Daczko Nathan R, Piazolo Sandra, Meek Uvana, Stuart Catherine A, Elliott Victoria
ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia.
Sci Rep. 2016 Aug 22;6:31369. doi: 10.1038/srep31369.
Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the 'missing' mass transfer zones.
整个分层地球的地球化学特征表明，需要通过下地壳进行大量的物质转移，但地质路径却未得到充分认识。在下部地壳的露头中，从基性到超基性岩石的细长岩体随处可见。位于新西兰峡湾地区彭布罗克谷麻粒岩相辉长质片麻岩中的超基性角闪石岩岩体是典型的例子，通常被报道为火成堆积角闪石岩。它们的火成特征与其寄主辉长质片麻岩的变质特征形成对比。这两种岩石类型有一个共同的母岩；野外关系表明寄主辉长质片麻岩转变为角闪石岩。这排除了任何涉及堆积过程形成角闪石岩的解释；这些岩体是假堆积体。相反，寄主辉长质片麻岩被替代形成角闪石岩，是由于高熔体通量通过下地壳形成通道所致。高熔体/岩石比率以及迁移的岩浆（花岗闪长岩）与其寄主辉长质片麻岩之间的不平衡导致片麻岩溶解（颗粒尺度的岩浆同化作用），并使迁移的岩浆中主要结晶出角闪石。这种反应 - 替代机制的程度表明，此类角闪石岩岩体勾勒出了重要的熔体通道。因此，下地壳中许多普遍存在的从基性到超基性的细长岩体可能标示出了“缺失”的物质转移带。
