西宁黄土碎屑锆石年龄特征及其 对黄土高原黄土物源的指示意义

物源研究是理解黄土高原风成沉积物中古环境记录的关键一环。众多证据表明,黄土高原西北部的阿拉善高原及其周边广袤干旱地区是黄土高原黄土的主要风尘源区。但是,基于锆石U-Pb同位素地质年代学的证据对此提出了质疑。黄土高原黄土与青藏高原北部物质中的锆石具有相似的年龄分布,预示黄土高原风尘主要以西风搬运的方式来自青藏高原北部,特别是柴达木盆地。本研究通过分析西宁黄土的锆石年龄特征验证此假说。西宁处于在从青藏高原北部到黄土高原的假想搬运路线上。西宁黄土主要来自青藏高原北部的干旱区。西宁黄土中锆石的主要年龄分布与黄土高原黄土相似,反映青藏高原北部确实为黄土高原黄土的主要物质来源。但是,与黄土高原黄土相比,西宁黄土缺少年龄在360Ma左右和年龄小于100Ma的锆石。这两组年龄的锆石可能来自其他地区,例如处于中亚造山带。由于阿拉善干旱区也通过河流搬运接收来自青藏高原北部的物质,锆石 U-Pb年龄证据并不能排除阿拉善干旱区作为主要风尘物源的可能。相反,阿拉善干旱区还接受来自其北部中亚造山带和其本身华北克拉通的物质,来自阿拉善干旱区的风尘可能更好地解释黄土高原黄土中的锆石年龄特征。

ZIRCON AGES OF XINING LOESS: IMPLICATION FOR THE PROVENANCE OF THE LOESS ON CHINESE LOESS PLATEAU

Deciphering the provenance of atmospheric dust is critical to understand the paleo-proxies archived in the eolian deposits on Chinese Loess Plateau.While most of evidences indicate major dust sources from the arid lands to the north and northwest of Chinese Loess Plateau,a recent research based on U-Pb geochronology of zircon renewed the hypothesis that the dust transported by westerly winds from north Tibetan Plateau and the Qaidam Basin might be the dominant eolian sources to Chinese Loess Plateau during glacial times.Here we test this hypothesis by analyzing ages of zircon in Xining loess.Xining(36.65°N,101.74°E) is on the hypothetical pass way of dust transportation from north Tibetan Plateau to Chinese Loess Plateau.Compared with the loess on Chinese Loess Plateau,loess in Xining lacks zircon population with age around 360Ma and <100Ma.In contrast,the materials eroded from Central Asian Orogenic Belt have large population of zircon aged ca.360Ma and <100Ma.Although the major peaks of the zircon age distribution of Xining loess are very similar to that of the loess on CLP,the materials from north Tibetan Plateau could also be transported northwardly to the Alxa arid lands on the northwest of CLP.We thus conclude that loess on CLP is mainly sourced from the Alxa arid lands where materials are received from both north Tibetan Plateau(Qilian Mountains)in the south and the Central Asian Orogenic Belt(Gobi Altay Mountains)in the north.