Study On Provenance Of Quaternary Loess In Longxi Area

The Longxi loess is an ideal geological material for studying the evolution of paleoclimate and environment in arid and semi-arid areas of China,but there have been great differences on the understanding of loess provenance in this area.Some scholars believe that the arid region deserts of northern China are the direct source of loess under the action of the East Asian winter monsoon,while others support that the Tibetan Plateau is the main original source area for aeolian deposits.In this thesis,the Huining profile of the Longxi area is chosen as the research object.Based on the regional comparison,the accurate time scale of this profile is established by means of paleomagnetism and ESR dating.Then,through the analysis of the characteristics of the heavy mineral assemblage and the U-Pb dating of detrital zircon grains,the source area and the changing history of Quaternary loess in this area are discussed,and the main insights gained are as follows:(1)By means of paleomagnetism and ESR dating,a reliable time scale of Huining profile is established.The result is validated by comparing the grain-size and magnetic susceptibility size of the existing profile.The results show that The Matuyama-Brunhes boundary(MBB)is located in the middle of L8(～104 m).The upper boundary of Jaramillo positive polarity subchron is located in the middle of L10(～125.5 m)and the lower boundary is at the top of L12(～133 m).The upper boundary of Olduvai positive polarity subchron is at S24/L25 boundary(～204 m)and the lower boundary is at the upper part of L27(～214 m).According to the sedimentation rate,the bottom age of Huining profile is ～2.13 Ma.(2)There are several heavy mineral assemblages in the Quaternary loess in Huining that can be indicative of the source region.The zircon + apatite + rutile assemblage may represent a moderately acidic volcanic source,the tourmaline +monazite + pyroxene assemblage may stand for a basal metamorphic source,the ilmenite + anatase + pyroxene + hornblende assemblage may signalize a basal volcanic source,and the chlorite + garnet + blue amphibole assemblage may symbolize a regional metamorphic origin.Compared with the rocks in the source area,it is inferred that the Tibetan Plateau is the main source area and the Gobi-Altay region is the secondary source area.The percentage content and characteristic index of different heavy minerals are different from other periods at 1.24-0.45 Ma,and these differences may be related to the changes in source contribution of the two major source regions.(3)The composition of the U-Pb age spectra of single-grain detrital zircons in the Quaternary loess of Huining is generally consistent,mainly including two main peaks in the 350-230 Ma and 550-350 Ma age groups,and several minor peaks in the 1300-550 Ma and 2800-1300 Ma age groups.Comparison with the single-grain zircon U-Pb age spectra of the two main source areas indicates that the dominant source area for the eolian soils in the Longxi area is the Tibetan Plateau for most of the Quaternary,with only a limited period at 1.5-1.4 Ma mainly originating from the Gobi-Altay region.(4)The Tibetan Plateau was the dominant source area of loess in Longxi in Quaternary,which indicates that this area received more near-source materials.During the period of 1.5-1.4 Ma,the Gobi-Altay region became the dominant source region,which is probably related to the end of the "C curtain of Tibetan Plateau" and the relatively increased contribution of winter monsoon.The limited increase of contribution of Gobi-Altay region at 0.89 Ma may be related to the intermission of "Kun-Huang Movement",while the limited increase at 0.45-0.27 Ma and 0.04 Ma may be influenced by the enhancement of East Asian winter monsoon and the intensification of drought in the source area.