Sedimentary layering in mafic intrusions: the Skaergaard trough bands

The trough bands of the Skaergaard Intrusion comprise individual vertical stacks of linear, upwardly concave, modally graded layers, separated by layers of homogeneous gabbro. The individual stacks are separated by homogeneous gabbro, and point towards the intrusion centre from the nearby vertical wall on the western margin of the intrusion.

The origin of the trough bands is contentious, with some advocating a sedimentary origin, whereas others argue for a formation by post-accumulation recrystallisation during localised compaction. Individual crescentic modally graded layers are present throughout the Layered Series below Upper Zone a (UZa), with clear erosional bases attesting to their sedimentary origin. Similar erosional surfaces are associated with changes in axial position or width of the troughs.

Electron backscatter diffraction analysis shows that modally graded layers exhibit either a strong mineral lineation parallel to the trough axis or a strongly defined foliation parallel to the modal layering. Conversely, the homogeneous gabbro, both that within the stacks of modally graded layers and that separating the stacks, shows either a weak foliated fabric or is entirely without fabric. The absence of any evidence of deformation microstructure and the dominance of facetted plagioclase crystals (precluding dissolution-reprecipitation) exclude significant compaction. Instead, the fabrics formed by grain re-arrangement in localised magmatic currents that formed long-lived channels: the layers of homogeneous gabbro within the stacks accumulated during relative lulls in the channelized flow, while that separating the stacks represents the channel banks. Evidence from dihedral angle variation and the morphology of late-stage gabbroic pegmatites suggests that the floor mush near the wall at UZa times was anomalously thick. We argue that crescentic modally graded layers are a normal component of sedimentation on the floor of the chamber, and that the unusually extensive stacks of trough bands in UZa are a consequence of temporary overthickening and steepening of the floor as a consequence of localised catastrophic collapse of the nearby wall mushy layer. Support for this suggestion is provided by the presence of highly elongate plagioclase crystals in the Marginal Border Series.