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首页> 外文期刊>Contributions to Mineralogy and Petrology >Assessing the origin of old apparent ages derived by Pb stepwise leaching of vein-hosted epidote from Mount Isa, northwest Queensland, Australia
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Assessing the origin of old apparent ages derived by Pb stepwise leaching of vein-hosted epidote from Mount Isa, northwest Queensland, Australia

机译：评估从澳大利亚昆士兰州西北部伊萨山的含铅附子的铅逐步浸出获得的表观年龄的起源

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摘要

Epidote metasomatism affected large areas of tholeiitic metabasalts of the similar to 1,780 Ma Eastern Creek Volcanics in the Western Fold Belt of the Proterozoic Mount Isa inlier. Hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant S-2 fabrics which formed during the regional metamorphic peak at similar to 1,570 Ma associated with the Isan orogeny. Previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin (with an evaporitic component). Application of the Pb stepwise leaching technique to the epidote does not separate radiogenic Pb4+ and common Pb2+, generating little spread in Pb-206/Pb-204 (between 16.0 and 30.5). The causes for this relatively low range are twofold: There is little radiogenic Pb in the epidotes (the most radiogenic steps account for <1 % of Pb released) and both Pb2+ and uranogenic Pb4+ substitute into the same site in the epidote crystal lattice. Consequently, age regressions using the Pb stepwise leaching data give ages between 150 and 1,500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak. These old ages are attributed to chemical inheritance from the host metabasalts, via radiogenic Pb release by breakdown of phases such as zircon, monazite, titanomagnetite, and ilmenite during metamorphism. This idea is supported by trace element data and chrondrite-normalized rare earth element patterns that are similar to both the metabasalts and epidotes (except for a variable Eu anomaly in the latter). Relatively high fO(2) during vein formation (Fe3+ dominates in the epidote crystal lattice) would allow the incorporation of Th4+ and exclusion of U6+ and would explain elevated Th/U ratios (up to 12) in epidote compared with the host metabasalts. Non-incorporation of U would explain the relatively low U/Pb ratios and non-radiogenic character of the epidote. This process may provide a source of metal for the small U deposits around Mount Isa and may also suggest a relationship between U mineralization and regional Cu mobilization during the Isan orogeny. Our work suggests that non-conventional geochronometers should be used only if additional geological information and geochemical data (e.g., mineral chemistry, trace elements) are available to evaluate any resulting age calculations.
机译：Epidote交代作用影响了大面积的变质玄武岩，类似于伊萨内里元古代元古代西部褶皱带中的1,780 Ma Eastern Creek火山。地热液化石通常发生在石英脉中，该脉与主要的S-2织物平行或包结在一起，该S-2织物是在与Isan造山运动有关的1,570 Ma的区域变质峰期间形成的。先前已发布的稳定同位素和卤素数据表明，负责形成附子的流体起源是变质的（具有蒸发成分）。将Pb逐步浸出技术应用于附子不会分离出放射性Pb4 +和普通Pb2 +，在Pb-206 / Pb-204中几乎不扩散（介于16.0和30.5之间）。产生此相对较低范围的原因有两个：放射性原子中的放射性Pb很少（放射性最高的步骤占释放的Pb的<1％），并且Pb2 +和尿素生成的Pb4 +都被替换到了Epidote晶格的同一位置。因此，使用Pb逐步浸出数据进行年龄回归分析得出的年龄比主体岩石年龄大150至1,500迈尔，而比热变质峰年龄高450迈尔。这些晚年归因于在变质过程中通过分解诸如锆石，独居石，钛磁铁矿和钛铁矿等相而导致的放射性Pb释放，从而使宿主从玄武岩中继承了化学物质。痕量元素数据和经玄武岩归一化的稀土元素模式（与后玄武岩和表象都相似）（后者中的变量Eu异常除外）支持了这一思想。在静脉形成过程中相对较高的fO（2）（Fe3 +在附子晶体晶格中占主导地位）将允许Th4 +的掺入和U6 +的排斥，并且可以解释与宿主偏玄武岩相比，附子中Th / U比率升高（最多12）。不掺入U可以解释相对较低的U / Pb比率和附子的非放射特性。这个过程可能为伊萨山周围的少量铀矿提供金属来源，也可能暗示伊桑造山过程中铀矿化与区域铜动员之间的关系。我们的工作建议仅在有其他地质信息和地球化学数据（例如矿物化学，微量元素）可用于评估任何由此产生的年龄计算时，才应使用非常规地球计时器。

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