中文摘要：

“万里长江源何时”已考问地质学家和地貌学家一个世纪，仍未有明确的答案。从云南石鼓至宜昌河段存在大量河流袭夺、掉向、下切等证据，成为考究长江上游水系东流出川、入海的热点，但因河流沉积地貌复杂、受后期改造明显，其古地理解释不同学者各有表述，在金沙江何时改道东流、三峡何时被贯穿问题上各烁其词，由此得出长江东西贯通时间上溯几千万年甚至上亿年，下至十几万年。利用新近发展起来的单颗粒碎屑矿物微区分析方法，由河口或海域沉积物直接追踪源区的变化，在国外已经成为研究流域演化和源区构造运动的重要手段。长江口地层中EMP独居石Th（U）-Pb年龄较好地限定了长江东西贯通时间在2．58MaBP前后，近年来河流沉积地貌的研究成果也认为长江贯通应发生在晚上新世一早更新世。激光剥蚀-电感耦合等离子体质谱（LA—ICPMS）不仅经济、快速，且测年精度高，将其应用于长江这样复杂的大河流域进行物源示踪、源区构造运动和流域演化研究，可望取得新的突破。今后关于长江贯通问题的研究，应该将流域演化视作一个系统，在大地构造、盆地分析和河流沉积地貌学研究基础上，充分利用近年来年代学和单矿物微区分析的新技术，不同研究方法相互补充、所得结果相互验证和约束，对争议的问题组织各方面专家进行系统研究，统一认识，不断逼近长江贯通时限的真实值。

英文摘要：

＂When the Yangtze was formed＂ has become a century question both to geologists and geomorphologists, but a well-constrained answer is still on its way. Enormous geomorphological and sedimentary features, left by river piracy, reversal, and incision in the river section from Shigu town in Yunnan to Yichang City, have been widely studied to reconstruct Yangtze drainage evolution history and timing of the capture of the upper Yangtze from south-flowing into east-flowing. However, the river sedimentation and geomorphology are very complicated by themselves, and exposed to external reformation, leading to great difference in paleoeological and paleo-geomorphological interpretations. There is no consensus on timing of the capture of the Jinsha River from south-flowing into east-flowing, and timing of the Yangtze channe- ling through the Three Gores area. It was therefore deduced that the Yangtze could be a very old river having existed since the Tertiary or the Cretaceous, or a very young river only hundreds of thousand years old. Microprobe dating of single detrital minerals, particular zircon and monazite, is considered to be a powerful tool in provenance study. Lots of successful case studies have been reported in foreign literatures to reconstruct drainage evolution history and tectonic activities in the drainage basins through detailed studies of zircon/monazite geochronology of the sedimentary succession in the river-mouth or marine realm. The dating method of electron microprobe （EMP） is rapid and inexpensive, although its resolution is relatively low, and can be widely applied to primary study of drainage evolution. EMP Th（U）-Pb ages of monazite grains of the late Cenozoic strata in the Yangtze River mouth, provide a key constraint on timing of the Yangtze formation in 42.58 MaBP, which follows within the initiation time of the Yangtze in late Pliocene to early Pleistocene and is proposed by lots of river sedimentological and geomorphological studies. Laser ablation-ICPMS is a newly developed mic