Analsis Of Heavy Mineral Composition And Influencing Factors Of Songhua River Sediment

The Songhua River is the most important fluvial system in northeast China and its heavy mineral composition is of great significance to the revealing of depositional environment,source-sink relationship and drainage evolution.However,the current study concerning the heavy mineral composition of the Songhua River system is very limited,and the factors affecting the composition of heavy minerals are not clear up to date.In order to evaluate the influences of sediment sources,fluvial processes and chemical weathering on the heavy mineral composition of the Songhua River system are discussed,as well as the application of artificial and TIMA methods in the identification of heavy minerals.So the sand samples are collected along the banks of the main stream and 10 major tributaries of the river system,in addition to the two from the T3 terraces of the Tonghe River.Using the new automated mineral identification technology TESCAN Integrated Minera Analyzer(TIMA)and the manual method respectively,heavy mineral analysis of the sample components in depth of 63～250 m is carried out for the samples.The results show that:(1)A total of 24 kinds of heavy minerals have been identified by artificial methods,including amphibole,epidote,sphene,ilmenite,hematite+limonite,pyroxene,zircon,apatite,rutile,anatase,leucoxene,tourmaline,magnetite,kyanite,monazite,pyrite,chalcopyrite,moissanite,spinel,chromium spinel,natural-zinc and cruciform,etc;A total of 39 heavy minerals have been identified by the TIMA method,and the detection results are very similar to those of the artificial ones.In addition to the absence of anatase and hematite+limonite,other occasional minerals include monazite,crossite,silicite,auburn,corundum,calcite,olivine,bastnaesite,iron spinel,aegirine,wazirite and serpentine.(2)The comparison between manual identification and TIMA technology shows that manual identification can obtain the surface characteristics of minerals more accurately,but it is time-consuming and laborious,and depends on the subjective judgment of the appraiser,and the identification of mineral facies is limited;TIMA technology has the characteristics of fast speed,automation and high resolution,and the identification of mineral types is more diversified,but there are deficiencies in identifying homogeneous,heterogeneous and polycrystalline minerals.Therefore,the combination of the two technical means can make the obtained information more comprehensive.(3)The composition of heavy minerals formed by tributaries in different source areas is different,indicating that the composition of heavy minerals in river sediments is controlled by the type of source rock.Amphibolite,sphene,epidote,dolomite,pyroxene,ilmenite and hematite+limonite are the characteristic minerals that distinguish the tributaries.The high content of pyroxene in the Nuomin River,the tributary of Nenjiang River,the high content of ilmenite in the Ganhe River,Dobukur River and Arun River,and the high content of hematite/limonite in the Ganhe River,Dobukur River and Yalu River reveal the control of local basic parent rocks on the composition of heavy minerals.(4)The composition characteristics of heavy minerals in the Great Xing’an Mountains and the Small Xing’an Mountains,such as the significantly enriched ilmenite(Gan River,Dobukur River and Allen River),hematite/limonite(Gan River,Dobukur River and Yalu River)and pyroxene(Nomin River)in the Great Xing’an Mountains,as well as the high sphene content in the Small Xing’an Mountains,have become quite vague in the main stream of the Songhua River.The composition of heavy minerals in the upper reaches of the Songhua River shows that it inherits the heavy mineral information of the Changbai Mountains.The above phenomenon shows that the control of heavy mineral composition by provenance is limited by river process,which makes the information of the parent rock in the source area of the Great Khingan Mountains significantly weakened in the main stream of the Songhua River.(5)Compared with the modern river sediments,the unstable rock-forming minerals pyroxene and amphibole in the Tonghe T3 terrace have completely disappeared,apatite and sphene have been strongly dissolved,and stable minerals(ilmenite,zircon,garnet and perovskite)have been significantly enriched,indicating that the sediments have undergone intense chemical weathering,and the heavy mineral composition and parent rock information have been seriously affected.Therefore,when using heavy minerals to study the source-sink process,we should pay attention to the impact of chemical weathering on the terrace sediments.