贵州松桃南华系大塘坡组凝灰岩锆石SHRIMPⅡ U-Pb年龄

SHRIMPⅡU-Pb Zircon Date from the Nanhuan Datangpo Formation in Songtao County, Guizhou Province

新元古代冰期和冰期发生的确切时代一直是全球前寒武纪地质学家关注的话题之一。本文应用高分辨率、高精度离子微探针(SHRIMP)锆石U-Pb定年方法,测定采自贵州松桃黑水溪锰矿大塘坡组底部凝灰岩锆石U-Pb年龄为667.3±9.9Ma(MSWD=1.6)。这一年龄结果与前不久Zhou等(2004)在“Geology”上报道的贵州松桃寨郎沟剖面大塘坡组下部凝灰质层的锆石U-Pb年龄662.9±4.3Ma(MSWD=1.24)完全一致,进一步限定了我国南华系大塘坡组间冰期的下限年龄。综合近年来同位素年代学新资料,认为南华系南沱冰期与新元古代马利诺(Marinoan)冰期相当,时限大致在660～630Ma之间。南华系的下冰期与斯图特(Sturtian)冰期相当,时限大致在750～670Ma之间。

The reported new U-Pb age by sensitive high-resolution ion microprobe on zircon （SHRIMP Ⅱ） was obtained from a tuff sample at the basal Datangpo Formation in the Hishuixi manganese ore in Songtao County, Guizhou Province, South China. Twenty-four spots of zircons were analyzed and a weighted mean ^208Pb/^238U age of 667.3±9.9 Ma （MSWD=1.6） was given. This age is quite similar to that reported in Geology by Zhou Chuanming et al. , which gives a U-Pb zircon age of 662.9±4.3 Ma （MSWD=1.94） from a tuffaceous bed in the lower part of the Datangpo Formation at the Zhailanggou section, Songtao County （Chuanming Zhou et al. , 2004）. Both of them form a robust age constraint on the timing of the Datangpo interglacial period. It also provides a maximum age constraint for the Nantuo （Mrinoan） glaciation. Together with other new isotopic ages in recent years, the new age suggests that the Nantuo glaciation is equivalent to the Mrinoan glaciation, which occurred at 660～630 Ma and the earlier glaciation is equivalent to the Sturtian glaciation, which occurred at 750～670 Ma.