末次冰期以来鄱阳湖东北缘下蜀黄土常量元素地球化学特征及其物源指示

Geochemical Characteristics of Loess from Northeast of Poyang Lake since Last Glacial and Its Provenance Implications

长江中游、鄱阳湖东北缘,末次冰期以来发育区域性风沙—风尘堆积体系。沿现代冬季风方向自北而南选取了10个剖面（通称D-D断面）采集下蜀黄土样品,并用湿筛法提取〈20μm粒级颗粒进行XRF元素分析,结果揭示：1自北而南,元素含量具有系统性变化特征,Sr O、Na2O、Ca O、K2O、Mn O、Ca O呈负对数函数关系,Si O2、Zr O2、Al2O3、Ti O2及化学蚀变指数（CIA）呈正对数变化;在距离长江河谷5~20 km的范围内,变化最为剧烈,随后趋于平稳。稳定元素—稳定元素、次稳定元素—稳定元素、活动元素—稳定元素对散点图同样具有系统的变化特点,显示D-D断面具有高度同源性。这些特征进一步揭示D-D断面属同一粉尘堆积体系。2现代长江河漫滩沉积物、渭南黄土、南京下蜀黄土与D-D断面上北端剖面稳定元素组成极其相似,与南端剖面则差异较大。这揭示了长江碎屑物质和风尘堆积一样具有广泛的来源及高度混合性,利用常量元素地球化学方法进行粉尘物源示踪需要谨慎。

The sandy hill sand and loess deposits constitute the regional sand-loess system,which are widely distributed in the middle reach of Yangtze River especially in the Poyang Lake area since the last interglacial. A transect named D-D transect was set from north to south along with the winter monsoon direction. The 〈20 mm grains extracted from bulk sample on the basis of wet-sieve methods were used to measure major and trace element abundances by XRF.Our results reveal that：（ 1） The content of major element has a systemic change from north to south. The changes are most pronounced within the range of 5 to 20 km away from the Yangtze River,and then level off. Sr O,Na2 O,Ca O,K2 O,Mn O,Ca O are of negative logarithmic function changes,Si O2,Zr O2,Al2O3,Ti O2 and CIA are of positive logarithmic function changes; The profiles of D-D transect seem to have a similar source displayed by the scatter diagrams of Zr O2/ Al2O3 vs. Zr O2/ Ti O2,Ti O2/ Al2O3 vs. Ti O2/ Zr O2,K2 O / Al2O3 vs. Zr O2/ Ti O2,K2 O / Ti O2 vs. Zr O2/ Al2O3,Sr O / Ti O2 vs. Sr O / Al2O3,Base /Zr O2 vs. Base / Ti O2,ect. These characteristics further reveals that all of the profiles in D-D transect belong to the same aeolian deposits system.（ 2） The stable elemental compositions of the D-D transect are similar to the WN loess,Xiashu loess,and sediment of flood land of the Yangtze River,these results reveal that aeolian deposits and the Yangtze River sediment have a rich source and over-commixing. Therefore,it is suggested that the factors on chemical composition of clastic sediment should be fully studied when geochemistry methods are used to analyze the province and depositional setting,and multiple methods can be synthetically used and the study on regional geology should be paid more attention.