Enormous numbers of basaltic dikes crosscut the Skaergaard pluton and surrounding rocks. They occur in many different orientations, and they range from centimeter thick dikelets through dikes of normal size to enormous "macrodikes" more than 100 m across. All of the dikes that cut the Skaergaard intrusion are, of course, younger than the pluton itself, though dikes that fed the Tertiary basalt lavas into which the Skaergaard intruded must be older. The Skaergaard therefore straddles the age of the numerous dikes in the area. Dikes are much more numerous and widespread than are indicated in the McBirney (1989) geologic map.
Dikes of normal size. These are ~0.2-10 m across and are common in the Skaergaard pluton and the surrounding Archean gneisses.
Thin basaltic dike crosscutting graded layering within the giant gabbro autolith.
Looking east along a 5 m thick plagioclase-rich dike in the Homestead area (dike labeled "1" in Irvine et al., 1998, just off their Figure 5 to the upper right). This dike is tied with the Campsite composite dike (below) for the oldest dike in the homestead area. Unfortunately the area with the required crosscutting relationships to break the tie is covered. This dike is unusual, containing abundant, large plagioclase phenocrysts in the more central portions of the dike. The dike has a chilled margin having few plagioclase phenocrysts, suggesting that the first batch of magma into this dike did not have so many phenocrysts. Notice the thin chilled margin and some contact-parallel layering in the lower middle part of the picture.
The central portion of the plagioclase porphyry dike in the photo above. Many phenocrysts are acicular and in radiating bunches. Thin section photos are here.
Looking east along the 4 m thick xenolith-rich Campsite composite dike in the homestead area (Dike is that labeled "2" in Figure 5 of Irvine et al., 1998, this photo and my samples are from near label "A" in that figure). This dike is tied with the plagioclase-rich dike (above) for the oldest dike in the homestead area; the area with the required crosscutting relationships to break the tie is covered. This dike is unusual, containing ~30% of xenoliths of sizes ranging from centimeters to meters. The xenoliths are mostly gabbros containing plagioclase and mafic minerals that are more An-rich and Mg-rich, respectively, than any found in the exposed Skaergaard layered series. They may be from the Skaergaard hidden zone, but if so geochemical modeling demands a much larger hidden zone than the more recent gravity modeling suggests. Gravity modeling suggests that there is very little hidden zone, and what there is of it is probably like UBZT and MBST. Thin section photos of the dike are here and of a xenolith are here.
Stuck for the day in the small fjord on the southwest side of Kraemer Island. The northwest side of this fjord, shown here, has Archean gneisses cut by numerous dikes of many widths and orientations.
Splays at the end of a basalt dike cutting the giant gabbro autolith on the east end of Kraemer Island adjacent to Uttental Sund. The en echelon continuation of the dike is to the upper left, which also ends a few meters beyond the photo to the lower left.
A basaltic dike cutting LZb on Kraemer Island, containing numerous xenoliths of felsic gneiss (not from Skaergaard). View is approximately to the east toward Uttental Sund, barely visible at the upper left.
One of the so-called "tramway dikes", wide basaltic dikes of great length that traverse the Skaergaard and surrounding gneisses, particularly on the western side of the pluton. This one has been broken up into gravel that makes a surprisingly smooth, flat roadbed. View is to the north, with the rise in the background being the Uttental Plateau. The western contact of the Skaergaard is visible on the upper left. The mountains in the distance are on the other side of Watkins Fjord.
A 10 m wide dike with a remarkably smooth, flat road-like surface made by oriented basaltic rocks. View is to the west.
Macrodikes. These are dikes of enormous size, 100 m or more thick, and are difficult to photograph from the ground. The Basistoppen sheet was probably emplaced by a macrodike that extends northeast from Douglas Plateau. The photos below are of the Mikis Fjord macrodike. These two macrodikes, at least, are coarse-grained and internally layered. A major problem of macrodikes is how they can be emplaced as single units.
View to the southwest of the macrodike surface and the ship beyond. The macrodike extends approximately straight ahead, on this photo, across the fjord. However, the macrodike is not seen again in this direction; it does not show up on the other side of the fjord. It is either terminated along a syn-emplacement fault, or runs along the fjord floor out to sea.
Closeup view of the macrodike interior rock, showing large plagioclase and pyroxene crystals. This area has a lot more vegetation than any place on the Skaergaard, and it was hard to find a place to photograph where there was no lichen.