【作者】 李玲；

【导师】 郑洪波；

【作者基本信息】 南京大学 ， 地球化学， 2011， 硕士

【摘要】 长江发源于青藏高原,是亚洲第一大河,它与新生代青藏高原隆升和亚洲季风演化密切相关。长江作为连接构造隆升与季风系统的纽带一直是地质学家和地貌学家研究的热点。众多学者对长江的形成与演化过程和时限至今未达成共识,这些问题的研究将有助于解决新生代亚洲地形格局和季风气候格局的形成与演化的过程与时限,具有重大科学意义。本文采用碎屑锆石U-Pb年代学的沉积物源示踪方法,将南京地区新生代沉积物和现代长江南京地区河砂沉积物对比分析进行物源示踪研究,并结合两者的重矿物特征,探讨了新生代长江的地质演化历史。同时利用粒度、磁化率和Rb/Sr值探讨了南京地区六合灵岩山剖面玄武岩下伏松散沉积物的成因及气候意义；利用X射线衍射方法对中新世以来长江三角洲沉积物、黄土高原红粘土和黄土-古土壤的粘土矿物组成变化进行了研究并探讨了其古环境意义。取得了以下结论：1)南京地区广泛分布的新生代沉积物,结合前人对其生物化石的研究和本文中对其上覆玄武岩Ar/Ar定年,表明其时代为中新世。根据地形分析,江宁方山剖面和浦口江浦砂矿剖面均位于白垩纪以来的古隆起区背离长江一侧,这两个剂面很可能与古长江无关；六合小盘山剖面、六合桂子山剖面、六合唐公山剖面发育的大型平行层理、交错层理,应是中新世的大河形成,结合前人研究把这条大河称为古长江。2)碎屑锆石U-Pb年龄谱系特征：南京地区六合小盘山、灵岩山、唐公山和桂子山剖面的中新世河流沉积物与现代长江南京地区河砂沉积物整体上相似。重矿物组合特征：南京地区中新世沉积物重矿物组合与现代长江南京地区河砂沉积物重矿物组合相似,但是其重矿物含量较低,且没有角闪石和辉石,化学风化系数较高,反映了其可能经过较强的风化作用。综合沉积物碎屑锆石U-Pb年龄和重矿物组合特征认为,中新世时长江应该贯通了三峡。南京地区中新世积物重矿物含量和组合与现代长江南京地区河砂沉积的差异不应是物源上的差异可能是风化的差异的结果。3)六合灵岩山剖面玄武岩下伏松散沉积物：从野外沉积剖面特征看,其20-110cm段非常类似黄土沉积,没有层理、质地均一、垂直节理发育,钙结核可见,应该不是流水作用和坡面过程作用下的沉积物。20-110cm段松散沉积物粒度分布特征与黄土高原黄土有很大的相似性。代表化学风化强度的Rb/Sr比值表明,20-110cm段松散沉积物Rb/Sr比值远大于第四纪黄土高原黄土,反映其历了较强的化学风化作用过程。综上所述,认为南京市六合区灵岩山剖面玄武岩下伏20cm-110cm段松散沉积物为风尘沉积,时代在10.32Ma以前,为迄今中国东部发现的新生代最老的风尘沉积物,其时代与秦安红粘土剖面的15-13Ma相当。4)长江三角洲和黄土高原中新世以来的沉积物粘土矿物都为伊利石+绿泥石+高岭石+蒙脱石组合,两地各连续沉积剖面沉积物粘土矿物存在伊利石逐渐增多的变化趋势,各地质时期长江三角洲和黄土高原不同沉积剖面沉积物有不同的粘土矿物组成。中新世以来两地逐渐增加的伊利石含量反映逐渐变冷的气候。伊利石/高岭石、伊利石/(蒙脱石+高岭石)可作为气候环境暖湿-干冷演化矿物学标志。中新世以来长汀三角洲、黄河高原由相对暖湿环境逐渐演变为相对干冷环境。更多还原

