为研究长江沉积物物质来源,对长江流域24个现代沉积物样品进行碎屑锆石U-Pb年龄测试,并对10个样品做Hf同位素分析,结果表明:长江河流沉积物的碎屑锆石多为岩浆成因.碎屑锆石U-Pb年龄主要有6组峰:2.4 ～2.6Ga,1.8 ～2.0Ga,700 ～ 1000Ma,400 ～ 550Ma,200～300Ma和＜65Ma,其中200～300Ma和700 ～1000Ma为主要的两组峰.锆石U-Pb年龄谱系可区分出长江4个区段,即金沙江段、川江段、长江中游段和下游段,与地理分段相吻合.结合Hf同位素研究,可认为长江流域沉积物中生代-新生代的锆石主要来自松潘-甘孜褶皱带和秦岭造山带;古生代的锆石多来自秦岭造山带;元古代-太古代的锆石则多来自扬子克拉通、华夏地块和大别造山带.%A total of 24 sand samples were collected in the mainstream and major tributaries of the Yangtze River drainage basin.The sample sites include the whole drainage,from the source "Tuotuohe" to the river mouth.Three samples were taken at the confluence of the mainstream and tributaries,avoiding potential contaminates.24 samples were dated by the LA-ICP-MS U-Pb technique and 10 samples were analyzed for Hf isotopes on the same spots of concordant grains.Our results show that most zircons are from igneous rocks,so the ages of these zircons could represent the time of their crystallization.The U-Pb age specturm define six age groups in mainstream of the Jinshajiang:2.4 ～2.6Ga,1.8 ～ 2.0Ga,700 ～ 1000Ma,400 ～ 550Ma,200 ～ 300Ma and ＜65Ma (Fig.7A ～ 7D),and two in the tributaries:700 ～ 900Ma and 100 ～ 300Ma(Fig.8A ～ 8D).There are four age groups in the Chuanjiang:2.4 ～ 2.6Ga,1.8 ～ 2.0Ga,700 ～ 1000Ma and 200 ～ 300Ma (Fig.7E ～ 7G,Fig.8E ～ 8F).Among which,2.4 ～ 2.6Ga are enrichment in the Jialingjiang and Wujiang.Five age groups display in the mainstream of the Midstream:2.4 ～2.6Ga,1.8 ～ 2.0Ga,700 ～ 1000Ma,400 ～ 500Ma and 200 ～ 300Ma (Fig.7H ～ 7K),whereas ＜ 1Ga age population in the tributaries:700～900Ma,400 ～ 500Ma and 200 ～ 300Ma(Fig.8G ～8I).400 ～500Ma age range is the main group in the Midstream.The Downstream is a little similar with the Jinshajiang,and exist ca.1％ age younger than 65Ma,but 2.4 ～2.6Ga age group is minor(Fig.7L ～7N).According to the zircon U-Pb age,the Yangtze River can be divided into four segments,which is consistent withthe geography division.Combined with the Hf isotope,it can be considered that zircons aged Mesozoic-Cenozoicwere mainly from the Songpan-Garze terrane,and zircons dating Paleozoic were from Qinling orogenic belt.TheYangtze Craton,Cathaysia Block and Dabie orogenic belt are the main source of Archean-Proterozoic zircons.
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