Gabbroic anorthosite replacement rock is more leucocratic (plagioclase-rich) than the host rock, and many of the mafic minerals appear as interstitial oikocrysts (sieve texture). Gabbroic pegmatites are very common throughout the pluton and have mineralogy identical to the host rock but are vastly coarser, commonly with a coarsening-inward texture. Next are so-called “Christmas tree” structures found as meter-scale, layer-parallel, dendritic concentrations of mafic or felsic minerals. I only saw these in the Homestead area in Layered Series UZa, but they may be more widespread. These are certainly related to the gabbroic anorthosite replacement rock, though stratabound and in more ornate shapes. Last are amphibolitized fractures, basically subsolidus cracks that had hot water percolating through them. They involved hydration of the adjacent host rock mineral assemblages, and deposition of amphibole and other minerals. This sort of replacement extends up to several centimeters from the fractures. How is it known that these are replacement features and not injected liquids or mush? They do not disrupt or displace the layering, but rather cut through it without disturbance.
Gabbroic anorthosite replacement rock
Replacement features are widespread. They generally occur as rather homogeneous, irregular bodies of plagioclase-rich rock that have replaced the host gabbro of any type. Replacement gabbroic anorthosite has the same mineralogy as the host rock, just more leucocratic. The mafic minerals, especially pyroxene, tend to occur as large sieve-textured oikocrysts. I fancy that replacement gabbroic anorthosite bodies form as residual liquid in the crystal pile becomes progressively plagioclase-richer as it becomes more hydrous (increasing fH2O displaces the liquidus cotectic toward plagioclase and to lower temperature). This migrating liquid can then replace rock. The margins of replacement anorthosite are amazingly sharp.
Irregular anorthositic replacement rock, just south of trough E in the Homestead area. On the left side of the image the replacement rock contains internal layering. I don’t recall if this layering is supposed to be relic from the replaced rock, or a feature of the replacement rock itself.
Irregular anorthositic replacement rock just west of trough C, Homestead area. Notice the dark, oxide-rich segregations associated with the margins of this body.
Replacement anorthosite body in LZa in the crossbedded belt, Uttental Plateau. Note the thin mafic layer at its base. The spotted texture of this and other similar bodies is caused by abundant mafic mineral oikocrysts (oinkocrysts if made of pigeonite…get it? Get it? Abbreviation: ‘pig’? Heh, heh). Thin section here of a replacement body near this one.
A larger gabbroic anorthosite replacement body, just northeast of the summit region on the western side of Uttental Plateau.
Another small replacement anorthosite also near the summit of Uttental Plateau. This one is sub-parallel to some crossbedded layering, and has a mafic layer on the bottom.
Close-up view of the replacement anorthosite in the photo immediately above, showing abundant oikocrystic pyroxene and a small, unreplaced island of gabbro.
The remarkable “gabbroic anorthosite diapir,” west side of Wagers Peak in LZb. This a complex structure is not fully understood. It has a flat base and a mushrooming top, with upturned beds on the right and less steeply upturned beds on the left. Just right of the top of the structure the beds are disturbed (see photo below). One idea is that this is a diapir of liquid or mush that pushed aside and broke the overlying layers. The resulting bump then caused turbulence in the magma that caused deposition of the disturbed bed visible in the photo below. The rising diapir must have been fed from considerable depth to give the anorthositic mass sufficient hydrostatic head to raise and break the 2-3 m of semiconsolidated gabbro crystal mush. For an alternative idea, see below.
