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首页> 外文期刊>Journal of Asian earth sciences >Age constraints on the Neoproterozoic Baikal Group from combined Sr isotopes and Pb-Pb dating of carbonates from the Baikal type section, southeastern Siberia
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Age constraints on the Neoproterozoic Baikal Group from combined Sr isotopes and Pb-Pb dating of carbonates from the Baikal type section, southeastern Siberia

机译：西伯利亚东南部贝加尔型剖面碳酸盐岩的Sr同位素和碳酸盐的Pb-Pb结合年龄对新元古代贝加尔族的年龄限制

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摘要

Depositional ages for the middle part of the Neoproterozoic sedimentary succession exposed along Lake Baikal in the southern margin of the Siberian Platform have been constrained by the application of Sr isotope chemostratigraphy and Pb-Pb dating of carbonate rocks. The thick silicidastic-carbonate Baikal Croup is the type section of the regional Siberian succession. Although many of the Baikal Group carbonate rocks have undergone pervasive dolomitization by mixed meteoric-marine fluids, oolitic limestones in the 800-m-thick carbonate member of the Uluntui Formation preserve primary textures and high Sr contents (up to 3200 ppm), suggesting that aragonite might have been the original precursor. The ~(87)Sr/~(86)Sr in the least altered high-Sr limestones (Mg/Ca < 0.08 and δ~(18)O > -8% PDB) range within 0.70842-0.70876, values we interpret as representing the best proxy for coeval seawater. Stepwise dissolution using 0.5 N HBr was applied to enrich samples in primary carbonate from the Uluntui Formation and nine data points from the five least altered, high-Sr limestones yielded a Pb-Pb isochron age of 560 ± 30 Ma with MSWD = 0.6 and probability = 0.8. This age matches that inferred from Sr isotope stratigraphy (580-550 Ma) and further supports the inference that the Baikal paleobasin was connected to the contemporary oceans. Our Sr chemostratigraphy and Pb-Pb isochron age of the Uluntui limestones cast doubt on previously proposed correlations of the Baikal Group as late Mesoproterozoic to Cryoge-nian. Instead, the data indicate that the entire silicidastic-carbonate succession of the Baikal Group was most likely deposited during the Ediacaran (or Vendian, of the Russian Stratigraphic Scale).
机译：西伯利亚平台南缘贝加尔湖沿岸暴露的新元古代沉积演替中段的沉积年龄受到了Sr同位素化学地层学和碳酸盐岩Pb-Pb定年的限制。较厚的硅质碳酸盐贝加尔湖群是西伯利亚地区演替的典型剖面。尽管贝加尔集团的许多碳酸盐岩已通过混合的流星-海洋流体进行了广泛的白云石化作用，但Uluuntui组碳酸盐岩段800米厚的碳酸盐岩中的石灰岩保留了原始的质地和高的Sr含量（高达3200 ppm），这表明文石可能是原始的前体。最少改变的高Sr石灰岩中的〜（87）Sr /〜（86）Sr（Mg / Ca <0.08和δ〜（18）O> -8％PDB）的范围在0.70842-0.70876之内，我们将其解释为代表的值中世纪海水的最佳代理。使用0.5 N HBr逐步溶解以富集Uluntui地层的一次碳酸盐中的样品，来自五个变化最小的高Sr石灰岩的9个数据点产生的Pb-Pb等时年龄为560±30 Ma，MSWD = 0.6，概率为= 0.8。这个年龄与从Sr同位素地层学（580-550 Ma）推断的年龄相符，并且进一步支持了贝加尔湖古青盆与现代海洋的联系。 Uluntui石灰岩的Sr地热地层学和Pb-Pb等时年龄使人们对先前提出的贝加尔岩群（晚中生代至冰冻年代）的相关性产生怀疑。取而代之的是，数据表明贝加尔湖群的整个硅碳酸盐岩碳酸盐岩层很有可能是在埃迪卡拉安（或俄罗斯地层规模的文迪安）期间沉积的。

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