By definition, xenoliths are foreign blocks entrained by a magma, blocks that are not derived from the magma itself (as are the autoliths). Xenoliths are therefore generally blocks of rock broken off of the walls or roof of the pluton. In the Skaergaard pluton itself, I saw four different kinds of xenoliths: basalt lava flow xenoliths in the MBS, sedimentary xenoliths in the MBS, wherlite (olivine-CPX) xenoliths in the MBS, and basalt lava flow xenoliths in the layered series. All of the wall xenoliths are in the marginal border series, MBST. The roof xenoliths we saw are in the layered series, specifically the MZ.
Basalt xenoliths in the marginal border series were best seen adjacent to the Skaergaard pluton contact on the southwestern point of Ivnarmiut Island. These xenoliths include angular blocks of massive basalt and basalt that appears to have white amygdules. I took no samples of these so I don't know what the white things are made of.
Photo of a large basalt xenolith, and several smaller ones, several of which are vesicular (bottom center). Skaergaard contact is a few meters to the upper left, under the wate.
Closeup of a vesicular basalt xenolith in the MBST Skaergaard gabbro.
Cretaceous sedimentary rocks were probably once extensive in the Skaergaard region. Now they are mostly eroded away or hidden beneath the Tertiary lavas (see geologic map). Blocks of these sediments were incorporated into the marginal border series, MBST. Unlike the rather refractory basalt xenoliths, the sediments underwent profound contact metamorphism, partial melting, and partial reaction with the enclosing magma.
Not a xenolith, but the best example we saw of the layered Cretaceous sediments, here at the Skaergaard gabbro contact on Mellemø Island. Dark rocks above are the Skaergaard chilled margin gabbros, below which are ~0.8 m of contact metamorphosed Cretaceous sediments, below which on the exposed subhorizontal surface is the Archean gneisses on which the Cretaceous sediments unconformably lie.
Ivnarmiut Island
A sedimentary xenolith several meters from the Skaergaard pluton contact. This is a small xenolith with a granofels interior and a partially melted and chemically modified margin. Note the reaction rim on the surface of the interior xenolith.
A somewhat larger xenolith than that in the photo above, with layering that has been differentially melted and reacted.
Closeup of the xenolith in the photo above. Though difficult to tell in the dark and in the rain, it looks like this sample contains cordierite, spinel, feldspar, and rutile.
Composite block of gabbro containing numerous sedimentary xenoliths, which was then broken off and engulfed by gabbroic liquid.
Southeastern Kraemer Island
Xenolith breccia at ~160 m near the western Skaergaard contact on southeastern Kraemer Island. Most of the smaller xenoliths here are ultramafic, probably similar to the wherlite on the Uttental Plateau. The large, light-colored, layered, elongate block in the center is metamorphosed Cretaceous sediment, containing spinel, magnetite, ilmenite, and cordierite. This is the same area as that shown in Irvine et al. (1998), their Figure 8.
Wherlite xenoliths occur in in MBST along the west and especially northwest side of the pluton. These rocks are much more primitive (i.e., Mg-rich) than any of the exposed Skaergaard pluton, and are considered to be blocks of a preexisting ultramafic body which the Skaergaard intruded.
Photo of the Marginal Boarder Series near the contact, showing ultramafic xenoliths (cumulate wherlite with interstitial plagioclase) included within the gabbro. The olivine in the xenoliths is Fo80-85, which is too Mg-rich for Skaergaard if the hidden zone is small, as recent gravity models indicate. It seems likely that wherlite is from ultramafic cumulates in a somewhat older Tertiary pluton that was intruded by the Skaergaard. Such bodies are found in the area.
Closer view of a fresher surface, showing densely packed wherlite xenoliths, larger grains visible are partially serpentinized olivine. Thin section photos here.
Closeup of one of the wherlite xenoliths, the larger crystals being partially serpentinized olivine, with smaller pyroxene crystals and interstitial plagioclase.
Photo of more of the ultramafic xenoliths near the contact at ~160 m elevation on southeastern Kraemer Island. The large, layered sedimentary xenolith exposed here is on the far left.
Photo of a nicely weathered outcrop at the same location as the photo above, showing the ultramafic xenoliths and a few xenoliths of partially melted sediment.
Basalt xenoliths in the layered series are especially abundant in one part of the MZ in the southeastern Kraemer Island. These are distinctive from the basalt xenoliths in MBZT in that their degree of contact metamorphism and recrystallization is much more severe. In thin section, textures are metamorphic.
Top of a large basalt xenolith block, showing the sharp contact, some magnetite in a reaction rind, and small magnetite-rich gabbro dikelets that were emplaced in fractures during or soon after block emplacement. Thin section here.
Closeup view of one of the larger magnetite-rich dikelets cutting the block in the photo above. These dikes essentially have MZ mineralogy, though they are somewhat richer in magnetite than most of the MZ.
A smaller, rusty, olivine-bearing xenolith is above the photo center, and another one is visible on the lower right. Thin section photos here.
Closeup of the xenolith in the photo above showing the rounded outer margin of a rusty, olivine-bearing basalt xenolith, showing a prominent oxide-rich reaction rind. This block has pits weathering out in a pattern that resembles amygdules, though they may be something else (see below).
Photo of a block bearing layered rocks of the UBS, presumably UBST, and attached basalt, therefore representing a foundered piece of the Skaergaard roof contact. The basalts are the irregularly mottled rock on the left, under and to the left of the woman in red, and the layered UBST rock is under all the people wearing blue. The contact is between white pants and red pants. The UBST layering in the block dips steeply to the left, whereas the layering in the host MZ rock dips shallowly down the slope in the background.
Photo of a large rusty basalt block above, with MZ gabbro below bearing small chips of rusty, fine-grained basalt.
Closeup of the pits in a rusty basalt block. These pits are not shaped like round amygdules, but rather look like irregular, perhaps radiating, crystals or oikocrysts.