巧家盆地河流沉积与重矿物特征对金沙江贯通的指示

The characteristics of fluvial deposition and heavy minerals in Qiaojia Basin:Indication for the transfixion of Jinsha River,Southwestern China

通过对云南巧家盆地第四纪河流沉积和重矿物特征的研究,探讨金沙江下游巧家段水系的发展和演化过程。研究结果表明:1.6 Ma B.P.之前巧家盆地开启后,古金沙江巧家段南流水系也随之形成,并在盆地内发育了至少3期河流沉积,当时为宽阔的辫状河沉积环境,物源区位于巧家以北至四川西昌-昭觉一带的黑水河和西溪河流域,河流将玄武岩、灰岩、砂岩等砂砾携带至盆地中堆积,重矿物组合为锆石-绿帘石-金红石-电气石-角闪石-磷灰石;在0.5 Ma B.P.之前,由于云南高原的隆升使得古金沙江巧家段水系南流受阻,并在古水系支流的溯源侵蚀下被袭夺,造成巧家河段的倒转北流,金沙江贯通,巧家段水系物源区发生巨大变化,河流开始将金沙江中上游流域的板岩、千枚岩、片岩以及石英岩等砂砾带至盆地,重矿物类型中也开始出现石榴子石,在构造活跃期河流不断向西迁移并进行下切,形成了金沙江4级阶地。巧家盆地河流沉积与重矿物特征及相关年代的研究显示,巧家段水系的袭夺倒流,金沙江的贯通应发生于0.5 Ma B.P.之前。

Based on the study of Quaternary fluvial deposits and the characteristics of heavy minerals in Qiaojia Basin,Yunnan Province,the development and evolution of the Qiaojia river system in the lower reaches of Jinsha River are discussed.The results show that after the initiation of the Qiaojia Basin before 1.6 Ma B.P.,the southward flowing water system of Qiaojia section of the ancient Jinsha River was formed,and at least three periods of fluvial deposits with broad braided river sedimentary environment were developed in the basin.It shows that the provenance is located in the Heishuihe and Xixihe drainage basin from the north of Qiaojia to the Xichang and Zhaojue area of Sichuan Province.The rivers carried basalt,limestone,sandstone and other sand and gravel to the basin,along with the heavy mineral assemblage of zircon-epidote-rutile-tourmaline-amphibole-apatite.Before 0.5 Ma B.P.,due to the uplift of the Yunnan Plateau,the southern stream of Qiaojia section of the ancient Jinsha River was blocked and captured under the erosion of the tributaries of the ancient water system,resulting in the reversing of Qiaojia section to the north and subsequent transfixion of Jinsha River.The provenance of the Qiaojia section of the river system changed greatly,and the river began to bring such metamorphic rocks as slate,phyllite,schist and quartzite from the upper and middle reaches of the Jinsha River to the basin.Garnet also began to appear in the heavy mineral assemblages.During the late tectonic active period,the stream trenching and westward migration led to the formation of four terraces along the banks of Jinsha River.The study on the characteristics of fluvial deposits and heavy minerals in Qiaojia Basin and their related ages shows that the recapture and backflow of the Qiaojia River and the transfixion of Jinsha River should have occurred 0.5 Ma ago.