Discussion on the Texture Features of Quartz Grains and Their Origin in the Xining Loess
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摘要: 利用电子扫描电镜分析了西宁黄土地层中的石英颗粒表面形态和机械结构特征,发现近1.2Ma以来西宁黄土石英颗粒形态以尖棱角—棱角为主,颗粒表面具有典型的剪切断口或大贝壳状断口或平行擦痕,且比例较高;有些颗粒表面还具有V形坑、水下磨光面,这类颗粒以次圆状为主,这些信息指示出,西宁黄土物质不是主要来源于沙漠环境,而是来源于青藏高原第四纪冰川和冰缘作用区.Abstract: It has been proved that quartz grains of different sources are characte rized with different surface textures and features,which has been successfully applied in d istinguishing the original source of Chinese loess.Seven groups of loess samples were retrieved from Dadunling loess section in Xining basin,Qinghai Province,No rhtwest China. Quartz grains were seperated by chemical processes and their surf ace texures and features were observed under SEM in this study to discover the o riginal source of loess on the Qinghai-Tibet plateau. It is found that the quartz grains in Xining loess since 1.2 Ma are mainly angul ar and subangular in features which are different form those of ordinary desert sands. Besides, these quartz grains display a distinct set of surface features including typical sheering fractures(with a frequency of 31%),big conchoidal br eakages(occurred on 50% of grains that are ＞1μm) and oriented striations(3%),w hich is generally considered to be the markers of glacial actions.At the same ti me,the sub-rounded quartz sands with typical V-shaped pits(12% in frequency)and polishe d surface(21%)under water environment are also found in the samples.All the abov e information indicates that the Xining loess material is not mainly originated from desert but from glacial and periglacial environment of the Tibetan plateau. It could be explained that a quantity of detrital material was formed by glacial grindings and freezing and melting processes when the Qinghai-Tibet Plateau rea ched an average altitude of 3 000 m 1.1Ma～0.7Ma ago,when most of the Plateau re ached the cryosphere.Under the Plateau winter monsoon or the westerly,the fine g rains were transported eastward and deposited in relatively lower basins such as Xining Basin.The quartz grains in Xining loess have documented not only the inf o rmation of global change but also the history of glacial actions on the Qinghai- Tibet Plateau.
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