GEOCHEMICAL CHARACTERISTICS OF EOLIAN DEPOSITS IN THE NORTHEASTERN MARGIN OF THE TIBETAN PLATEAU AND IMPLICATIONS FOR PROVENANCE AND WEATHERING INTENSITY
The Longxi basin of the Chinese Loess Plateau (CLP) is located in the northeastern margin of the Tibetan Plateau (TP),which is also in the desert-loess boundary zone of Northern China (NC). The climate in this region is controlled by the East Asian Monsoon,the Westerly Circulation,the TP monsoon and the India southwest monsoon, so the environmental response to climatic change is very sensitive in this area. The eolian deposits widely distributed in this region bear information about climate change for the arid or semi-arid zone of NC,and it is also an ideal material for studies on climatic response to uplift of the TP. In this study, geochemical compositions of an eolian sequence in the northeastern margin of the TP are studied and compared with loess and paleosol samples from the Longdong basin of the CLP. The main objectives are: (1) to characterize the elemental geochemistry of the eolian deposits in the northeastern margin of the TP and compare with the loess-paleosol sequence in the Longdong basin of the CLP; (2) to interpret the geological implications for dust sources; (3) to compare the chemicalweathering characteristics of the eolian deposits in the Longxi and Longdong Basins. The studied section is from Huining (HN) County of Gansu Province. The present-day climate at HN County is temperate with mean annual precipitation of 330mm and mean annual temperature of 7.9℃,respectively. The HN section (36°15 'N, 105°07'E) is 224.25m in thickness. In this study, only the upper part of this sequence (since L_9 which is about 125m in depth) is studied, 15 samples were taken for major and trace elemental measurements, including 8 loess and 7 paleosol samples. 2 loess samples are from L_1,and the other 6 loess samples are from L_2, L_3,L_4,L_6,L_8, and L_9; the 7 paleosol samples are from S_1, S_2,S_3,S_4,S_5,S_7,and S_8. All the samples were measured at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences. The geochemical data of the Longdong basin loess are from Lingtai section (34°59'N, 107°45'E) in Lingtai County, Gansu Province. The results show that geochemical characteristics of the eolian deposits from the Longxi and Longdong basins are similar to the average UCC on the whole, indicating that the dust materials have experienced numerous upper crustal recycling processes. However, they also show obvious differences, reflecting different chemical weathering intensity and different eolian dust sources. The loess in the Longxi basin have higher CaO, MgO,Na_2O and lower Fe_2O_3 concentrations in comparisonwith the Longdong loess samples, indicating lower weathering intensity of the Longxi loess. According to the CIA value, the Longxi loess was in the stage of incipient chemical weathering, while the Longdong loess was in the stage of intermediate chemical weathering intensity. For the relatively invariant elements, compared with the Longdong loess, the Longxi loess samples have higherZr, Hf and lower Ta, Y value, higher SiO_2/Al_2O_3,SiO_2/TiO_2,TiO_2/Al_2O_3, U/Pb and lower Rb/Sr, Ba/Sr, Ce/Yb, Eu/Yb,LREE/HREE ratios, reflecting different eolian dust sources. Due to the influence of geographical location,the desert in NC,the glacial and periglacial sediments in the TP and the Quaternary sediments in the adjacent area is likely to become the potential sources of the Longxi loess. Further studies show obvious changes in geochemical characteristics of the HN loess since about 300ka B.P. Our geochemical data, combined with previous results of magnetic fabric and quartz grain surface morphology,further prove that the sources of the eolian deposits distributed in the northeastern margin of TP maybe obviously changed during this period. Eolian dust contribution of glacier and periglacial sediments from the TP significantly increased after 300ka B.P. This change may be related to the increased plateau monsoon caused by the rapid uplift of TP in this period.