西宁地区新近纪风尘堆积的元素组成特征及物源指示意义

ELEMENT COMPOSITIONS OF NEOGENE EOLIAN DEPOSITS IN XINING AREA AND THEIR IMPLICATION FOR PROVENANCE TRACING

西宁地区的新近纪风尘堆积是研究青藏高原东北部古环境变化和地貌演化的良好对象。本文基于X射线荧光（XRF）光谱分析方法，对西宁地区的新近纪风尘堆积、第四纪黄土、河流沉积、湖泊沉积代表性样品中小于75μm组分的元素含量进行了对比研究。结果显示：1）在常量元素平均含量上，西宁地区的新近纪风尘堆积与第四纪黄土大体相似，与西宁地区的河流、湖泊沉积也大体相似。与上地壳（UCC）平均成分相比，各类沉积物样品的CaO、MgO、MnO、Zr、Nb含量较高，Na2O、K2O含量较低。2）与西宁第四纪黄土、河流沉积和湖泊沉积物相比，西宁新近纪风尘堆积具有较高的Al2O3、Fe2O3、K2O、MgO、TiO2、Rb、Nb和较低的CaO、Sr、Zr。3）西宁地区的新近纪风尘堆积和第四纪黄土的Rb-Sr含量散点图可以拟合成一条直线，指示二者的母岩在物质成分方面可能具有相同的特征。然而河流、湖泊沉积的Rb-Sr含量散点图特别分散。西宁新近纪风尘堆积的Rb/Sr比值高于第四纪黄土的Rb/Sr比值，指示新近纪风尘堆积为较强风化成壤作用下的产物，支持以往新近纪风尘堆积在沉积前后经历了较强化学风化作用的研究结论。4） K2O/Al2O3（分子比）-TiO2/Al2O3（分子比）、K2O/Al2O3（分子比）-Zr/Nb图解显示西宁地区的新近纪风尘堆积与第四纪黄土一致，但是它们与西宁地区的河流、湖泊沉积显著不同。上述特征表明西宁地区的新近纪风尘堆积和第四纪黄土可能具有相似的源区。在西风环流盛行时，位于上风向地带的柴达木盆地可能是西宁地区风尘堆积的主要源区。

Xining area is located on the northeastern Qinghai-Tibetan Plateau. The eolian deposits in Xining area are composed mainly of the Quaternary loess and Neogene loess, which are of importance for studying the environmental change and geomorphological evolution of the northeastern Qinghai-Tibetan Plateau. The Neogene eolian deposit accumulated on the high terraces of the Huangshui River （a main tributary of the Yellow River）, and it lies between the fluvial sediments and Quaternary loess. Similar to the Quaternary loess and paleosol sequences, the Neogene eolian deposit in Xining displays clear loess and paleosol alternations. Previous paleomagnetic dating results and biochronology of micromammalian assemblages show that the depositional ages of the Neogene eolian deposit in Xining area are around 7.4Ma to 13.5Ma ago. The provenance of eolian deposit is closely linked to changes in source aridity and dust transport dynamics. However, the source area of the Neogene eolian deposit in Xining area is still unknown, hindering the research progress on arid zone reconstruction and paleomonsoon variation by investigating the Neogene eolian deposit. In the Xining area, 9 Neogene loess samples, 24 Quaternary loess samples, 8 fluvial sediments and 13 lacustrine sediments are collected. Fluvial sediments are sampled from terraces of the Huangshui River （e.g. the highest terrace T11 and the lowest terrace T1）, and also from the modern floodplain of the Huangshui River. According to previous studies, the age of the lacustrine sediments should belong to Late Eocene to Early Miocene. Based on the X-ray fluorescence （XRF）analysis method, the element compositions of less than 75μm fractions in representative samples of Neogene and Quaternary eolian deposits, fluvial and lacustrine sediments in Xining area have been determined. The elemental comparison results for all studied samples show that：（1）The contents of major elements in Xining Neogene eolian deposit are not only generally similar to those in Late Quaternary loess, but also are broadly similar to those in fluvial and lacustrine sediments in the Xining area. Compared to the average content of the upper continental crust （UCC）, the contents of CaO, MgO, MnO, Zr and Nb in the studied sediments are higher, Na2O and K2O contents are lower. （2）Compared to other studied sediments, Xining Neogene eolian deposit has higher Al2O3, Fe2O3, K2O, MgO, TiO2, Rb and Nb content, and lower CaO, Sr and Zr content. （3）The scatter plot of Rb and Sr contents of eolian deposits in Xining area can be fitted into a straight line, indicating that their source rocks may have common characteristics in composition. While the scatter plot of Rb and Sr contents in the fluvial and lacustrine sediments are very dispersed. The Rb/Sr ratios of Xining Neogene eolian deposit are higher than those of Xining Quaternary loess, indicating the Neogene eolian deposit is a production formed by relatively strong pedogenic weathering process, consistent with the previous conclusion by investigating the Qin＇an Miocene loess deposit in Tianshui area, Gansu Province. （4）Plots of K2O/Al2O3 （molecular ratio）-TiO2/Al2O3 （molecular ratio）, and K2O/Al2O3 （molecular ratio）-Zr/Nb show that the Neogene eolian deposits are similar to the Quaternary loess, but they are obviously different from the fluvial and lacustrine sediments in Xining area. The above characteristics show that the Neogene eolian deposit and Quaternary loess in Xining area are from remote areas by wind transportation, and they perhaps have similar source areas. Due to Qaidam Basin is located in the upwind area of the Xining area when dominated by the Westerly circulation, it may be the main source area of the eolian deposits in Xining area.