长江三峡地区埃迪卡拉(震旦)系锆石U-Pb新年龄对庙河生物群和马雷诺冰期时限的限定

摘要

首次报道应用高分辨率、高精度离子微探针(SHRIMPⅡ)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549.9 Ma±6.1 Ma(MSWD=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784Ma±15Ma(MSWD=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628.3Ma±5.8Ma(MSWD=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SHRIMp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630 Ma非常接近,表明中国修订后的震旦系/南华系界线年龄应为630Ma左右.%New U-Pb ages by sensitive high-resolution ion microprobe (SHRIMP Ⅱ) on zircons were obtained from two tuff samples: one from the boundary of the Dengying and Doushantuo formations, the other from the basal Doushantuo Formarion in the Jiuqunao section, which situated at the western limb of the Huangling anticline in the Yangtze Gorges in Zigui, Hubei Province, South China. Analyzed zircons from the boundary of the Dengying and Doushantuo formations form two dusters. The younger cluster includes 16 spots and after removing one spot deviated from the population mean gives a weighted mean 206Pb/238U age of 549.9±6.1 Ma (MSWD=1.48), which represents the age of the volcanic tuff in this horizon and forms an age constraint on minimum age of Miaohe biota. The other from the basal Doushantuo Formation includes two clusters: one includes three spots, with an inherited age of 784±15 Ma (MSWD=0.05); the other consists of 15 spots and gives a weighted mean 206Pb/238U age of 628.3±5.8 Ma (MSWD=0.86). It is the first SHRIMP U-Pb zircon age obtained nearly the base of the Sinian (Ediacaran) Doushantuo Formation and represents a maximum age of the Doushantuo Formation.The sampling site lies just above the Doushantuo cap carbonate, so the data basically forms an age constraint on the lower boundary of the Sinian (Ediacaran) System. In addition, this age is very close to 630 Ma for the base of the Ediacaran System in the International Stratigraphic Chart published by the International Committee on Stratigraphy in 2004, indicating that the boundary age of the revised Sinian and Nanhuan systems in China should be ～630 Ma.