湖北峡东地区“盖帽碳酸盐岩”中燧石条带的地球化学特征及其古环境意义

Geochemistry of Bedded Cherts in the Cap Carbonates in Three Gorges Region,Hubei Province,and Its Paleoenvironmental Implications

湖北峡东地区新元古代陡山沱组一段的"盖帽碳酸盐岩"中发育有保存良好,与白云岩界线截然,顺层产出的燧石条带。利用硅酸盐的BrF5硅氧同位素分析方法对燧石条带进行测试,得到较高的硅同位素(δ^(30)Si_(NBS-28)为0.4‰~1.7‰)和氧同位素(δ~18O_(V-SMOW)为24.0‰~26.6‰)组成。燧石条带可能为温暖的滨海相化学沉积产物,氧同位素测温得到的沉积海水温度为35.4~47.7℃。燧石条带的Al/(Fe+Mn+Al)为0.43~0.50,Si/Al比值为257~269,表明燧石条带形成过程中有少量的陆源物质参与,而较低的∑REE(2.19×10^(-6)~3.59×10^(-6)),Ce正异常(1.20~1.24)和U/Th比值(3.75~14.45)上则明显带有热水沉积的特征。燧石条带的硅质可能主要来自远源火山热液活动,在温暖的滨海环境达到硅质饱和而形成的化学沉积,同时验证了早期研究者认为的"雪球事件"冰后期环境温度曾一度极为温暖的猜想。

The bedded cherts, which is in conformity with the stratum of the first Member of Doushantuo Formation and chemical precipitation originally, were discovered in the ＂Cap Carbonates＂ of Three Gorges region of Hubei Province. This paper used BrF method to analyse the silicate and oxygen isotopes of the cherts. High compositions of 30SiNBS 28 （0. 4% 1. 7%O） and 8 18OvsMow （24. 0%-26. 6%） values of the bedded cherts indicate that the environment of silicate precipitate are warm littoral facies, and geothermometers of quartz-seawater oxygen isotope fractionation estimate that the seawater temperature of chert sedimentary is 35.4--47.7~C. The ratios of A1/（Fe＋Mn＋A1） and Si/A1 are respectively 0.43~ 0.50 and 257~269, indicate a few terrigenous matters mixed in the progress of silicate sedimentation, whileCe positiveanomalies（1.20~1.24）, the values of ~,REE（2. 19X10 6~3. 59X10 6） and U/Th （3.75~ 14. 45） indicate a hydrothermal sedimentary genesis. Therefore, the origin of silicate maybe supplied from a distal volcanic hydrothermal activity and precipitated as bedded cherts in warm littoral facies when the concentration of silicate basic saturated,and this work verifies a guess proposed from early researchers that the environmental temperature in the cap carbonates was once hit to 50℃.