Bushveld anorthosites commonly contain the so-called “mottles” comprising irregular, typically centimetric domains of oikocrystic pyroxene or olivine enclosing small, embayed plagioclase grains. The mottles were traditionally interpreted to result from solidification of trapped intercumulus liquid or via in situ crystallisation at the top of the crystal mush. Here, we present microtextural and compositional data of a mottle to place further constraints on the formation of anorthosite layers. Element maps generated by scanning electron microscopy reveal that plagioclase within and around the mottle has markedly elevated An contents (up to An95) relative to the host anorthosite and is strongly reversely zoned. Other unusual features, some of which were reported previously, include a halo of sub-vertically oriented, acicular phlogopite around the mottle, elevated contents of disseminated sulfides, and relatively evolved yet Ni-rich olivine (Fo71–75, 3000 ppm Ni). These features are interpreted to result from reactive porous flow of hot, acidic fluid enriched in nickel and sulfur through proto norite. The fluids dissolved mafic minerals and leached alkalis from the outer rims of plagioclase grains. Reconnaissance studies suggest that reversed zoning of plagioclase is a common feature in Bushveld norite and anorthosite. This implies that reactive porous flow could have been far more pervasive than currently realised and that Bushveld anorthosite layers formed through recrystallisation of norites.

中文翻译：

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反应性多孔流形成布什维尔德斜长岩

Bushveld 斜长岩通常包含所谓的“斑驳”，包括不规则的、典型的厘米范围的 oikocrystic 辉石或橄榄石，包围着小的、斜长石颗粒。这些斑点传统上被解释为由被困的积云液体凝固或通过在结晶糊顶部的原位结晶造成的。在这里，我们展示了斑驳的微观纹理和成分数据，以进一步限制斜长岩层的形成。扫描电子显微镜生成的元素图显示，斑驳内部和周围的斜长石相对于寄主斜长岩显着升高了 An 含量（高达 An95），并且强烈反向分区。其他不寻常的特征，其中一些以前曾报道过，包括一个次垂直方向的光环，斑驳周围的针状金云母，弥散硫化物含量升高，相对进化但富含镍的橄榄石（Fo71-75，3000 ppm Ni）。这些特征被解释为由富含镍和硫的热酸性流体通过原苏长岩的反应性多孔流动引起的。这些流体溶解了镁铁质矿物并从斜长石颗粒的外缘浸出碱。勘察研究表明，斜长石的反向分带是布什维尔德紫长石和斜长岩的共同特征。这意味着反应性多孔流动可能比目前意识到的要普遍得多，而且布什维尔德斜长岩层是通过蓝长岩的再结晶形成的。这些特征被解释为由富含镍和硫的热酸性流体通过原苏长岩的反应性多孔流动引起的。这些流体溶解了镁铁质矿物并从斜长石颗粒的外缘浸出碱。勘察研究表明，斜长石的反向分带是布什维尔德紫长石和斜长岩的共同特征。这意味着反应性多孔流动可能比目前意识到的要普遍得多，而且布什维尔德斜长岩层是通过蓝长岩的再结晶形成的。这些特征被解释为由富含镍和硫的热酸性流体通过原苏长岩的反应性多孔流动引起的。这些流体溶解了镁铁质矿物并从斜长石颗粒的外缘浸出碱。勘察研究表明，斜长石的反向分带是布什维尔德紫长石和斜长岩的共同特征。这意味着反应性多孔流动可能比目前意识到的要普遍得多，而且布什维尔德斜长岩层是通过蓝长岩的再结晶形成的。