The upper parts of the floor cumulates of the Skaergaard Intrusion, East Greenland, contain abundant features known as troughs. The troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase-rich material. The origin of the troughs and layering is contentious, attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls, or to in situ development by localised recrystallisation during gravitationally-driven compaction. They are characterised by outcrop-scale features such as mineral lineations parallel to the trough axis, evidence of erosion and layer truncation associated with migration of the trough axis, and disruption of layering by syn-magmatic slumping. A detailed microstructural study of the modal trough layers, using electron backscatter diffraction together with geochemical mapping, demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution–reprecipitation. Instead, the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning. We argue that the lineations and foliations are, therefore, a consequence of grain alignment during magmatic flow. Post-accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid, with upwards migration of the Si-rich conjugate into the plagioclase-rich upper part of the layers, whereas the Fe-rich immiscible conjugate remained in the mafic base. Both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal-rich currents descending from the nearby chamber walls.

中文翻译：

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Skaergaard 槽分层：对流岩浆房中的沉积

东格陵兰 Skaergaard 侵入体的上部积层包含丰富的特征，称为槽。低谷是缓慢倾斜的同形结构，包括堆叠的新月模态渐变层，具有清晰的基性基底，向上渐变为富含斜长石的材料。槽和分层的起源是有争议的，不同地归因于从附近墙壁下降的岩浆流沉积矿物颗粒，或在重力驱动的压实过程中通过局部再结晶进行的原位发育。它们的特点是露头尺度特征，例如平行于槽轴的矿物线、与槽轴迁移相关的侵蚀和层截断的证据，以及同岩浆坍塌对层状的破坏。模式槽层的详细微观结构研究，使用电子背散射衍射和地球化学绘图，表明这些岩石没有记录由位错蠕变或溶解-再沉淀引起的变形的证据。相反，槽的特征是自形斜长石晶体与未修改的初级火成岩成分分区的排列。因此，我们认为线理和叶理是岩浆流动过程中晶粒排列的结果。模态分层的累积后放大是由于未混合的不混溶间隙液体的差异迁移而发生的，富硅共轭物向上迁移到层的富含斜长石的上部，而富铁不混溶共轭物仍然存在在黑手党基地。