Supplementary files for: Petrology of the Skaergaard Layered Series

The Skaergaard intrusion is a layered, ferrobasaltic intrusion emplaced during the Early Eocene into the rifting volcanic margin of East Greenland. The magma chamber crystallised in response to cooling from the roof and margins upwards and inward, forming upper, marginal and bottom series, the latter referred to as the Layered Series. The phase layering in the bottom series suggests an evolved, olivine-normative tholeiitic melt saturated in plagioclase and olivine, followed by augite, and then simultaneously by ilmenite and magnetite forming primocrysts. Pigeonite appears in the lower parts and continues until the centre of the series. Apatite appears in the upper part concurrently with liquid immiscibility. Cryptic variations of the individual primocrysts record a systematic upward increase in iron and decrease in magnesium for the mafic minerals and a systematic increase in sodium and decrease in calcium for plagioclase. The appearance of pigeonite is caused by reactions and crystallisation in the trapped melt and by subsolidus adjustments without this phase reaching liquidus saturation. The high mode of olivine at the base of the upper part with the appearance of apatite is interpreted to mark the onset of liquid immiscibility. This may have led to the separation of conjugate melts with granophyre migrating upward and the basic component largely staying stationary or sinking. Petrologic and geochemical observations indicate differentiation in the lower part of the intrusion, principally controlled by crystal fractionation with the efficiency of fractionation controlled by the evolution and escape of liquid from the solidifying mush. During the final stages of solidification, the onset of liquid immiscibility and termination of melt convection impeded differentiation. Modelling by perfect Rayleigh fractionation shows that major and included trace elements conform reasonably to observations, while excluded elements deviate from model predictions. This decoupling is caused by the mobility of a granophyre component formed in the trapped melt and in the main residual magma chamber. Consequently, the sampled gabbros may not be representative of the final solid-melt mush. By restoring the gabbros to their original mush compositions, it is possible to constrain granophyre migration pathways. We suggest that the granophyre formed in the trapped melt in the lower part of the intrusion mostly migrated laterally through pressure release pathways to form lenses and pockets with only limited upward migration into the main magma reservoir. Near the end stage of differentiation, the residual magma exsolved and formed complex mixtures of ferrobasaltic and granophyric melts. Estimates predict that a substantial amount of the granophyric melt penetrated as sills into the downward crystallising, upper part of the body as well as into the host rocks. The redistribution of granophyric melts within the solidifying crystal mush complicates predictions of trapped-melt content and mass-balance calculations but helps to explain apparent decoupling of included and excluded trace elements, especially towards the end stages of evolution. Final crystallisation was controlled mostly by in situ crystallisation leaving complex mixtures of ferrodiorite and granophyre components.

斯凯尔加德侵入体（Skaergaard intrusion）是形成于始新世早期的层状铁铁质基性侵入体，侵入于东格陵兰的裂谷型火山陆缘。岩浆房因从顶部、边缘向上并向内的冷却作用而发生结晶，形成了上部、边缘与底部岩系，其中底部岩系被称为层状岩系（Layered Series）。底部岩系中的矿物相层理表明，演化后的以橄榄石为标准矿物的拉斑玄武质熔体首先达到斜长石与橄榄石饱和，随后析出普通辉石，接着同时析出钛铁矿与磁铁矿作为早期晶斑（primocrysts）。易变辉石（Pigeonite）出露于该岩系下部，并延伸至岩系中部；磷灰石（Apatite）则于上部出现，同时伴随熔体不混溶作用（liquid immiscibility）。单个早期晶斑的隐秘变异记录了：镁铁质矿物的铁含量随向上方向系统升高、镁含量系统降低，斜长石的钠含量系统升高、钙含量系统降低。易变辉石的出现是由捕获熔体中的反应与结晶作用以及固溶线下调整所致，该矿物并未达到液相线饱和。上部底部的高橄榄石含量与磷灰石的出现，被认为标志着熔体不混溶作用的起始，这可能导致共轭熔体分离：花斑岩（granophyre）熔体向上迁移，而基性组分基本保持静止或下沉。岩石学与地球化学观测表明，侵入体下部的分异作用主要受结晶分异作用控制，而分异效率由液体从固结晶粥（solidifying mush）中演化与逸出的过程决定。在固结的最终阶段，熔体不混溶作用的起始与熔体对流的终止阻碍了分异作用。采用理想瑞利分馏（Rayleigh fractionation）的模拟结果显示，主量元素与伴生微量元素与观测结果基本吻合，而排除型微量元素则偏离模型预测。这种解耦现象是由捕获熔体与主残余岩浆房中形成的花斑岩组分的迁移性所致，因此所采样的辉长岩（gabbros）可能无法代表最终的固结晶粥。通过将辉长岩恢复至其原始晶粥组成，可限定花斑岩的迁移路径。我们提出，在侵入体下部捕获熔体中形成的花斑岩，大多通过泄压通道横向迁移，形成透镜体与囊状体，仅有限的部分向上迁移至主岩浆房。在分异作用的临近末期，残余岩浆出溶，形成铁铁质基性熔体与花斑岩熔体的复杂混合物。估算结果表明，大量花斑岩熔体以岩床（sills）的形式侵入至向下结晶的岩体上部以及围岩中。花斑岩熔体在固结晶粥中的再分布，使得捕获熔体含量预测与质量平衡计算变得复杂，但有助于解释伴生与排除型微量元素之间的表观解耦现象，尤其是在演化的末期阶段。最终的结晶作用主要受原地结晶作用控制，形成了铁英闪长岩（ferrodiorite）与花斑岩组分的复杂混合物。