Another idea is that this body is actually a surface deposit of relatively hydrous, plagioclase-rich residual liquid that percolated upward through the crystal mush floor cumulates. As the residual liquid percolated to the top of the cumulate pile, the plagioclase-rich liquid froze as it lost water to the less hydrous magma above. As more plagioclase-rich liquid percolated to the surface, the mushroom-shaped body grew, forming a pedestal-like obstruction that resulted in the deposition of onlapping layers, and the disturbed layer on the downstream side. In this model the upturned beds therefore are primary depositional features, like upturned beds on the flanks of autoliths. This photo is from slightly to the south of the mushroom, showing another body of gabbroic anorthosite, and the striking disturbed bed to the right of the mushroom. There are also oddly, somewhat dendritic shaped replacement bodies, below the large gabbroic anorthosites, that are made of upper felsic material and lower mafic material.
Photo of the rock that makes up the body of replacement anorthosite below and to the right of the mushroom, in the photo immediately above.
Small mushroom-shaped anorthositic replacement (?) bodies in a medium size autolith block on southeastern Kraemer Island. These replacement bodies predate detachment, and so formed in the roof region. Notice how the anorthositic material has more mafic margins in their lower parts, but not at the caps.
Gabbroic pegmatites
The gabbroic pegmatites consist of irregular to stratiform bodies of coarse-grained gabbro, having mineralogy like the host rock. Commonly these bodies are inward-coarsening.
Mafic pegmatite replacement bodies in the Homestead area. Stratigraphic tops are to the upper right.
Close-up of the mafic pegmatite in the photo above. White and gray are plagioclase, black is pyroxene and oxides, and rusty patches are olivine. These pegmatites are frequently associated with the anorthositic replacement features.
Stratabound mafic pegmatite in the Homestead area.
Close-up of the mafic pegmatite in the photo immediately above. Visible are white and gray plagioclase, brown pyroxene, black magnetite, and small quantities of very rusty interstitial olivine.
Small mafic pegmatite on the Uttental Plateau. Stratigraphic top is toward the top of the photo.
Unusual small mafic pegmatite in LZa on Uttental Plateau, that grades from mafic to felsic, base to top, and appears to be continuous with a replacement gabbroic anorthosite body at its top. The mafic pegmatites and replacement anorthosites appear to be related in many places, possibly related to the availability of water and its control on residual liquid phase relations.
Colliform layering in LZb* on Mellemø Island, with pegmatites close to the coliform margin. The pluton interior is to the right.
A mafic pegmatite in UZc. Note that the interior of this pegmatite has abundant, fine-grained, gray granophyre.
Another photo of the same mafic pegmatite as in the one immediately above, showing contact with host gabbro. An angular notch in the contact suggests, though does not prove, an intrusive relationship.
An autolith block bottom, with slightly deformed layers below and mafic pegmatite along the bottom contact. Pegmatite on block bottoms was fairly common, implying that the blocks were relatively impermeable to upward-percolating liquids. Stratigraphic top is to the right.
Christmas tree structures
These were seen by me only in UZa in the Homestead area, though their distribution may be more widespread. These structures are stratabound, confined to particular horizons that are typically the felsic tops of graded layers. There are both felsic varieties (most common) and mafic varieties (less common and less well developed).
“Christmas tree” anorthositic replacement features, confined to bedding planes. The fluids (probably plagioclase-saturated relatively H2O-rich residual liquid) may have been confined to the bedding planes in this region because of particularly impermeable layers.
Extensive “Christmas tree” anorthositic replacement features on a bedding surface. These typically occur at the top of a leucocratic graded bed.
Mafic Christmas tree structures in the same bedding plane as the anorthosite Christmas tree structures in the photo above, which are also visible at the top of this photo. Note that the mafic trees are in the same bedding plane as the anorthositic trees.
Amphibolitized fractures
These are subsolidus fractures to which hot water gained access to the Skaergaard rock. Hydrothermal alteration of the gabbro adjacent to the fractures ranges from ~0.5 to several centimeters. There are apparently several different orientations of these fractures.
Close-up of some of the pervasive subsolidus fractures that are variously filled with pyroxene, amphibole, and other minerals that formed after the solid pluton was cracked, allowing fluids to penetrate. These cracks occur in many different azimuths, though most are subvertical.
