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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Time constraints for Mesoproterozoic upper amphibolite facies metamorphism in NW Namibia: a multi-isotopic approach
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Time constraints for Mesoproterozoic upper amphibolite facies metamorphism in NW Namibia: a multi-isotopic approach

机译：纳米比亚西北部中元古界上闪石岩相变质的时间限制：一种多同位素方法

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This study presents the chronological evolution of the upper amphibolite facies Orue Unit in NW Namibia. Metasedimentary and meta-igneous rocks of the Orue Unit were investigated using the Pb-Pb stepwise leaching technique on garnet and rutile, U-Pb multi-grain analysis on rutile, Sm-Nd-Lu-Hf leaching technique on gamet, SHRIMP analysis on zircon and Ar-Ar dating on amphibole. Each of these techniques pertains to different processes that occurred before, during, or after the metamorphic peak. Our age data can be integrated with petrological constraints to provide a more complete understanding of the metamorphic cycle. Our pre-peak metamorphic zircon ages, peak metamorphic garnet ages and peak to late peak metamorphic amphibole Ar-39-Ar-40 ages bracket the upper amphibolite facies metamorphic event including hydration or dehydration processes into a time span of only ca. 20 Ma. The age data obtained by peak metamorphic mineral analyses cluster around 1340-1320 Ma. Based on age data and field observation, we interpret the upper amphibolite facies metamorphism as a large-scale regional mid-crustal event. Spot analyses of inherited zircon cores obtained by SHRIMP reflect the sedimentary origin of the respective rocks of the Orue Unit and derivation from Palaeoproterozoic protoliths. The metamorphic rocks south of the anorthositic Kunene Intrusive Complex (KIC) have previously been ascribed to the Palaeoproterozoic Epupa Complex at the SW margin of the Congo craton and were thus thought to be older than the Mesoproterozoic KIC. Our data show that the high-grade metamorphic overprint took place 30-50 Ma after emplacement of the KIC. Rutile growth ages of 1248 Ma in one sample reflect fluid activity which seems to be a local phenomenon since there is no other evidence of geological activity throughout the Orue Unit at that time. The rutile ages predate the emplacement of satellite intrusions in that area by 30 Ma and there is no causal relation between these two events. (C) 2004 Elsevier B.V. All rights reserved.
机译：这项研究提出了纳米比亚西北部上角闪石相Orue单元的时间演化。利用石榴石和金红石上的Pb-Pb逐步浸出技术，金红石上的U-Pb多颗粒分析，配子上的Sm-Nd-Lu-Hf浸出技术，SHRIMP分析对Orue单元的准沉积和准火成岩进行了研究。闪石上的锆石和Ar-Ar约会。这些技术中的每一种都与在变质峰之前，之中或之后发生的不同过程有关。我们的年龄数据可以与岩石学约束相结合，以提供对变质周期的更完整的了解。我们的峰前变质锆石年龄，峰变质石榴石年龄和峰到晚峰变质闪石Ar-39-Ar-40年龄将上层角闪岩相变质事件包括水合或脱水过程转变为仅约一个时间跨度。 20 Ma。通过峰值变质矿物分析获得的年龄数据聚集在1340-1320 Ma附近。基于年龄数据和现场观察，我们将上闪石相变质解释为大规模的区域性中地壳事件。通过SHRIMP获得的继承锆石岩心的现场分析反映了Orue单元各个岩石的沉积成因和古元古代原生岩的衍生。以前在刚果克拉通西南缘的古生代古生代埃普帕复合体被归因于正寄生的库内内侵入复合体（KIC）以南的变质岩，因此被认为比中元古生界的KIC年龄大。我们的数据表明，高阶变质叠印发生在KIC放置30-50 Ma之后。一个样品中金红石的生长年龄为1248 Ma，反映出流体活动，这似乎是一种局部现象，因为当时在整个Orue单元中没有其他地质活动的证据。金红石时代早于该地区入侵卫星的时间为30 Ma，这两个事件之间没有因果关系。 （C）2004 Elsevier B.V.保留所有权利。

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