Typomorphic Characteristic Of Magnetic Minerals Of The Late Cenozoic Sediments From The Yangtze River Mouth And Its Source Implications

The evolution of the Yangtze River is closely linked with many important scientific issues, such as the Qinhai-Tibet Plateau uplift, the origin of large river basins in Asia and the paleoclimate change. The theory of "source" to "sink" is by tracking down the sediment provenance of the Yangtze Delta back to the evolution of Yangtze River Basin. Numerous experimental results from previous studies have proved that the elemental composition of magnetite in sediments can be used as a proxy for the mother rock types, and further to indicate the sediment provenance.To get the knowledge of the rock type distribution of the Yangtze River catchment, and look for the effective diagnostic source are the first step to apply for the Late Cenozoic core in the Yangzte Delta. Thus, firstly, this paper identified 47 samples by magnetic mineral collected from different locations in the middle and lower reaches of the modern Yangtze River, and measured the magnetic mineral form of scanning electron microscopy (SEM) and chemical composition of the electron probe (EPMA) test.Secondly, based on the results of the chemical composition of the electron probe (EPMA) test on the 440 samples from different sediment layers in the Late Cenozoic sediment core (SG7,336.62m long) in the Yangtze estuary, this paper picked up 54 samples with strong magnetic properties for the measurements of magnetic mineral form of scanning electron microscopy (SEM) and chemical composition of the electron probe (EPMA) test. Among of them,9 samples were tested for heavy mineral compositons. Meanwhile, other materials incorporated into the present study are 430 samples of particle size,267 samples of paleomagnetic analysis,10 samples of optical dating and 3 samples of U-series dating.Through the comparison analysis between the modem sediments of the Yangtze River of EPMA results in SG7. the present study is trying to reveal the provenance evolution of the Yangtze River and the related driving forces, which will also provide a new insight into the evolution of the Yangtze River drainage basin.The EPMA results on the modern sediments of the Yangtze River showed that: the dominant contribution of magnetite source in upper Jinsha River (basin above Shek Kwu Section) is characterized by high Fe. Co, high Cr, Mg. Al, V, low Ti, Mn. Zn; various tributaries of the Yangtze River also contributed a lot of magnetite with high Ti. Cr: the increasing Zn were significantly derived from Qingjiang River: Hanjiang River also have some contributions to the sediment source with high-Zn magnetite; Rivers from Dabie Mountain provide a lot of large iron ore with high-Mn magnetic; Rivers from Poyang Lake Basin largely contributes sediments with high-Ti, Al and Cr; Qingyi River Basin has contributed sediments with high Mg magnetite; the local provenance from the estuary contributes the local source by the low-Fe, Co, high-Ti, Mg, Al, Cr magnetite. EPMA analysis of surface samples also showed that output of magnetite from the upper Yangtze River basin are more likely to accumulate when it is discharged into the Jianghan basin.The elemental compostion of magnetite in SG7 can be divided into five sections according to EPMA results. Section I (Pliocene-the bottom of Early Pleistocene) is characterized by high Ti, Mg, Mn low-Cr, Zn, Al; Sectionâ…¡(the lower part of the Early Pleistocene) is featured with high in Fe, and low in trace element contents; Section III is the next middle Pleistocene, characterized by high Ti, Mg, Mn, low-Cr, Zn, Al, V; Section IV (the upper part of Early Pleistocene-late Pleistocene) is characterized by the strong elemental fluctuations; Section V (the Holocene) is characterized by low Mg, Mn, Cr, and even lower Al, V.Typomorphic characteristics of magnetite from sediments of the SG7 core in the Yangtze Delta and modern river sediments of Yangtze River show that:From the Pliocene to the beginning of Pleistocene, magnetite in sediments of Yangtze River Delta region mainly come from Bailonggang Ti-rich basalt rock. Ancient topography is undulating, and sediments from a narrow range of local. Magnetite of Early Lower Pleistocene sediments mainly from proximal sedimentary rock and metamorphic rocks of Fujian and Zhejiang Uplift.It is characterized by high Fe. the content of other elements is low.Middle Lower Pleistocene sediments have a large number of Mn-rich magnetite particles. It indicates that the basin occurred at least once a significant expansion Event. It reflects the area become to an open depositional system under the tectonic subsidence movement.The sediments may come from the Dabie Mountains. Zn-rich magnetite particles appear in the Late Lower Pleistocene sediments. It indicates the sediments reach to this area most likely come from the Middle Yangtze River Region before Wuhan at this time. The basin expanded once again.The Yangtze River has been completed throughout at Holocene.Holocene sediments of the Yangtze River Delta comes from distal upper reaches steadily.The Evolution of Yangtze River water system gets into the mature period.