuo And Doushantuo Formations In The Three Gorges Area

The Neoproterozoic sedimentary rocks are fully developed and widely distributed in the Yangtze block. The paleoclimate during the Neoproterozoic time has changed drasticly and several glacial events have happened. The Nantuo Formation and the overlying Doushantuo Formation own special sedimentary texture, isotopic geochemical characteristics and a lot of paleontologic fossil, hence, it is important to study the two formations not only to provide constraints on their provenance and depositional environment, but also to help us for our understanding in palaeoenvironment, paleoclimate and the evolution of life in Neoproterozoic.The Nantuo tillites exposed in the Jiulongwan section of the Yichang area are glacial diamictites from northern Yangtze block in Nanhua period, which unconformably overlie the Neoproterozoic Huangling granites. The compositions of the Nantuo tillites are complicated, and the sedimentary time and components of source are rarely studied and in dispute. The zircon U-Pb isotopic dating results of Nantuo tillite in this study show youngest zircon age of 706 ± 7 Ma. Combining with previous results about zircon U-Pb isotopic ages of the tuff from the basal Doushantuo Formation and top of the Datangpo Formation, the sedimentary ages of the Nantuo Formation are limited to 654-635 Ma, which can be equal to the Marinoan event. The peaks of zircon U-Pb isotopic age-spectrum are ～3.0 Ga,2.5-2.3 Ga,2.05-1.95 Ga, and 900-700 Ma, implying four crustal-reworking events in the Yangtze block. The ages of Paleoproterozoic and Archean are similar to the Kongling complex, meanwhile 900-800 Ma and 800-700 Ma are the same to the formation time of the Huangling granites nearby and the rocks from the northern margin of the Yangtze block. The trace element and isotope compositions of moraine sandstones are obvious differentiation, maybe due to different degree of mixture by the materials from the northern margin of the Yangtze block, Huangling granite and Kongling complex. The granitic conglomerates in the Nantuo tillites have different features of trace elements, such as low REE contents and depleted HREEs. Associated with zircon U-Pb isotopic ages and signature of Sr-Nd-Pb isotopic compositions, these granitic conglomerates should mainly come from the Huangling batholith and the Kongling complex nearby. The provenance characteristics of the Nantuo tillite reflect intensive rift related magma activities in the Yangtze block at Neoproterozoic, which caused the terrain geographically lower in the south and higher in the north.Base on different lithologies, the Doushantuo Formation in the Jiulongwan section can be devided into four rock units, from bottom to top, carbonates, black shale and dolomite interbedding, dolomite and thin layers of limestone interbedding, black shale. Pure carbonates were separated from siliciclastic components by leaching in 3.4 M acetic acid solution. Geochemical features of the carbonate components, such as slightly enrichment of MREE, negative to positive Ce anomalies (Ce/Ce* of 0.86 to 1.40), positive La anomalies (La/La* from 1.02 to 1.42) and sub-chondiritc Y/Ho ratios (36.8-49.5), indicate that the Doushantuo cap carbonates were deposited in a fluctuating inner-shelf environment, where freshwater and seawater mix together and the influence of hydrothermal fluid from the sea floor was absent. All the Doushantuo carbonates and shales have variable Sr isotopic compositions with initial Sr isotopic ratios ranging from 0.704030 to 0.712720. Moreover, high initial Sr isotopic composition represents intense weathering condition from continent and supports significant contribution from river water during the deposition of the Doushantuo Formation. From buttom to top, initial 143Nd/144Nd values of sedimentary rocks gradually decree and their TDM values increase. This indicates the change in crustal formation ages of the sedimentary sources during the deposition. Based on geochemical characteristic, the Doushantuo Formation can be divided into lower and upper parts. Rocks from the lower Doushantuo Formation have less radiogenic Nd isotopic composition (εNd(t):-2.05--7.35) and lower Th/Sc ratios (0.14-1.41) relative to the upper Doushantuo rocks (eNd(t):-9.26～-11.48, Th/Sc:0.61-1.67), indicating that the lower Doushantuo Formation received relatively juvenile materials (e.g. Neoproterozoic rocks of the northern Yangtze margin), whereas the upper Doushantuo Formation received relatively mature material (e.g. the Kongling complex). Moreover, Sr-Nd isotopic compositional analyses of black shales from the Doushantuo Formation indicate a mature old upper curst as the provenance. TDM of black shales ranges from 2.06-2.24 Ga and ENd(560 Ma) spans at-11.48 to -9.26, both of which are similar with the signature of the Kongling complex, indicating Kongling complex offered the major material for deposition of black shales in the Doushantuo Formation. Pb isotopic composition of the sedimentary rocks also indicates that old crustal section provided the major material for the Doushantuo Formation during the deposition. Combining the previously reported carbon isotopic data, the variation of sedimentary facies and related elemental and isotopic compositions are likely associated with the change of palaeoenvironment and paleoclimate, and thereby reflecting changes on sea level and the oxygen content in the atmosphere.