Skaergaard intrusion photomicrographs

Mineralogy: plagioclase, olivine, primary augite (brown), augite inverted from ferrobustamite (green), oxides, apatite. Cumulus phases are plagioclase, olivine, ferrobustamite, oxides, augite, and apatite. Olivine has a dark, finely speckled appearance because of small, brown and black oxide plates, probably unmixed from the olivine during subsolidus oxidation. Notice that the green augite is generally in crystal cores, and the brown augite is on the rims. The two are optically continuous. No separate grains of relict ferrobustamite were identified by me. In cross-polarized light augite cores and rims have slightly different birefringent colors. Olivine birefringence shows through well in the cross-polarized light view. Field width is 6 mm.

Detail of some of the augite, with strongly contrasting core and rim colors and sharp contacts. The distinction between green augite cores and brown rims is more clear in this view. Birefringence varies but the extinction angles of the brown and green augite are similar. Field width is 3 mm.

Mineralogy: plagioclase, augite, olivine, oxides, and apatite. Cumulus phases are plagioclase, olivine, augite, oxides, and apatite. This sample is somewhat altered. Though it contains no orthopyroxene or inverted pigeonite, the augite has abundant, very thin pigeonite exsolution lamellae. Olivine crystals are visible at the left-center and bottom-left of the image. Field width is 6 mm.

Mineralogy: plagioclase, augite, olivine, oxides. Cumulus phases are plagioclase, olivine, augite, and oxides. Augite is light brown, and olivines are slightly lighter in color and slightly greenish, and several grains of each are visible. Field width is 6 mm.

Mineralogy: plagioclase, augite, inverted pigeonite, and oxides. Cumulus phases are plagioclase, augite, pigeonite, and oxides. Parallel alignment of plagioclase crystals is prominent in this sample. Field width is 6 mm.

Mineralogy: plagioclase, augite, olivine, inverted pigeonite, oxides. Cumulus phases are plagioclase, augite, olivine and oxides. Augite is light brown, olivine is lighter, as is the interstitial inverted pigeonite, which is now mostly orthopyroxene with augite exsolution lamellae. In cross-polarized light, notice interstitial olivine rimming Fe-Ti oxides, optically continuous with cumulus olivine. Field width is 6 mm.

Mineralogy: plagioclase, augite, olivine, inverted pigeonite, and oxides (interstitial only). Cumulus minerals include plagioclase, olivine, and augite. Olivine is just above center, augite crystals are brownish, and inverted pigeonite (probably interstitial) is at the lower right. Field width is 6 mm.

Mineralogy: plagioclase, olivine, augite, inverted pigeonite, oxides. Cumulus phases are plagioclase and olivine. Augite crystals here are clearly interstitial, with lighter-colored olivine crystals. Field width is 6 mm.

This is from the southwest flank of Basistoppen. Mineralogy: plagioclase, olivine, augite (in part inverted ferrobustamite), oxides, and apatite, with abundant interstitial granophyre and subsolidus alteration. Like the UZc sample shown above, the augite here includes brown and green color varieties. I think that the green augite, mostly in augite cores and generally optically continuous with adjacent brown augite, is inverted ferrobustamite. No zircon was seen though the sandwich horizon is supposed to have up to 1500 ppm Zr. Field width is 6 mm.

Same as above showing detail of olivine (lower left) and the brown and green colors in the augite. In this image the brown augite is in the grain core, which is unusual. Perhaps augite nucleated and grew first, and was overgrown by ferrobustamite (now inverted to green augite). Field width is 3 mm.

Sandwich horizon candidate 2 taken from the east flank of Nunatak 1. Mineralogy: plagioclase, augite, olivine (mostly altered to a mass of serpentine, talc, and magnetite), oxides, apatite. The apatite is not particularly abundant, and no zircon was seen. Again, the sandwich horizon is supposed to have up to 1500 ppm Zr. Field width is 6 mm.

Gabbro ~10 cm from the contact with Archean gneiss on Mellemø Island. Mineralogy: plagioclase, augite, orthopyroxene(!), inverted pigeonite, oxides. Olivine was not found, possibly because of contamination of the border zone by silica-rich liquids derived from the adjacent gneiss. Field width is 6 mm.

Contact zone gabbro taken from between wherlite xenoliths at the base of Uttental Plateau, near Uttental Sund. Mineralogy: plagioclase and olivine with interstitial orthopyroxene, augite, and oxides. This sample is remarkable in having large orthopyroxene crystals and no pigeonite, inverted or otherwise. The orthopyroxene is in the center and lower parts of this image and is pinkish, the augite is mostly at the upper right, and is greenish, and the olivine crystals are all rather round and colorless. Field width is 6 mm.

"Tranquil zone" sample from the western margin of Uttental Plateau at ~150 m elevation. Mineralogy: plagioclase and olivine with interstitial augite, orthopyroxene, and oxides. Orthopyroxene is the faintly pinkish mineral at the top center and right, augite is the mineral with exsolution lamellae in the center and bottom left, and olivine crystals are scattered throughout. No inverted pigeonite was found. Field width is 6 mm.

Perpendicular feldspar rock from ~10 m inside the contact on Mellemø Island. Mineralogy: plagioclase, olivine, interstitial augite, and oxides. The largest plagioclase crystal visible here, extending from the picture center into the upper-right corner, is one of the perpendicular feldspars. The scale of the perpendicular feldspars is difficult to see even on the scale of a whole thin section. For other examples, see Irvine et al. (2001, Figure 47). field width is 6 mm.

Replacement gabbroic anorthosite from the crossbedded belt, LZa, on Uttental Plateau. Mineralogy: plagioclase appears to be cumulus like the plagioclase elsewhere in the pluton. All other phases (augite, minor inverted pigeonite, and oxides) appear to be interstitial. I wonder if this replacement gabbroic anorthosite isn't basically a framework of the original rock with some mafic minerals dissolved out and more plagioclase precipitated in by migrating plagioclase-rich residual liquids. Field width is 6 mm.

Gabbroic anorthosite from a small autolith block in the Homestead area, UZa. Mineralogy: plagioclase with interstitial augite, olivine, oxides, and biotite. Interstitial olivine is on the right, augite is on the left. Field width is 6 mm.

Gabbroic anorthosite from a medium size autolith from the Homestead area, UZa. Mineralogy: cumulus plagioclase and olivine with interstitial augite, oxides, and biotite. Olivine is on the left, augite is on the bottom of the image. Field width is 6 mm.

Gabbro autolith from the giant block on the east end of Kraemer Island on the shore of Uttental Sund, in the MZ. Mineralogy: plagioclase, augite, inverted pigeonite, and oxides. No olivine was seen, so this rock is mineralogically equivalent to the MZ (roof facies called UBZβ). field width is 6 mm.