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首页> 外文期刊>Geosciences >Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland
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Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland

机译：评估负高场强度元素异常作为古生俯冲代理的有效性：来自苏格兰西北部本·斯特罗姆综合体的证据

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摘要

The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km 2 ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H 2 O and CO 2 -rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data.
机译：在归一化的痕量元素图上，诸如Nb，Ta和Ti之类的高场强元素（HFSE）的相对耗竭是常规用于与生代俯冲有关的岩浆岩浆过程的地球化学代理。这种替代物已越来越多地应用于古生界克拉通中的超镁铁质-镁铁质单元，但是由于这些组合通常受到高级变质作用和热液蚀变/致突变作用的影响，相对于生代时代的例子而言，元素迁移的可能性很高。为了评估HFSE异常作为古生俯冲的可靠替代手段的有效性，我们在这里调查其起源于Ben Strome岩体的超镁铁质岩石，Ben Strome岩体是苏格兰西北部Lewisian片麻岩复合体中的7 km 2超镁铁质-镁铁质复合体。最近被解释为变形的层状侵入体，Ben Strome Complex已经历了高级变质的多个阶段，包括独立的粒状和角质岩相变形事件。除块岩地球化学外，我们还针对35个超镁铁质样品提供了详细的岩石学以及主要元素和微量元素矿物化学，其中15个显示负HFSE异常。我们的数据表明，本·斯特罗姆综合体中HFSE异常的程度与与闪石化相关的富含H 2 O和CO 2的热液相互作用时可能产生的轻稀土元素（LREE）富集相关，而不是与原岩浆（与俯冲有关的过程）。因此，我们认为仅是块状岩石HFSE异常是经历了多个高级变质阶段的古生界中古生俯冲的不可靠替代，对元素移动性和岩石学的综合评估是对地球动力学解释进行分配之前的最低要求。块状岩石地球化学数据。

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