【摘要】 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。更多还原

【Abstract】 The M iocene 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 sed imentological,pedological and land-snail fossil evidence,and by the spatial stratigraphic correlation and magnetic susceptibility time series over long d istances.These loess-soil sequences ind icate that sizable deserts in Asian inlands and the Asian winter monsoon system were already formed by 22 m illion years ago while the alternations between loess and soil layers ind icate cyclical climate changes within the orbital bands.Here,we exam ine the geochem ical properties of the most complete QA-I sequence located in Q in′an County covering the interval from 22.0MaB.P.to 6.2MaB.P.,with special emphasis given to the loess layers.E ighteen 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 E lement concentrations.In order to compare the geochem ical properties,representative samples from the Quaternary loess and from the LateM iocene-Pliocene red clay atX ifeng are selected formajor,trace and Rare Earth E lement analyses.Our results show a high sim ilarity of geochem ical properties for these eolian deposits of d ifferent ages,and also a sim ilarity to those of the upper continental crust(UCC).Good agreements in both major and trace element compositions are observed between the Q in′an and X ifeng eolian deposits.The REE d istribution patterns are also remarkably sim ilar,with enriched LREE and fairly flat 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 Q in′an and X ifeng samples.The Na2O /A l2O3,K2O /A l2O3,U /Pb and Th /Pb ratios suggest that the M iocene eolian deposits must have been experienced a thorough sed imentary d ifferentiation and a moderate chem icalweathering 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 sim ilar to those of the average UCC.These results ind icate that the dust materials form ing these eolian deposits were all derived from well-m ixed sed imentary protoliths,which underwent numerous upper-crustal recycling processes,as are characteristic of eolian deposits.The M iocene loess d isplayed also some d ifferences 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 M iocene 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 d istant dust transportation from the remote deserts to the loess depositional region.更多还原