中新世风尘堆积的地球化学特征及其与上新世和第四纪风尘堆积的比较

GEOCHEMICAL CHARACTERISTICS OF THE MIOCENE EOLIAN DEPOSITS AND COMPARISON WITH THE PLIOCENE AND QUATERNARY EOLIAN DEPOSITS

文章对秦安中新世黄土-古土壤序列中黄土层样品的常量、微量和稀土元素地球化学特征进行研究,并与西峰第四纪黄土和晚中新世-上新世的风成三趾马红土样品进行对比。结果表明,不同时代的风尘堆积具有大致相似的元素配分模式,并与上地壳平均成分高度接近,表明它们的物质均来自很广阔的物源区,经过多次循环和远距离搬运,并在搬运过程中实现了多种矿物成分高度混合,是远源风尘物质的典型特征。秦安中新世黄土与西峰第四纪黄土相比,常量元素组配特征也有一些区别,主要表现为K,Fe,Mg的轻微富集、Na元素的轻微亏损及相对较高的烧失量,与中新世黄土较细的粒度和相对温湿的气候条件有关。与世界上一些物源较近较小的黄土相比,我国北方不同时代的风尘堆积均具有Cs相对富集、Zr和H f相对亏损的特征,与粉尘远距离传输过程中的物质分异有关,是沙漠黄土的特点。

The Miocene loess-soil sequences in northern China represent the oldest loess deposits reported to date, and are of great values in the study of Cenozoic paleoclimate in Asia and the links with global climate changes. The eolian origin of these sequences has already been demonstrated by sedimentological, pedological and land-snail fossil evidence, and by the spatial stratigraphic correlation and magnetic susceptibility time series over long distances. These loess-soil sequences indicate that sizable deserts in Asian inlands and the Asian winter monsoon system were already formed by 22 million years ago while the alternations between loess and soil layers indicate cyclical climate changes within the orbital bands. Here, we examine the geochemical properties of the most complete QA-I sequence located in Qin＇an County covering the interval from 22.0MaB. P. to 6.2MaB. P. , with special emphasis given to the loess layers. Eighteen loess samples are selected from the QA-I for the analyses of major elemental concentrations, and six samples are analyzed for trace- and Rare Earth Element concentrations. In order to compare the geochemical properties, representative samples from the Quaternary loess and from the Late Miocene-Pliocene red clay at Xifeng are selected for major, trace and Rare Earth Element analyses. Our results show a high similarity of geochemical properties for these eolian deposits of different ages, and also a similarity to those of the upper continental crust （UCC）. Good agreements in both major and trace element compositions are observed between the Qin＇an and Xifeng eolian deposits. The BEE distribution patterns are also remarkably similar, with enriched LREE and fairly fiat HREE profiles, clear negative Eu anomaly, as are also the REE properties of the upper continental crust （UCC）. The negative Eu anomalies, expressed in the Eu/Eu^＊ ratios, vary from 0.6 to 0.7 for both Qin＇an and Xifeng samples. The Na2O/Al2O3, K2O/Al2O3 , U/Pb and Th/Pb ratios suggest that the Miocene eolian deposits must have been experienced a thorough sedimentary differentiation and a moderate chemical weathering in the source area. The La/ Th ratios, which are not fractionated in size-fractions relative to the whole-rock values, are very constant in all the eolian deposits and are nearly similar to those of the average UCC. These results indicate that the dust materials forming these eolian deposits were all derived from well-mixed sedimentary protoliths, which underwent numerous upper-crustal recycling processes, as are characteristic of eolian deposits. The Miocene loess displayed also some differences from the Quaternary loess, characterized by slightly higher K, Fe, Mg and LOI content, and by lower Na content. These are attributable to the finer grain size and stronger weathering of the Miocene loess. In comparison with the other small-scale loess formation of the world, one of the features for the eolian deposits in northern China is the higher Cs, lower Zr and Hf contents. This is attributable to the sorting processes during the distant dust transportation from the remote deserts to the loess depositional region.