摘要
通过对江汉平原周老孔第四纪岩芯的22个重砂样品中0.125～0.063mm粒级的重矿物中的普通辉石进行实验分析,发现从钻孔深度104m开始向上,普通辉石的含量明显增多(辉石在该粒级沉积物中的丰度从平均3.2×10^(-4)增加到10.9×10^(-4)),种类也明显多样(从单一的墨绿色普通辉石到多种颜色的普通辉石都有相当数量的出现),特别是含钛普通辉石(玫瑰红色调)首次出现。通过对江汉平原物源分析,特别是近源(三峡以下,含三峡)和远源(三峡以上)产出普通辉石的岩体的面积及类型的比较分析(近源与远源的基性、超基性岩分布明显不同),认为江汉平原沉积物中普通辉石的变化是由长江上游下金沙江-三峡贯通后远源物质进入所引起的,这个发生变化的钻孔深度(104m)的古地磁年龄为1.1Ma。
The formation of the Three Gorges was a major event in the evolutionary history of the Yangtze River. Although many researches of using different methods have addressed the environmental conditions of its formation, the chronological framework of its formation is still a matter of debate. This paper contributes to this debate by analyzing the content of heavy minerals in the deposits of the Jianghan Plain. This plain was a depression east of the Three Gorges. Heavy minerals are a good evidence for studying the age and provenance of sediments there. In this study,the heavy mineral analyzed is augite. It was obtained from 22 heavy placer samples （0.125-0.063mm） collected from cores of the Zhoulao borehole（ about 300m long）. The augite came mostly from basic and uhra-basie intrusions. The analyzing results show that the content of augite increases from the bottom to the top of the borehole and that the proportion of pyroxene changes from 3.2×10^-4 to 10.9×10^-4. Moreover,There are different types of augite to occur： from single bottle green to other colors. Yellowish and reddish Ti-augite appears in the whole sediments above 104 meters. These differences should be due to the change of source rock. By the comparative analyses of the distribution and type of near and far source rocks of augite, it can be believed that augite should come both from near source rocks and from far source rocks. The near source rocks should be mostly mafic dykes in the Huangling area and the far source rocks should be west of Three Gorges. Because Ti-augite is a characteristic mineral of the Panxi area, its existence in the samples might prove that there were sediments from west of Three Gorges to reach the Jianghan Plain. And because the change of the content and color of augite occurs around the depth of 104meters of the Zhoulao borehole,this depth can be considered to correspond to the formation of Three Gorges. The paleomagnetie age of this depth is 1.1 Ma. This means that the formation age of Three Gorges should be 1.1Ma.
出处
《第四纪研究》
CAS
CSCD
北大核心
2009年第5期1000-1007,共8页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40771213和40672206)
国家重点基础研究发展规划项目(批准号:2004CB720204)资助
关键词
辉石
含钛普通辉石
江汉平原
周老孔
物源示踪
pyroxenes, Ti-augite, Jianghan Plain, Zhoulao borehole, source tracing
