The Provenance Study On The Yangtze River Sediments, Based On Mineralogy, Geochemistry And Detrital Zircon Dating

The large river systems are the major transfer of continental masses to the ocean, playing significant roles in global geochemical cycles. The Yangtze River is the longest river in Asia, originating from the Tibetan Plateau. As the linkage of both tectonics and climate, the Yangtze River has been of great interests to many geologists and geomorphologists. The sediments in the Yangtze River record the information of several geomorphology and tectonic units along the drainage. Thus, the research on the sediment provenance of the Yangtze River could give important implications of river sediment erosion and transport pattern, and also could provide better understanding of the evolution of the drainage, and the response to the tectonic uplift and the monsoon system.During recent years, various approaches on the sediment provenance in the Yangtze River have been performed, but most of the previous studies only focused on the mid-lower reaches or the delta area of the Yangtze, with only simply one method. There still lack of the investigation on the whole Yangtze River, including the mainstream and major tributaries, and lack of studies using the combined approaches to talk about the sediment source. Furthermore, the differences between diverse methods and the relationship between the Yangtze and other rivers which are from the Tibetan Plateau are still unknown. The Yangtze River sediment source is still obscure. Based on the complex "source to sink" process of the Yangtze River, this thesis focus on the provenance study of sediments collected from the whole Yangtze River by using combined methods including mineralogy (clay mineral, detrital mineral), detrital zircon U-Pb ages and Hf isotope and geochemistry (major element, trace element and Sr-Nd isotope) to understand the sediment source of the Yangtze River and to establish a sediment provenance system.Samples are collected within several field trips. The sampling sites cover the whole drainages, mainly near the confluence between the trunk stream and tributaries. We took sand and clay samples at each point and soil sample in the tributaries in the upstream. Most samples were collected from deposits within the channel such as mid-channel bars and lateral bars, few samples from river dredges or stockpiles of dredged sand when river levels were high, surely avoiding cities, dams and other sources of possible contamination. Totally, we have114sand samples,74clay samples and10soil samples.The results of sediment provenance from different methods are distinct. The clay mineral composition and detrital mineral composition indicate that contribution of sediments from the tributaries like the Yalong Jiang, Dadu He, Min Jiang, Jialing Jiang and Han Jiang is larger, but that of the Wu Jiang, Yuan Jiang, Xiang Jiang and Gan Jiang is less. There are representative minerals in each tributary. Glaucophane is the characteristic mineral of the Tuotuohe. Cyanite is the typical mineral in the Min Jiang. Barite and cassiterite abound in the Wu Jiang and Xiang Jiang respectively. Muscovite could be the typical minerals in the drainage to trace the provenance. Detrital zircon dating shows sediments are mainly from the Hanjiang, Jialingjiang, Xiangjiang and Panzhihua-2to Yibin in the mainstream. The Ganjiang and Wujiang are the minor contributor to the drainage. It could also be deduced that different age range zircons are from different geological units. The Sr-Nd isotope analysis indicates sediments from the Jinshajiang and the Chuanjiang are mainly from the Songpan-Ganze fold belt and the Middle and East Yangtze Craton, repectively. The geochemistry compositon of sediments in the middle-lower stream have homogenization trend. Totally speaking, the sediment source is mainly from the Jialingjiang and Hanjiang, but the Wujiang, Xiangjiang and Ganjiang contribute little to the minstream.Sediments source are controlled by several factors, so are provenance-tracing methods. The rock types are the most impotant factor on different techniques. The clay mineral and detrital mineral compositon are controlled by steepness, tectonic and the specific stream power (the combination of discharge and steepness). Element and Sr-Nd isotopes compositon are influenced by the discharge and rainfall. However, the discharge and the specific stream power impact the detrial zircon dating mostly. In conclusion, the rock types along the drainage and the specific stream power (the combination of discharge and steepness) are the most aspects to the Yangtze River sediments.Accroding to the results from the clay mineral, the heavy mineral and the geochemistry, we know that the weathering intensity increases along the drainage, with the upstream is controlled by the physical weathering and the mid-downstream is governored by chemical weathering. The average weathering strength of the Yangtze River is between the Yellow River and the Pearl River, which demonstrate different climates of these regions.The detrital zircon U-Pb dating and Hf isotope analysis also shows that there are several age peaks in the Yangtze sediments which could correspond to known granitoid magmatic events within the drainage (Luliangian, Jinningian, Caledonian, Hercynian and Indo-Sinian/Yanshanian). Zircon Hf model ages display two major ranges of crustal growth, one being Mesoproterozoic (1.0-1.5Ga) and the other Archean-Paleoproterozoic (2.5-2.8Ga).Based on the provenance system we established, researchers could choose appropriate approach to investigate the provenance of river sediments according to the background of the drainange, the purpose of the study and the property of sediments. This system could be as reference in research of river sediment source.