重庆秀山寒武系锶同位素演化曲线及全球对比

Evolution of Sr Isotopes of the Cambrian Sections in Xiushan, Chongqing and Related Global Correlation

本文评述了国际上已有的寒武纪海相碳酸盐的锶同位素演化曲线,公布了重庆秀山寒武系海相碳酸盐的锶同位素分析结果。尽管对样品进行了成岩蚀变检测和必要的筛选,但样品仍可能不同程度地经历了成岩蚀变,锶同位素组成与地层厚度投点图显得较为离散。笔者主要利用^(87)Sr/^(86)Sr比值较低的样品建立的寒武纪海水的锶同位素演化曲线在长期旋回上与国际上已公布的同时代锶同位素演化曲线仍具有较好相似性和可对比性,说明全球事件是海水锶同位素组成的主要控制因素,海水的^(87)Sr/^(86)Sr比值在盆内对比、盆间对比、以及全球对比中都具有潜在价值。根据锶同位素地层学的基本原理,笔者尝试确定了重庆秀山寒武纪剖面一些关键点的年龄,累积厚度1340m处的年龄为503～509 Ma,累积厚度1950m处的年龄为496～498 Ma,这对秀山寒武纪剖面下、中寒武统和中、上寒武统界线的确定具有参考价值。

94 marine carbonate samples were collected from the sedimentary section of Cambrian in Xiushan of Chongqing, the Upper Yangtze Platform, Southwest China. Samples for strontium isotopic analysis were selected by way of the test of cathodoluminescence, observation of thin sections, and analysis of Sr and Mn. The Mn/Sr ratios of all samples presented in this paper are less than 2, and the cathodoluminescence is dull, but the samples from the Cambrian underwent diagenetic alteration to a certain degree, and the plotted points of the thicknesses and 87Sr/86Sr ratios are somewhat discrete. The strontium isotope evolution curve drawn using the data with relatively low 87Sr/86Sr ratios are consistent with other curves based on samples from different areas of the world, supporting the notion of global consistency of strontium isotope composition of marine carbonates. The strontium isotope variations through time reflect the relative importance of global events, and will become a potential useful tool for intrabasinal and interbasinal correlations. According to this fundamental principle, the authors have dated some key points of the Xiushan Cambrian section. The age of the point with an accumulating thickness of 1340 m from the base of the Cambrian System is 503-509 Ma, and that for the point of 1950 m is 496-498 Ma, which should be valuable for determining the boundaries of Middle/Upper Cambrian and Lower/Middle Cambrian.