【作者】 王节涛；

【导师】 李长安；

【作者基本信息】 中国地质大学 ， 第四纪地质学， 2009， 硕士

【摘要】 长江何时贯通东流入海这一难题已经困扰学术界长达一个世纪。长江所携带的碎屑物质并没有在四川盆地大量的沉积,江汉平原成为长江上游物质在流经三峡后最先沉积的地点,其中的沉积物记录了长江演化的重要信息。但至今仍无人对位于长江中游的江汉平原中的沉积物的物源区进行过系统的研究。本文试图利用江汉平原周老孔中碎屑锆石U-Pb年龄及其微量元素特征来推测江汉平原沉积物的物源进行判别,进而为长江贯通这一难题的最终解决提供一定参考。利用碎屑锆石进行物源示踪研究首先要掌握的是上游地区的构造热事件及一些重要岩体中的锆石年龄分布特征。长江主要流经羌塘-昌都地块、松潘-甘孜板块和扬子克拉通三个构造分区。由于长江在羌塘-昌都地块的流域面积小,且位于青藏高原高原面上,河流纵比降小,河流的侵蚀能力不强,所携带下来的少量沉积物可能很快就被“淹没”,而在年龄谱上没有任何体现。松潘-甘孜复理石带是世界上最大的三叠系复理石沉积区。松潘-甘孜复合岩体中的长英质火成岩侵入体的年龄集中在晚三叠纪(～214-211Ma),少量年龄集中在中三叠纪(220Ma)。义敦岛弧的年龄显示了中晚三叠纪的年龄,从225-215Ma,这一年龄早于松潘-甘孜发生大规模变形的时间。松潘-甘孜复合岩体中的碎屑锆石主要集中在四个主要的年龄峰值:2.4-2.5Ga和1.85-1.95Ga,认为来自于华北板块;第三个年龄峰值(400-450Ma)是北秦岭常见的岩体年龄。第四个峰值(ca.250-280 Ma)与已知的秦岭-大别造山带、华北板块以及华南板块的构造热事件年龄不符。源区可能为龙门山地区的瓜德鲁普统(Guadalupian)-乐平统(Lopingian)的流纹英安岩,时代为250-270 Ma,可能与松潘-甘孜盆地的裂解有关。扬子克拉通的演化历史较为复杂,太古代、元古代扬子克拉通经历了四次重要的岩浆事件720-910 Ma,1.90-2.05 Ga,2.40-2.55 Ga和2.60-2.70 Ga,还发现少量＞3.2 Ga的锆石。扬子克拉通内部的中生代火成岩可以分成三组:(1)扬子火成岩群。它由闪长岩、石英闪长岩和花岗闪长岩岩株组成,分布于长江中下游地区,位于扬子克拉通的东北边缘(大概位于南京和武汉之间)。这些岩体在早白垩纪时期(127Ma到135 Ma)侵入到古生代沉积岩中。(2)江南火成岩群。它分布于扬子克拉通的东南缘,包括中生代的花岗岩和花岗闪长岩,以岩基、岩株以及长英质岩浆岩形式出现(120Ma到140Ma)。(3)克拉通西部的侵入体。它们以花岗岩岩基和岩株形式出现,侵入的时代是从晚古生代到早中生代。长江的上游沉积岩分布广泛,尤其是靠近的江汉平原的三峡地区,大部分地区是古老的沉积岩。对长江中下游的沉积物进行物源示踪是件非常复杂的事情。不能简单分析上游的岩浆构造热事件后,就直接与沉积物中碎屑锆石的年龄峰值对比,还应该考虑河流侵蚀量,上游不同岩性的分布特点,构造抬升速度等因素。结合周老孔中砾石层的砾组分析及沉积物磁学特性等方面的证据,本文取得的主要认识或发现的问题如下:(1)六个样品中的碎屑锆石形貌和阴极发光图像特征指示其大多数为岩浆锆石。它们具有典型的震荡环带;轻稀土元素相对亏损,重稀土元素强烈富集;具有明显的正Ce异常和负Eu异常;Th/U比值也大多数大于0.1。由于几乎所有的锆石形貌、阴极发光图像、稀土元素含量和Th/U比值都极为相似,这些特征难以成为判断物源的判别标志。(2)周老孔中六个锆石的U-Pb年龄分布特征相似,均集中在阜平期(＞3.0Ga),吕梁期(1600-2500Ma),扬子期(720-850Ma),印支期(212Ma)。喜山期年龄罕见,仅在Gsh6中见到两颗。Gsh6中新生代年龄锆石的出现表明,Gsh6沉积的时代长江源头已经到达青藏高原。(3)六个样品的U-Pb年龄显示江汉平原的沉积物可能从2.2MaBP以来并没有重大的变化。但是仍然可以看出从下往上锆石年龄中古老锆石所占比重逐渐降低,而相对年轻的锆石所占比重逐渐增加。由于这些相对年轻的锆石所代表的岩体均分布在松潘-甘孜板块,这表明长江在青藏高原地区发生溯源侵蚀,上游流域面积逐渐扩大。江汉平原中的212Ma和＜17Ma的锆石的出现,预示着上游物质已被长江通过三峡搬运至江汉平原。长江可能自Gsh19沉积的时代以来,除在Gsh6与Gsh10之间金沙江加入到长江,其余水系并没有发生过重大变化。长江在2.2MaBP时已经与现代长江水系特征相似,三峡贯通可能发生在更为古老的年代。这预示着我们应该在前第四纪地层中寻找长江切穿三峡的证据。(4)＞3.0Ga,2490～2386Ma,2006～1784Ma,850～720Ma这些年龄峰值的出现表明江汉平原的沉积物中来自崆岭群的沉积物可能占了很大的比重。更多还原

