Mafic sills of the Marginal Zone are widespread within the footwall of the Bushveld Complex with some of them being enclosed in later Bushveld intrusions. In the central sector of the northern limb of the Bushveld Complex on the Turfspruit farm, a sill of norite enclosed within the Lower Zone cumulates contains numerous semi-assimilated fragments of anorthositic and magnetite-orthopyroxenitic lithologies that are interpreted to be the partially melted xenoliths of local argillaceous, magnetite-bearing, and anhydritic shales. Almost pure anorthite with eutectic inclusions of microgranular Al-rich orthopyroxene composes the interstitial material in the Lower Zone chromite-olivine cumulates at the contact with the magnetite-bearing norite. An identical association of anorthite+Al-rich orthopyroxene occurs in the overlying Platreef pyroxenite at the contact with anhydritic hornfels suggesting that an exchange reaction has likely involved anhydrite in both cases. Strontium isotope signatures of plagioclase from the norite support its hybrid origin, whereas Sr isotopes in anorthite from harzburgite is within the range of the most primitive Bushveld rocks indicating a predominant contribution of magmatic Sr. We suggest that the observed mineral assemblages of Al-rich and Cr-poor orthopyroxene, Fe-rich anorthite with elevated Ga, and Al-rich, Cr-depleted spinel, as well as the changes in mineral chemistries towards those compositions, are indicative of assimilation of evaporites and argillaceous shales by Bushveld melts on Turfspruit and elsewhere in the northern limb.

中文翻译：

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Bushveld 熔体同化的混杂紫砂岩和泥质页岩到硬质页岩的归宿

边缘区的镁铁质基岩广泛分布在布什维尔德复合体的下盘内，其中一些被封闭在后来的布什维尔德侵入体中。在 Turfspruit 农场的 Bushveld 复合体北翼的中央部分，下带堆积体中的一块紫砂岩基石包含许多半同化的斜长岩和磁铁矿-正辉石岩岩性碎片，被解释为部分熔融的捕虏体当地的泥质、含磁铁矿和硬石膏页岩。几乎纯的钙长石与微颗粒富铝斜方辉石的共晶包裹体构成了下带铬铁矿-橄榄石中的间隙材料，在与含磁铁矿的紫长石接触处堆积。在与硬石膏角岩接触的上覆 Platreef 辉石中出现了相同的钙长石 + 富含铝的斜方辉石的关联，这表明在两种情况下交换反应都可能涉及硬石膏。来自蓝长石的斜长石的锶同位素特征支持其混合起源，而来自斜长石的斜长石中的 Sr 同位素在最原始的 Bushveld 岩石范围内，表明岩浆 Sr 的主要贡献。我们建议观察到的富铝和富铝矿物组合贫铬的斜方辉石、具有升高的 Ga 的富铁钙长石和富铝、贫铬的尖晶石，以及矿物化学对这些成分的变化，表明 Bushveld 熔体在 Turfspruit 和在北翼的其他地方。