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首次报道应用高分辨率、高精度离子微探针(SH R IM PⅡ)定年方法,对采自三峡地区黄陵背斜西翼湖北省秭归县九曲脑剖面震旦系灯影组与陡山沱组界线和陡山沱组底部的凝灰岩锆石进行的U -Pb定年研究。分析结果,灯影组与陡山沱组界线的凝灰岩锆石测点形成2组,较年轻的一组由16个测点组成,舍去偏离较大的1个点,给出的206Pb /238U 比值年龄加权平均值为549.9M a±6.1M a(M SW D =1.48),代表该层位的实际年龄并限定了庙河生物群的最小年龄。采自陡山沱组底部的另一样品测点同样分为2组:一组由3个点组成,给出784M a±15M a(M SW D =0.05)的继承年龄值;另一组由15个测点组成,给出206Pb/238U比值年龄加权平均值为628.3M a±5.8M a(M SW D =0.86)。这一定年结果是目前中国震旦(埃迪卡拉)系陡山沱组底部首次获得的SH R IM P锆石U -Pb年龄。采样点紧邻作为震旦系底界的“盖帽碳酸盐岩”之上,基本可以限定震旦系的底界年龄。该结果与国际地层委员会2004年发布的《国际地层表》中埃迪卡拉系(Ediacaran)的底界年龄630M a非常接近,表明中国修订后的震旦系/南华系界线年龄应为630M a左右。
For the first time, the SHRIMPⅡ dating method was used to study the relationship between the Sinian Dengying Formation and the Doushan Tuo of the Jiuqu’lian section in the Zigui County of the Huangling anticline in the Three Gorges area U-Pb dating of tuff zircons at the bottom of the Doushantuo Formation and the bottom of the Doushantuo Formation. As a result of the analysis, the zircon measuring points of the tuff in the boundary between Dengying Formation and Doushantuo Formation formed two groups, the younger one consisted of 16 measuring points, and the deviations from the larger one were given. The 206Pb / 238U ratio The age-weighted average is 549.9 Ma ± 6.1 Ma (M SW D = 1.48), representing the actual age of the horizon and defining the minimum age of the Miao River biome. Another sample measuring point taken from the bottom of the Doushantuo Formation is also divided into two groups: one consists of three points, giving an inherited age value of 784M ± 15M a (M SW D = 0.05); another group consists of 15 test points, giving a weighted average age-weighted average of 206Pb / 238U ratio of 628.3 Ma ± 5.8 Ma (M SW D = 0.86). The result of this certain year is the SH R IM P zircon U-Pb age at the bottom of the Doushantuo Formation at the present Sinian (Edicala), China. The sampling point immediately above the “capped carbonate rock”, which is the bottom of the Sinian system, can basically define the bottom boundary age of the Sinian system. This result is very close to 630Ma of the bottom boundary of Ediacaran in the International Stratigraphic Table published by the International Stratigraphy Commission in 2004, which indicates that the revised Sinian / South China line boundary age should be about 630Ma .