【作者】 孙博亚；

【导师】 张云翔；

【作者基本信息】 西北大学 ， 第四纪地质学， 2011， 硕士

【摘要】 中国黄土因分布广泛,沉积连续,携带环境信息丰富,与深海沉积物,极地冰芯并称为全球变化研究的三大支柱。洛川剖面作为黄土高原中部的标准剖面之一,地层序列完整,出露良好,长期以来受到国内外学者的广泛关注,并进行了大量的研究。锆石作为第四纪风成黄土沉积中普遍存在的一种重矿物,其物理化学性质稳定,封闭温度高,抗干扰能力强,沉积后不易受到风化和成壤作用的影响,可进行U-Th-Pb同位素体系定年,是目前定年的首选矿物,并已运用到沉积物物源示踪研究中。本文试图利用洛川黄土-古土壤剖面中11个层位以及下伏红粘土中的碎屑锆石的表形特征,粒度特征,U-Pb年龄组合特征,结合中国西北主要造山带出露岩体的锆石U-Pb年龄组合特征,探讨洛川黄土-古土壤的搬运动力和物源的变化。得到以下几点认识：(1)第四纪黄土-古土壤及新近纪红粘土中含有大量碎屑锆石,其表面和内部结构特征显示出洛川剖面中的碎屑锆石是经过长期的风力搬运形成的,为多个物源区混合的产物。(2)通过对洛川剖面碎屑锆石的粒度研究表明,其主要粒级分布于20-60μm之间,细颗粒百分含量(<20μm)和粗颗粒含量(>60μm)较少。锆石中值粒径在各个层位变化较小,全岩则变化较大,尤其是在黄土层和古土壤层全岩粒度有较大差异,原因可能是由于成壤作用对全岩粒度影响较大,对锆石粒度影响较小。L24层黄土碎屑锆石粒径的粗颗粒含量、中值粒径、平均粒径较其他层位明显增大,可能是由于青藏高原的阶段性隆升；但是新近纪红粘土中粗颗粒含量的增加,可能是由于第四纪黄土与新近纪红粘土物源区的差异。(3)通过对洛川黄土-古土壤中的8个样品和下伏红粘土中的1个样品的碎屑锆石U-Pb年龄分析表明,洛川碎屑锆石的年龄峰值为250-450Ma,750-900Ma,1800-1900Ma和2500Ma。结合第四纪以来的古风向和中国西北主要造山带锆石年龄进行分析对比,认为第四纪黄土-古土壤沉积的物源区在稳定中有小幅度变化,其物源区可能分布在中亚造山带,内蒙古北部造山带,祁连造山带和青藏高原东缘造山带；新近纪红粘土和黄土的沉积显示出一定的连续性,其物源区可能包括内蒙古北部造山带,祁连造山带和青藏高原东部造山带。更多还原

【Abstract】 Luochua loess-paleosol sequence that lies on the Luochuan tableland, the main body of the loess plateau, is known as the direct product of the climate change in the Quaternary, the study of which on sedimentology, geochemical and paleomagnetic is the effectual approach to research the climate change of the quaternary. As common existing mineral in the eolian loess of quaternary, zircon’s physical chemistry characteristics is stable, it has high closure temperature and strong ability of anti-jamming, it cannot be easily effected by airslake and pedogenesis once deposited, it is the most preferable mineral used to do the U-TH-Pb isotopic dating, and has been used into the REE tracer research. This article is trying to research the provenance evolution of Luochuan loess-paleosol and its climate change of the time, using the phenotype, Granularity, U-Pb ages integrated of detrital zircon in the red clay in the 11th and lower layers of Luochuanloess-paleosol with the exposed rock U-Pb zircon age combination in the Orogenic belt in northwest China.Understandings are obtained by following:a). Quaternary loess-paleosol and the Neogene red clay contains large amounts of detrital zircons, which has greater hardness.chemical stability, Its surface eolian transport character-istics and CL images shows that the zirons in Luochuan is formed by long-term eolian transpirt and mixture source.b).Luochuan detrital zircons has low percentage of<20μm fine particles,mainly distri-buted in the grain size between 20-60μm,there is larger size difference with the whole-rock, the reason of which may be due topedogenes is a greater impact on the whole rock particle size, rather than the size of zircon, In the detrital zircon of L24 loess, the particle content of coarse particles larger than 60μm, the median diameter, average particle size significantly increased comparing to other layers, which may be due to the phased uplift of the Tibetan Plateau; Neogene red clay increased in large size zircon content, it should be relevant to Exposed basement rocks in near source.c). Through the analysis on detrital zircon ages in 8 samples of loess-paleosol show that the peak age of zircons from Luochuan loess are in four time periods 250-450Ma, 750-900Ma,1800-1900Ma and 2500Ma. With the comparison analysis between Quaternary paleo wind direction and the zircon age in the orogenic belt in northwest China, we can find that provenance of loess deposition in a stable minor changes, its provenance area may be located in the Central Asian Orogenic Belt, Mountains in Inner Mongolia, eastern margin of the Qilian Mountains and the Qinghai-Tibet Plateau, and the red clay and loess provenance area has most of theoverlap, the provenanc earea may be locatedin theteritory of Inner Mongolia Mountains,Qilian Mountains and the eastern edge of Qinghai-Tibet Plateau, the lack of the Central Asian Orogenic Belt Source.更多还原