Abstract

Rocks with chemical compositions similar to Cenozoic boninites occur in many Archean cratons (boninite-like rocks), but they are rarely well-preserved, well-sampled, or presented within chrono- and chemo-stratigraphic context. This study provides a detailed description of the most extensive and well-preserved Archean boninite-like rocks reported to date. Within the 2820 to 2740 Ma magmatic suites of the northwest Youanmi Terrane, Yilgarn Craton, boninite-like rocks occur as two distinct units. The first boninite-like unit is thinner (several 10 s of m thick), occurs close to the base of the 2820–2800 Ma Norie Group and includes both volcanic flows and subvolcanic intrusions. The second boninite-like unit is thicker (locally several 100 s m), occurs near the base of the 2800–2740 Ma Polelle Group and consists of mainly fine-grained volcanic flows with local cumulate units. On average, major and trace element compositions for Youanmi Terrane boninite-like rocks are marginal between basalt, picrite and boninite and they have asymmetrically concave REE patterns, and Th–, Zr–Hf enrichments, similar to many Phanerozoic low-Si boninite suites, but at generally higher MREE–HREE contents. We report over 300 new whole-rock geochemical analyses, and 16 new Sm–Nd isotopic analyses, and associated petrographic evidence, including representative mineral compositions, which we support with published geochemical analyses and several decades of fieldwork in our study area. Comparison between Archean boninite-like rocks and Cenozoic boninites shows that most Archean examples had less depleted sources. We consider two possible petrogenetic models for the Youanmi Terrain examples: (1) they reflect variably contaminated komatiites, or (2) they reflect melts of metasomatised refractory mantle, analogous to Phanerozoic boninites. Trace element modelling indicates that crustal contamination could potentially produce rocks with boninite-like compositions, but requires an Al-enriched komatiitic parent liquid, for which there is no field evidence in our study area. Initial ε_NdT values in pre-2800 Ma rocks (ε_NdT -0·4 to +1·2) are on average slightly higher than those in 2800–2733 Ma examples (ε_NdT -3·2 to +1·2), compatible with increasing mantle metasomatism involving recycling of ≥ 2950 Ma crust. Integration of trace element and Nd isotopic data demonstrates that significant direct crustal assimilation was restricted to felsic magmas. The Th–Nb and Ba–Th systematics of mafic-intermediate rocks reflect fluid- and sediment-derived processes in the mantle, with boninite-like examples being linked primarily to fluid metasomatism. We compare the well-preserved igneous textures and mineralogy of Youanmi Terrane boninite-like rocks with those of their Phanerozoic counterparts, and based on studies of the latter, suggest that former had similarly hot, H₂O-rich parent magmas. The association of boninite-like rocks in the Norie and Polelle Groups with coeval high-Mg andesites, sanukitoids and hydrous mafic intrusions of the Narndee Igneous Complex strongly suggests a metasomatised mantle source and subduction operating in the Yilgarn between 2820 and 2730 Ma.

中文翻译：

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西北尤安米地貌的太古代博尼石样岩石，伊尔加尔邦克顿：地球化学和成因

摘要

化学成分类似于新生代邦尼石的岩石出现在许多太古宙克拉通中（类似邦尼石的岩石），但很少保存完好，取样良好，也不存在于时间地层和化学地层中。这项研究对迄今报道的最广泛和保存最完备的太古宙邦尼石样岩石进行了详细描述。在西北尤安米地带（Yilgarn Craton）的2820至2740 Ma岩浆组中，类似邦尼石的岩石是两个不同的单元。第一个类似贝尼石的单元较薄（厚约10 s m），位于2820–2800 Ma Norie群底部附近，包括火山岩流和次火山岩侵入。第二个类似boninite的单元较厚（局部为数百平方米），发生在2800-2740 Ma Polelle群底部附近，主要由细粒火山流和局部累积单元组成。平均而言，Youanmi Terrane boninite样岩石的主要元素和微量元素组成在玄武岩，picrite和boninite之间，并且具有不对称的凹面REE模式，以及Th-，Zr-Hf富集，类似于许多杂多生低硅boninite套件，但通常MREE–HREE含量较高。我们报告了300多项新的全岩石地球化学分析，16项新的Sm-Nd同位素分析以及相关的岩相学证据，包括代表性的矿物成分，我们将在已发表的地球化学分析和研究区域的数十年实地研究中提供支持。比较Archean boninite样岩石和新生代boninites可以看出，大多数Archean实例的耗竭源较少。对于Youanmi地形实例，我们考虑两种可能的岩石成因模型：（1）它们反映了不同污染的科马提岩，或者（2）它们反映了交代化的耐火地幔的熔融，类似于Phanerozoic boninites。痕量元素模拟表明，地壳污染可能会产生具有类似次锰酸盐成分的岩石，但需要富铝的Komatiitic母液，在我们的研究区域内没有现场证据。初始ε 但是需要富铝的Komatiitic母液，在我们的研究区域内没有现场证据。初始ε 但是需要富铝的Komatiitic母液，在我们的研究区域内没有现场证据。初始ε2800 Ma以前的岩石中的_NdT值（_εNdT -0·4至+ 1·2）平均略高于2800–2733 Ma例子中的NdT值（_εNdT-3·2至+ 1·2），与不断增加的地幔交代作用有关，涉及≥2950 Ma地壳的再循环。痕量元素和Nd同位素数据的整合表明，重要的直接地壳同化作用仅限于长英质岩浆。镁铁质中间岩石的Th–Nb和Ba–Th系统反映了地幔中流体和沉积物衍生的过程，类似石的例子主要与流体交代作用有关。我们将保存完好的Youanmi Terrane boninite样岩石的火成质地和矿物学特征与其Phanerozoic对应的岩石相比，并基于后者的研究表明，前者具有类似的热H ₂富含O的父母岩浆。Norie和Polelle组中的邦尼石状岩石与Narndee火成岩时代的同时期高镁安山岩，sanukitoids和含水镁铁质岩浆侵入之间的联系强烈暗示了亚尔加尔河在2820至2730 Ma之间发生了交代化的幔源和俯冲作用。