云南保山中晚志留世碳酸盐岩的锶同位素组成及海平面变化

本文以云南保山地区中上志留统栗柴坝组海相碳酸盐岩作为研究载体,运用锶同位素比值方法,结合岩石学、古生物学等,对该区志留系中晚期的海平面变化情况及其影响因素进行了研究。其中锶同位素比值数据为该区首次获得,其所反映的海平面变化趋势与同时期美国中西部的海平面演化曲线具有良好的耦合性。研究结果表明;剖面第一段至第二段,~(87)Sr/~(86)Sr值逐渐减小,表明此时海平面相对逐渐升高;第二段~(87)Sr/~(86)Sr值整体处于相对低的水平,总体呈现出逐渐减小趋势,且在21号样品处达到最低值(0.710 5),说明此阶段海侵作用明显;第三段,在较短的地质时期内,~(87)Sr/~(86)Sr值表现出相对强烈的正偏移,达到极值0.728 49,说明海平面相对下降明显。总之,志留系中晚期海平面表现为先升后降的过程。锶同位素的强烈偏移以及岩石内较高的~(87)Sr/~(86)Sr值,正是同时期造山运动和海平面下降事件的反映。

Strontium Isotope Composition and Sea Level Changes of Middle-late Silurian Carbonate in Baoshan Area, Yunnan Province, China

The marine carbonate rocks of Middle-Late Silurian Licaiba Group in Baoshan area, Yunnan Province, China were collected as the research object to investigate the changes of sea level of this area during the Middle-Late Silurian and its causes, by using the methods of Sr isotope ratio, combining with the petrology and paleobiology. Among those methods, it is the first time to achieve strontium isotopic ratios in this area and the variation trend of sea level reflected by this method has a good coupling with the middle west of the United States. Results show that ⁸⁷Sr/⁸⁶Sr values decrease gradually from first paragraph to the second, indicating that the sea level had gradually risen up during the same period of time; In the second paragraph,⁸⁷Sr/⁸⁶Sr ratio is at a low level as a whole, showing obvious decreasing trend, and sample No.21 reaches the minimum value (0.710 5), indicating the occurrence of obvious transgression. In the third paragraph, during a short geological periods, ⁸⁷Sr/⁸⁶Sr values had exhibited the positive offset strongly, reaching the maximum value(0.728 49), indicating the strong decline of sea level. In short, from the mid to late Silurian, sea level had showed the process of rising first and then falling. The strong deviation of Sr isotopes and the higher ⁸⁷Sr/⁸⁶Sr values in rocks are the reflection of orogenic and sea-level decline events during the same period.