The so-called "tranquil zone" is the steeply dipping marginal border series (MBS) between the chilled margin (MBST) and the shallowly dipping layered series adjacent to and continuous with the MBS. Our tour of this region suggests that it was dominated by growth of crystals on the steeply dipping walls, principally as "colliform layering". Perpendicular feldspar rock, wavy pyroxene rock, and possible slumps also occur in this zone. This zone was probably not particularly tranquil, but rather had fast-moving currents flowing down it.
Colliform layering is not smooth and planar, like that in the layered series, but instead is made up of inwardly convex bulges typically 0.5 to 2 m across with intervening, outward-pointing cusps. Vertical sections show that these layers are almost straight and parallel, so these colliform layers are essentially nested corrugations of otherwise parallel continuous layers.
This next set of photos are from Mellemø Island
Photos here and below are from Mellemø Island. Colliform layering, probably in the Marginal Border Series unit LZb*. The pluton interior is to the left, so this colliform layering has the rounded parts pointing inward, and the cusps pointing outward. These are growth features and not erosional.
Colliform layering also in LZb*. The layering here is nearly vertical.
Colliform layering in LZb*, with pegmatites close to the coliform margin. The pluton interior is to the right.
Looking southeast across the plateau on top of Mellemø Island, showing extensive near vertical coliform layering in LZb* extending into the distance.
Vertical cross section through steeply dipping colliform layers, showing that in this section the layers are nearly parallel. The colliform layers are therefore like parallel, nested lengths of celery. There are replacement pegmatites on the left and right.
Impressive colliform layering, showing the concentric layers that represent nested growth structures.
Photo of very nice colliform layering in LZb*, pluton center is to the right.
Interesting texture of perpendicular feldspar rock in colliform layers, showing the perpendicular feldspars in one of the narrow colliform layer cusps, which are sharp compared to the convex parts but certainly not points. The feldspars crowd toward the center of the cusp. Very much like Figure 45 in Irvine et al. (2001b).
Perpendicular feldspar rock refers to rock in the marginal border series with tree-like dendritic growths of plagioclase crystals that are more or less perpendicular to the layering surfaces. It is not supposed that these were growing as dendrites far out into the liquid, but rather were probably growing at or very close to the solid surface. These feldspars tend to be more An-rich than the typical feldspars in the host rock.
This set of photos are from Mellemø Island.
Perpendicular feldspar rock ~10 m in from the Skaergaard contact with Archean gneisses. These probably grew very close to the rock-magma interface and probably did not protrude far into the magma. Note that they grew in dendritic forms, rather than as straight single crystals. Maybe perpendicular feldspar rock represents the formal end of the contact or chilled zone and the start of the "Tranquil Zone", which despite its name appears to have had a lot of action. Thin section photos here.
More perpendicular feldspar rock from a few meters away from that in the photo above.
More perpendicular feldspar rock from a several meters farther into the pluton than the two photos above.
This photo is from Ivnarmiut Island. Perpendicular feldspar rock from the colliform layering section, from near the same location as this photo.
This photo is from southeastern Kraemer Island. Photo of the very common and dense perpendicular feldspar rock in this area. Notice that the feldspars are not quite so perpendicular here as in some places.
This photo is from Uttental Plateau. Perpendicular feldspars growing upward from the base of the scoured crossbed surface.
Wavy pyroxene rock is rock in the marginal border series that has numerous, small, irregular mafic pegmatite segregations. This rock can also have the larger replacement mafic pegmatite bodies that are common elsewhere in the pluton.
The next several photos are from Mellemø Island.
This and the following photos are from Wavy pyroxene rock, which here is an olivine gabbro with small, wavy, pegmatitic stringers within it. The pegmatitic zones contain abundant large pyroxenes. Bedding here is dipping 70-80° to the east (left).
Closeup of one of the wavy pyroxene stringers, showing thin but irregular shape, large pyroxenes and plagioclase, and the matrix olivine gabbro.
More wavy pyroxene rock.
This photo is from Ivnarmiut Island. Photo of large pyroxenes in wavy pyroxene rock ~10 m from the Skaergaard contact.
This photo is from southeastern Kraemer Island. Spectacular wavy pyroxene rock ~10 m inside the pluton margin at an elevation of ~70 m. Layering is nearly horizontal, possibly suggesting a ledge. There is lots of perpendicular feldspar rock in this area.
This photo is from Mellemø Island. Apparent slump folds in the Tranquil Zone 20 or 30 m from the Skaergaard contact.