湖北宜昌第四纪沉积物中铁质重矿物特征对三峡贯通的指示*

长江三峡的演化问题一直是地学界争论的焦点和热点。近年来,不少学者通过锆石、石榴子石、角闪石等重矿物的同位素和微量元素组成来判断长江物源,取得了较好的成果,但将单颗粒铁质重矿物作为物源指示矿物并应用于三峡贯通方面研究较少。因此作者运用电子探针、扫描电镜和能谱仪对湖北宜昌地区第四纪沉积物中的铁质重矿物进行化学特征和形貌分析,并与可能的物源区样品进行对比,来分析物源演化过程及判定三峡贯通时间。研究结果显示,宜昌地区第四纪沉积铁质重矿物的主要源岩为位于三峡以西的攀枝花钒钛磁铁矿、峨眉山玄武岩和位于三峡以东的黄陵花岗岩,在善溪窑组沉积结束以后,宜昌地区第四纪沉积物的物源发生了明显改变,出现来自三峡以西的物质,表明长江三峡此时发生了贯通。结合ESR测年数据和前人研究结果可知,长江三峡的贯通发生在0.75—0.73Ma B.P.。

Implication of iron rich heavy minerals in Quaternary sediments in Yichang area of Hubei Province for cutting through time of Three Gorges

The evolution of the Three Gorges has been the focus of debate in the field of geosciences. In recent years,many scholars have successfully analyzed the provenance of the Yangtze River through the isotope composition and trace elements in heavy minerals such as zircon,garnet and amphibole. However,the single-grain iron-rich heavy minerals are seldom used as provenance indicators and applied in the determination of the cutting-through time of the Three Gorges. Therefore,in this paper,the chemical properties and morphologic analysis of iron-rich heavy minerals in Quaternary sediments of Yichang area were analyzed by electron probe,scanning electron microscope and energy disperse spectroscopy,which were compared with the analytical results of possible provenance samples. The evolution process of provenance and the cutting-through time of the Three Gorges were analyzed based on the occurrence of materials in Yichang area that may belong to the west of the Three Gorges. The results show that the source rocks of the iron-rich heavy minerals in the Quaternary sediments in Yichang are mainly from the Panzhihua vanadium titanomagnetite,Emeishan basalt that are located to the west of the Three Gorges and Huangling granite located to the east of the Three Gorges. After the deposition of the Shanxiyao Formation,the provenance of the Quaternary sediments in Yichang area changed obviously,and the material that belong to the west of the Three Gorges appeared in this area,which indicates that the Three Gorges was cutted-through. Combined with ESR dating data and previous studies,it can be assumed that the cutting-through time of the Three Gorges occurred in 0.75-0.73Ma B.P..