Magnetic And Geochemistry Characteristics Of Loess Deposits At Northern Foot Of Tianshan Mountains

Loess played a significant role in the progress of rebuilding the paleoclimate. The Tianshan Mountains which located in non-monsoon region,is one of important distribution areas of eolian loess in the Xinjiang region. The study of Xinjiang Tianshan Loess has an important significance not only in understanding the characteristics of the local geographic environment but also in exploring ancient climate change of the arid areas and modern dust storm and global loess characteristics.We carried out systemic rock-magnetic and Geochemistry investigations on the loess/paleosol sequence in the Zhongliang section, which is representative loess sequences northern foot of Tianshan Mountains. Grain size index and the X-ray diffraction were also used in research. The main indicators were obtained through the analysis as follows:(1)Results show that the main magnetic minerals of this sequence are magnetite, maghaemite, haematite, and the main magnetic grain size of magnetic minerals are PSD. The magnetic susceptibility of the loess layer in the Tianshan Mountains is higher than paleosoil,which is different from the loess in Loess Plateau. Magnetic susceptibility of the sediments is controlled by native ferrimagnetic minerals, and the change of magnetic susceptibility is mainly affected by wind speed.(2)Compared with constant element of the UCC, the results showed that the content of Al2O3is closer to UCC; SiO2,MgO,TiO2and Fe2O3enriched; Na2O and K2O showed relatively weak loss to UCC.The chemical index of alteration(CIA) distributed between45to55,indicating that the loess was in the early stages of chemical weathering. Al-Ti-Zr ternary diagram shows that the distribution Al-Ti-Zr ratios of S0, L1and S1are closer to the upper continental crust and PAAS, which means that the sediment has experienced a long distance transport and mixed process.(3)The grain sizes of loess is composed mainly of silt and the grainsize percentage frequency curve has a typical bimodal distribution characteristics. X-ray diffraction analysis of typical samples shows that the loess sediments are mainly composed of quartz, feldspar, mica and calcite and dolomite minerals. No significant magnetite, maghemite and hematite were found from diffraction spectrum of the samples, which means their contents are very limited.(4)The proxy of Rb/Sr ratio and (CaO+Na2O+MgO)/TiO2ratio which are able to reflect summer monsoon and the weathering intensity of the loess in the Loess Plateau have a poor corresponding relation with the strata in the Tianshan Mountains region. As a magnetic parameter, χARM/χlf can response the strength of pedogenesis and paleoclimate more accurately. Compared with the magnetic parameters, there is a good correspondence between the chemical index of alteration CIA and the strata. The chemical index of alteration CIA can reflect the weathering intensity and the strength of pedogenesis of the loess,which exempt from impact of differences of provenance and depositional environment.