The rock type most commonly associated with komatiite throughout Earth’s history is tholeiitic basalt. Despite this well-known association, the link between komatiite and basalt formation is still debated. Two models have been suggested: that tholeiitic basalts represent the products of extensive fractional crystallisation of komatiite, or that basalts are formed by lower degrees of mantle melting than komatiites in the cooler portions of a zoned plume. We present major and trace element data for tholeiitic basalts (∼7.5 wt% MgO) and dunites (46 – 48 wt% MgO) from the Palaeoproterozoic Winnipegosis Komatiite Belt (WKB), which, along with previous data for WKB komatiites (17 – 26 wt% MgO), are utilised to explore the potential links between komatiite and basalt via crystallisation processes. The dunites are interpreted as olivine + chromite cumulates that were pervasively serpentinised during metamorphism. Their major and immobile trace element relationships indicate that the accumulating olivine was highly magnesian (Mg# = 0.91 – 0.92), and that small amounts (4 – 7 wt% on average) of intercumulus melt were trapped during their formation. These high inferred olivine Mg#s suggest the dunites are derived from crystallisation of komatiite. The tholeiitic basalts have undergone greenschist facies metamorphism and have typical geochemical characteristics for Palaeoproterozoic basalts, with the exception of high FeO contents. Their REE patterns are similar to Winnipegosis komatiites, although absolute concentrations are higher by a factor of ∼2.5. The ability of thermodynamic modelling with MELTS software to reproduce komatiite liquid lines of descent (LLD) is evaluated by comparison to the crystallisation sequence and mineral compositions observed for Winnipegosis komatiites. With minor caveats, MELTS is able to successfully reproduce the LLD. This modelling is extended to higher pressures to simulate crystallisation of komatiitic melt in an upper crustal magma chamber. All major and rare-earth element characteristics of the tholeiitic basalts can be reproduced by ∼60% crystallisation of a Winnipegosis komatiite-like parental melt at pressures of ∼1.5 – 2.5 kbar at oxygen fugacities between QFM – 1 and QFM+1. Winnipegosis basalts have low Mg#s that are not in equilibrium with mantle peridotite. They therefore cannot represent primary mantle melts derived from cooler mantle than the komatiites, and require fractional crystallisation processes in their formation. Furthermore, their trace element characteristics indicate a depth of melting indistinguishable from that of the Winnipegosis komatiites, and derivation from an identical depleted mantle source. All geochemical and geological evidence is therefore consistent with their derivation from a komatiitic melt, and the presence of a large komatiite-derived dunite body in the WKB provides evidence of extensive fractionation of komatiite in the upper crust. The observed uniform basalt compositions are interpreted as the result of a density minimum in the evolving komatiitic melt at temperatures between clinopyroxene and plagioclase saturation, with efficient extraction of melt from a mixture containing ∼60% crystals. We conclude that the WKB basalts formed by fractional crystallisation of a komatiitic parental melt, and suggest that this model may be more broadly applicable to other localities where komatiites and associated basalts show similar geochemical or isotopic characteristics.

中文翻译：

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加拿大马尼托巴古元古代温尼伯病科马蒂岩带的科马蒂岩，玄武岩和dunits之间的部分结晶链接

在整个地球历史上，与高锰铁矿最常见的岩石类型是高辉石玄武岩。尽管存在这种众所周知的联系，但仍在争论科马铁矿与玄武岩形成之间的联系。有人提出了两种模型：玄武岩代表高岭土的广泛分步结晶产物，或者玄武岩是由地幔融化程度低于高岭土在层状羽流较冷部分形成的。我们提供了古元古代温尼伯病科马蒂岩带（WKB）的玄武岩玄武岩（〜7.5 wt％MgO）和杜尼二烯（46 – 48 wt％MgO）的主要和微量元素数据，以及WKB科玛蒂岩的先前数据（17 – 26） wt％MgO），用于通过结晶过程探索科马铁矿和玄武岩之间的潜在联系。dunites被解释为在变质过程中普遍蛇纹石化的橄榄石+亚铬酸盐累积物。它们的主要元素和不可移动的痕量元素关系表明，积累的橄榄石是高镁质镁（Mg＃= 0.91 – 0.92），并且在形成过程中捕获了少量（平均4 – 7 wt％）的积云。这些高推断的橄榄石Mg＃s提示，杜尼二烯衍生自科马铁矿的结晶。除高FeO含量外，高生玄武岩经历了绿片岩相变质作用，并具有古元古代玄武岩的典型地球化学特征。它们的REE模式类似于温尼伯蠕虫科玛氏体，尽管绝对浓度高约2.5倍。通过与观察到的温尼伯病科玛替身的结晶顺序和矿物组成进行比较，评估了使用MELTS软件进行热力学建模以再现科玛替身下降血统（LLD）的能力。MELTS可以在不加警告的情况下成功复制LLD。该模型被扩展到更高的压力，以模拟上地壳岩浆室内的科迈特熔体的结晶。可以通过在QFM-1和QFM + 1之间的氧逸度下，在约1.5-2.5 kbar的压力下，温尼伯象氏钾铁矿样母体熔体的约60％结晶，来重现玄武岩玄武岩的所有主要和稀土元素特征。温尼伯戈斯玄武岩的Mg＃s低，与地幔橄榄岩不平衡。因此，它们不能代表比科玛铁矿更冷的地幔衍生的地幔初熔体，并且在其形成过程中需要分步结晶过程。此外，它们的痕量元素特征表明其融化深度与温尼伯巨噬球菌的融化深度没有区别，并且来自相同的贫化地幔源。因此，所有的地球化学和地质证据都与它们从胶体熔体中衍生而来的一致，并且在WKB中存在大量的源自钾铁矿的榴辉岩体，这提供了上部地壳中钾铁矿的广泛分馏的证据。观察到的均匀玄武岩组成被解释为，在斜辉石和斜长石饱和度之间的温度下，演化的科迈特熔体中的密度最小，从含有约60％晶体的混合物中有效提取熔体。我们得出的结论是，由科迈特亲代熔体的分步结晶形成的WKB玄武岩，并表明该模型可能更广泛地适用于科马铁矿和相关玄武岩表现出相似地球化学或同位素特征的其他地区。