【Abstract】 The Yangtze River originates from the Tibetan Plateau and is the longest river in Asia. As the product of both tectonics and climate, the evolution history of the Yangtze River has been of great interest to many geologists and geomorphologists. Much work has been carried out, but the history of Yangtze River is still a mysterious problem owing to the complex of the river.In this thesis, the author focused on the provenance change of sediments near Nanjing in the lower Yangtze valley using combined methods of detrital zircon U-Pb ages and heavy minerals in order to discover some clues for the evolution of the Yangtze River. In order to investigate the palaeoclimate, the proxy of grain-size, susceptibility and the value of Rb/Sr was applied to study the sediments near Nanjing, and the X-ray diffraction analysis (XRD) was applied to study the clay minerals in the sediments in the Yangtze Delta and the red clay-loess sequences in the Chinese Loess Plateau (CLP)1) Late Cenozoic fluvial sediments are widely distributed near Nanjing. Most of them are overlain by basalts, from which preventing the underlain to be eroded. Lingyanshan section (LYS)、Xiaopanshan section (XPS)、Tanggongshan section (TGS)、Guizishan section (GZS)、Jiangpu sections (JP) and Jiangning Fangshan section (FS) around Nanjing are all covered by basalts with the age of 12.2-8.6 Ma. Because of the terrain and geographical location of JP and FS, we think that the sediments in JP and FS were not formed from the ancient Yangtze River. According to the typical sedimentary strata and sedimentary facies of large river, we believe that the Miocene fluvial sediments of the XPS、GZS and TGS should be the product of the ancient Yangtze River at that time.2) In order to uncover the characteristics and provenance of these sediments, heavy mineral composition and detrital zircon U-Pb ages were measured. Compared with the modern deposits of the Yangtze River, we found the detrital zircon U-Pb age patterns, especially XPS、GZS and TGS, are similar to the modern Yangtze sediments, with slightly difference.The content of heavy minerals in the Miocene fluvial sediments was lower than the modern sediments of the Yangtze River. At the same time no hornblende and pyroxene had been found in the Miocene fluvial sediments, which is contrary to the modern sediments of the Yangtze River;As hornblende, pyroxene and other heavy minerals are unstable and easy to be reconstructed by post-diagenesis and weathering, it is hard to justify the difference of provenance between ancient and modern Yangtze River. As the resistance of detrital zircon in the process of physical and chemical weathering, its U/Pb ages are a common tool in provenance studies for fluvial sediments. The patterns of detrital zircon U-Pb age of XPS、GZS and TGS are similar to the modern sediment, which indicated that the Three Gorge must have been cut through and sediments originated from the upper Yangtze Block could be carried to the Yangtze delta through the Three Gorges.3) On the basis of field investigation near Nanjing, the section of Lingyanshan (LYS) Hill was discovered and the loose sediment layer was covered with basalt layers of the age of 10.32Ma. Field investigation and sampling were carried out on the whole sedimentary section. Under the basalt layer were loose sediments. A general analysis about the loose-like sediments was carried on the proxies of grain size, magnetic susceptibility and the value of Rb/Sr.According to the sedimentary characteristics of the loose sediments and laboratory analysis results, the loose sediment was mainly composed of silt and clay, and no level bedding in the upper part of loose sediment. The 1.1m thick deposits in the section of LYS Hill were regarded as the Aeolian sediments of the age more than 10.32Ma, which was the earliest Aeolian deposits in east China. The deposit was corresponding with the 15-13Ma strong deposition of Acolian sediments in the Qin’an section in Loess Plateau.4) The X-ray diffraction analysis (XRD) was applied to study the clay minerals in the Yangtze Delta and the red clay-loess sequences in the Chinese Loess Plateau (CLP). The results show that the clay mineral compositions of the Yangtze and Yellow River drainages are mainly consisted of illite, chlorite, kaolinite and smectite. During each geological time, the sediments of different deposition profiles along the Yangtze River and Yellow River have different combinations of clay minerals. In each continuous deposition profile, there exists a phenomenon that illite trends to gradually increase which reflects the gradual cooling of climate. The ratios of illite to kaolinite (Ⅲ/Kao) and illite to smectite plus kaolinite(Ⅲ/(Sm+Kao)) may serve as a proxy to evaluate the paleoclimate.更多还原