Pb Isotopic Compositions Of Detrital K-feldspar Grains Of The Late Cenozoic Sediments From The Yangtze River System

Originating from the eastern Tibetan Plateau, after draining approximately one fifth of the continental area of China, the Yangtze River finally enters the East China Sea near Shanghai. However, the Yangtze’s origin and evolution have been vigorously debated for more than 100 years and still generates controversy. The Yangtze River delivers great volumes of water, sediment, and associated chemicals from its headwater regions to the middle-lower reaches and the East China Sea, significantly influencing the evolution of sedimentary systems in these areas. Recognition of the source to sink transport pattern of the Yangtze sediments, therefore, allows us to better understand these phenomena.When conducting sediment provenance work within continental-scale drainage basin, more attentions should be paid on chosing suitable provenance tool because of the diverse rock compositions and strata. As one of the major minerals of the crust, K-feldspar provides a means of assessing first-cycle sand-grain provenance. Furthermore, where it occurs as a significant framework component (c.20% modal abundance), K-feldspar must presumably be more representative of the source area than relatively minor components, such as zircon, which typically make up << 1% of the mode. Several distinct Pb isotopic provinces have been recognized in eastern Asia, suggesting that a detrital Pb isotopic signal may be appropriate for tracing the sediment source and/or understanding erosion-transportation processes within the Yangtze River Basin.In this study, Pb-in-K-feldspar was applied in the Yangtze River drainage system to trace sediments provenance. Firstly, modern sand samples of the Yangtze River, including the major tributaries and mainstream were analysed to characterize Pb isotopic range of different sources. In addition, several typical late Cenozoic deposits were investigated. Based on them, we reconstructed the drainage evolution history of the Yangzte River since late Cenozoic.Pb isotopic compositions of K-feldspar grains in the major tributaries were compared with those of basement rocks in the Yangtze drainage basin in order to assess their provenance. Results show that the detrital Pb isotopic signals from different tributaries provide a good proxy for their respective source regions, especially for those terranes that have been well characterized, such as the Qinling and Qiangtang blocks.According to Pb isotopic compositions of K-feldspar grains in the mainstream samples, Pb isotopic composition is a stable provenance signal within the Yangtze River. Based on detrital K-feldspar data from the trunk stream and the tributaries of the upper Yangtze, we propose that the Jinshajiang River is not a major sediment contributor to the Yangtze trunk stream. The tributaries flowing into the Sichuan Basin seem to the dominant sediment suppliers as the sample YC-YZB yields grains broadly consistent with the Pb isotopic ranges from these rivers.We determined Pb isotopic provenance signal of the Xigeda Formation to investigated its relationship with drainage evolution of the Jinsha River. New Pb isotopic results demonstrate that the Jinsha River had achieved its current sediment-transport pattern by late Pliocene, indicating a similar drainage character to nowadays.Pb isotope compositions of detrital K-feldspars from Eocene-Pleistocene sediments in the western and central Jianghan Basin, located just downstream of the Three Gorges. Our new Pb results indicate that western Jinghan Basin was fed by local sources, like Huangling Missif, but no Tibet grains during Eocene. Feldspars from the Songpan-Ganzi were already being delivered to the Jianghan Basin by～3.4 Ma. Therefore, we suggested that the Three Gorges was cut through prior to the late Pliocene, but no earlier than Eocene.Sediment provenance results demonstrated that the Jinsha River had achieved its source-to-sink processes by late Pliocene. Thus, we proposed that the Jinsha had a stable drainage pattern since then. Moreover, Pb isotopic results indicate that detritus from the southeastern Tibet had been transported to the Jianghan Basin between Eocene and late Pliocene. The only way that the sediments from this region could be transported to the Jianghan Basin is by the Yangtze River. Therefore, we propose that the Three Gorges had been cut through by the upper Yangtze River prior to the late Pliocene.To test whether the Songpan-Ganzi and Yangtze Craton used to be the source regions of the paleo-Red River system, K-feldspar grains from the Hanoi Basin are compared with the fields defined by the modern upper-middle Yangtze River system. K-feldspar grains from late Cenozoic sediments in the Hanoi Basin are distinct from the Songpan-Ganzi terrane but provide a good match with the Yangtze Craton in its range of lower 206Pb/204Pb ratios. These observations support the idea that the "Middle Yangtze" used to be a tributary of the paleo-Red River system and there has been no drainage linking the Songpan-Ganzi terrane and the Red River since the Eocene.