青藏高原东缘宜宾地区第四纪河流沉积物中铁质重矿物特征及物源意义

大河流域的发育史和演化在地学界广受关注。探讨宜宾地区第四纪沉积物中铁质重矿物的特征,可为长江流域的演化提供重要证据,同时对于了解青藏高原东缘的隆升及沉积响应具有重要意义。本文以金沙江和岷江交汇处的宜宾地区第四纪沉积物中的铁质重矿物为研究对象,分析其在电子探针和背散射图像中的化学和形貌学特征,探究长江及其支流的物源演化过程。研究结果显示:岷江下游Ⅴ级阶地至Ⅳ级阶地沉积以磁铁矿为主,物源主要来自龙门山构造带;Ⅲ级阶地沉积以钛铁矿为主,物源仍然主要来自龙门山构造带;现代沉积中蓝晶石、磷灰石等包裹体的铁质矿物增多,其物源来自龙门山构造带和松潘—甘孜褶皱带。岷江Ⅴ级至Ⅲ级阶地形成时,岷江的源头主要位于龙门山构造带;Ⅲ级阶地形成以后,岷江不断向上溯源侵蚀,深入松潘—甘孜褶皱带内,逐渐形成现代岷江。金沙江Ⅴ级阶地和长江Ⅴ级阶地中均未出现攀枝花钒钛磁铁矿,此时金沙江攀枝花—宜宾河段未贯通,长江此时在研究区的物源主要由发源于龙门山构造带的岷江提供。金沙江Ⅳ级阶地中攀枝花钒钛磁铁矿大量出现,表明此时金沙江攀枝花—宜宾河段已经贯通,时间为Ⅳ级阶地开始沉积的时间,即0.5~0.3 Ma BP。

Characteristics and provenance significance of iron-rich heavy minerals in Quaternary fluvial sediments in Yibin area,eastern margin of Tibetan Plateau

The development and evolutionary history of large river systems have attracted much attention from geoscientists. Exploring the characteristics of iron-rich heavy minerals in the Quaternary sediments in Yibin area where the Jinsha and Minjiang Rivers meet may reveal important evidence for the evolution of the Yangtze River; it can also be of great significance for understanding the uplift and sedimentary response of the eastern margin of the Tibetan Plateau. The chemical and morphological characteristics of iron-rich heavy minerals were analyzed by electron probe and backscatter imaging to explore the provenance evolution of the Yangtze River and its tributaries. Sediments of terraces Ⅴ and Ⅳ of the lower reaches of the Minjiang River contained a lot of magnetite from mainly the Longmenshan tectonic belt. Although sediments of terrace Ⅲ contain plentiful ilmenite, their provenance is still mainly in the Longmenshan tectonic belt. More iron-rich heavy minerals with inclusions such as kyanite and apatite can be found in the modern sediments, which proves that both the Longmenshan tectonic belt and the Songpan-Ganzi fold belt provided materials to the sediments. So we can conclude that when terraces V to III of Minjiang River formed, the source of Minjiang River was located in the Longmenshan tectonic belt. After the formation of terrace Ⅲ, the Minjiang River continued to erode upward to the source area and entered the Songpan-Ganzi fold belt to gradually form the modern Minjiang River. In terrace Ⅴ of the Jinsha and Yangtze Rivers the Panzhihua vanadium titanomagnetite was not found, which means the Panzhihua-Yibin section of the Jinsha River was not cut-through, and the provenance of the Yangtze River in the study area at this time was mainly the Minjiang River sourced from the Longmenshan tectonic belt. However, the Panzhihua vanadium titanomagnetite is abundant in terrace Ⅳ of the Jinsha River, indicating that the Panzhihua-Yibin section of the Jinsha River was cut-through at the time (~0.5-0.3 Ma B.P.) of sediment deposition in terrace Ⅳ.