Heavy Mineral Assemblages And Provenance Analysis Of Eolian Sand In The Alashan Desert, Northwestern China

Deserts in northern China are located in the mid-latitude arid region and play an important role in shaping terrestrial hypergene processes.Furthermore,these deserts are the main source of dust in eastern Asia and even throughout the Northern Hemisphere.Therefore,accurate identification of the provenance of eolian sand in deserts is essential to studying sedimentation processes in the earth system.Eolian sediment is the material basis for the formation and development of aeolian geomorphology.Its characteristics record significant information in the formation and development of the desert.Thus,determination of the sedimentary environment and material provenances of the desert have great significance for desertification control and ecological construction.The Alashan Desert is an important part of the deserts of northern China with strong sandstorm activity.It is the representative of the typical arid desert with various landforms inside the desert and the sedimentary environment and sediment types is complex and varied.In addition,the Alashan Desert is surrounded by bedrocks,wilderness,fluvial sediment and lacustrine sediment and affected by both fluvial and eolian effects,which leads to quite complex provenances of the Alashan Desert and no unified understanding.Studies of the provenance and transportation processes of eolian sand can not only provide further understanding for the regional and global land desertification and the formation of aolian geomorphology,but also have important guiding significance for land desertification control.And it can provide new evidence for the interaction process and mechanism of various earth surface layers.Hence,in this study,we selected the Alashan Desert in northern China as the study area,and a total of 433 samples were collected throughout the desert and nearby regions to study the provenance of the eolian sand based on their heavy mineral assemblages and contents and combined with wind direction datum and regional geological background.The results show as follows:(1)The heavy mineral assemblages of eolian sand in the Alashan Desert is dominated by magnetite,hematite,garnet,amphibole,zircon,tourmaline,rutile,ilmenite,pyroxene,epidote,apatite,titanite and monazite.However,different deserts and different locations of the same desert have different relative content of heavy minerals.The relative content of stable heavy minerals gradually increases from the periphery of the desert to the hinterland on the whole,and the unstable heavy minerals have opposite trends,but the spatial changes of different deserts are different.Additionally,the heavy mineral assemblages of eolian sand is very similar to that in the bedrock,wildness and the fluvial-lacustrine sediments around the desert.(2)ZTR index and stability coefficient have different changes in different deserts.the spatial variability of the ZTR index and the stability coefficient of the eolian sand in the Badain Jaran Desert and the Tengger Desert are relatively small.That gradually increases from the northwest to the southeast of the desert on the whole.However,the stability coefficient tends to decrease in the areas near the northern Gobi and the bedrocks in the southwestern and the southern desert.The spatial variability of the stability coefficient and ZTR index in the Tengger Desert show a gradually increasing trend from the northwest to the southeast of the desert.And the low value appears in the areas closed to the Helan Mountains and the wildness in the southern of the desert.The ZTR index and stability coefficient of eolian sand in the Ulan Buh Desert gradually increased from the periphery of the desert to the desert hinterland,but the spatial variability in different regions was different.(3)The GZi index of eolian sand in the Alashan Desert are all very high.The GZi index have not changed much in the Badain Jara Desert and the Tengger Desert,while the GZi index in the Ulan Buh Desert have gradually decreasing trend from the northwest and west to the east of the desert with local high values.The ATi index of the eolian sand in the three major deserts is large at the edge of the desert and small in the desert hinterland,indicating that the weathering degree of the eolian sand at the edge of the desert is lower and the desert hinterland is higher.Through systematic and detailed analyses of the above results,the following conclusions are reached:(1)The rich and conplex heavy mineral assemblage of the Alashan Desert indicates that the provenance of eolian sand was not sourced from a single source rock.The sources of the eolian sand in the Badain Jaran Desert mainly include bedrock,gobi,fluvial-lacustrine sediments in the west and northwest of the desert,and the contribution of bedrock in the southeast was less.The eolian sand of the Tengger Desert mainly comes from bedrock and gobi along the west-north-northeast route,the quicksand of the Badain Jaran Desert and the paleolake sediments inside the desert,while the contribution of the gobi in the south of the desert was little for the eolian sand.The main provenances of eolian sand in the Ulan Buh Desert include the bedrock in the northwest of the desert,gobi to the west of the desert,and the exposed bedrock and paleolacustrine deposits in the desert hinterlands.The fluvial deposits in the Yellow River near the desert and bedrock in the Helan Mountains contributed little to the eolian sand in the Ulan Buh Desert.(2)It is found that the bedrocks,fluvial and lacustrine sediments around the desert,the wildness surrounding the desert,and the paleolacustrine sediments inside the desert were important provenances for eolian sand through heavy mineral analysis.But the above provenances are distributed in different locations in the desert,making every provenance have different contributions to eolian sand.(3)The characteristic index distribution of eolian sand in the Alashan Desert shows that the eolian sand in three desert were transported shortly,and the provenance was mainly near-source.In addition,the provenances of eolian sand were not only the gobi and bedrock around the desert,but also has common source where has rich-garnet bedrock.