峨眉山玄武岩作为长江上游特征源岩对三峡贯通的指示

Emeishan Basalts as Provenance Indicators:Implications for Formation of the Three Gorges

峨眉山玄武岩作为长江上游攀西地区广泛分布的岩类，具有形成环境独特、出露面积大、岩石易鉴定的特点，可作为长江三峡贯通物源示踪研究的指示标志．通过对长江中上游阶地和江汉平原周老孔第四纪岩心沉积物中的玄武岩砾石和岩屑（1～2mm）进行研究发现，峨眉山玄武岩砾石在长江上游很常见，在三峡以下很少；玄武岩岩屑在长江上游沉积物中非常普遍，而且在周老孔岩心沉积物中的很多层位都有出现．对玄武岩岩屑基质中的斜长石做了微量元素微区原位分析（LA_ICP_MS），分析表明，在周老孔中含峨眉山玄武岩岩屑的岩心层位最大深度为156m，该层位古地磁年龄约为1．7Ma，此时长江已经形成目．三峡已经贯通．

The Emeishan basalts are typical rocks of the Yangtze River basin because of their unique origin, the broad distribu-tion and simplicity of identification, which can be used as an indicator of tracing the source of the upstream Yangtze sediments after cutting of the Three Gorges. The gravel and debris （between 1--2 mm） of basalts in the sediments of the middle and upper reaches of the Yangtze terraces（above Fuling City） and the Quaternary sediments of the Zhoulao core in the Jianghan Plain are mainly studied in the article. The gravels of the lithic basalts are abundant in the floodplain sediments and the terrace deposits along the upper reach of the river （upstream of Fuling City）, and are rare along the downstream of the Three Gorges. The basalt debris are very common on the terraces of the upper reach of the Yangtze River, and are also evaluated by the loose sedi ments of the Zhoulao core. These lithic debris were identified as the Emeishan basalts by LA-ICP-MS and appear at the core depth of 156 m （up）. This line of change （156 m depth） dated of about 1.7 Ma attests a provenance change. Meanwhile, a wide drainage basin similar as today＇s dimension of the Yangtze River had formed and the Three Gorges had formed around 1.7 Ma.