【Abstract】 The study of the formation time of the modern Yangtze River has been a difficult problem for about a century.Because the sediments carried by the Yangtze River does not deposit in the Sichuan basin,the Jianghan Plain is one of the largest sedimentary sink in the catchment of the Yangtze River.The sediment in the Jianghan Plain records the evolutional history of the Yangtze River.But there is still no one have studied the provenance of the sediments.The author is using LAICPMS to study the U-Pb and trace elements of the detrital zircons which recovered from the Zhoulao core,and to predict the provenance of the sediments in the Jianghan Plain.This study will must be give the reference for the further study of the evolution of the Yangtze River.We should know the tectonic-thermal events of the upper reaches of the Yangtze River.The Yangtze River flows through the Qiangtang-Changdu block,Songpan-Ganze block,and the Yangtze Craton.Because of the small area,the sediments eroded from the Qiangtang-Changdu block must be very little,and then they will be drowned in the sediments from the lower reaches. Songpan-Ganze flysh zone is the largest Triassic flysh sedimentary area in the world.The felsic igneous intrusions in the Songpan-Ganze area are in the Late Triassic(～214-211Ma),some ages are Middle-Late Triassic(220Ma).The age of the Yidun Island Arc is appears Middle to Late Triassic,from 225-215Ma.This age is earlier than the time of the mass deformation in the area. There are four age populations in the detrital zircon ages of the Songpan-Ganze sediments: 2.4-2.5Ga and 1.85-1.95Ga,which is thought from the North China Craton;the third age peak (400-450Ma) is very popular in the North Qinling area.The fourth age peak(ca.250-280Ma) is different from the age peaks of the Qinling-Dabie zone,North China Craton,and the South China Craton.The provenance area of them maybe is the Guadalupian and Lopingian formations in the Longmenshan area,and is linked to the breakup of the Songpan-Ganze basin.The tectonic-thermal history of the Yangtze Craton is very complicated.U-Pb ages of detrital zircons from three Nanhua Formations of the Yangtze craton reveal four major age groups of 720-910 Ma,1.90-2.05 Ga,2.40-2.55Ga and 2.60-2.70 Ga with few＞3.2Ga grains.Three groups of igneous rocks occur in the Yangtze craton in the Mesozoic:(1) The Yangtze Igneous Group.It consists of diorite;quartz diorite and granodiorite stocks distributed along the middle and lower reach of the Yangtze River in the northeastern margin of the craton.These rocks intruded into Palaeozoic sedimentary rocks during Early Cretaceous times (127 to 135 Ma),and are closely associated with the important copper,iron,sulphur and gold ore deposits in eastern China.Some A-type granite intrusions also occur in this region.(2) The Jiangnan Igneous Group.It is distributed near the southeastern margin of the craton and includes Mesozoic granites and granodiorites,occurring as batholiths,stocks as well as felsic volcanic rocks(120 to 140Ma).(3) The intrusions in the western part of the craton,including granitic batholiths and stocks emplaced in late Palaeozoic to early Mesozoic times.There are many sedimentary rocks in the upper reaches of the Yangtze River catchment especially in the Three Gorges area.Using the U-Pb ages of the detrital zircon in the provenance study is very complicated problem.We should think about the erosion rate of the Yangtze River, the areas of the different rock types in the catchment,the velocity of the tectonic uplift.The conclusions conjectured from this study are:(1) The morphology and CL images of the detrital zircons show that majority of the detrital zircons are igneous zircons.There are obviously oscillations in the zircons.The LREE is rare and the HREE is enriched.And the content of the element Ce is positive and the Eu is negative.Most of the ratio of the Th/U is less than 0.1.Because most of them are the same,these characteristics cannot be used as a tracer in the provenance study.(2) The age peaks of the 6 detrital zircon samples are quite similar.They are concentrated in the Fuping period(＞3.0Ga),Lvliang period(1600-2500Ma),Yangtze period(720-850Ma),and Indo-china period(212Ma).Himalayan period is very rare;there are only two zircons in the Himalayan period.They all in the Gsh6,and the depositional time is ca 800ka BP,which indicates that the Yangtze River reached the Tibet Plateau at that time.(3) The U-Pb ages of the six samples do not change significantly,suggesting that the Yangtze River catchment did not change greatly fro 2.2MaBP.The older ages of the detrital zircon are becoming less and less,and the younger ages of zircons are becoming more and more,indicating the area of the Yangtze River are becoming larger and larger.Because the younger ages rocks are all in the Songpan-Ganze area,this shows that the Songpan Ganze block is one of the most important source of the sediments in the Jianghan Plain.And the formation time of the Three Gorges maybe very older than we thought before.We should search the evidence in the Pre-Quaternary sediments.The appearance of 212Ma and＜17Ma age peaks suggest that the detrital sediments of the upper reaches of the Yangtze River have been transported into the Jianghan Plain.(4) The appearance of the＞3.0Ga,2490～2386Ma,2006～1784Ma,850～720Ma age peaks show that most of the sediments are from the Kongling terrain.更多还原