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Diorite is common near the borders of plutons, but is volumetrically insignificant. Analyzed specimen no. 3 is medium-dark-gray, medium-grained diorite from the west shore of Beyer Bay. It came from an outcrop containing abundant xenoliths and large tabular septa of basaltic country rock. The xenoliths and septa have a conspicuous hornfels texture. Clinopyroxene, amphibole, orthopyroxene, biotite, magnetite, and sodic labradorite are common in the recrystallized rock. Large biotite flakes rim the xenoliths, and magnetite crystals are visible in them. Under the microscope a few large crystals of plagioclase can be seen in the xenoliths. These crystals are not zoned, but they show interior and zonal patterns of minute inclusions suggesting that the original rock contained zoned feldspar and was porphyritic. Xenoliths occur in all stages of fragmentation and change toward diorite, and those that have been greatly made over appear as vague dark-colored clots in the diorite. The pluton here is low-silica diorite whose dotted amphibole and poikilitic biotite suggest contamination of an originally granodioritic magma. The estimated mode of the diorite (specimen no. 3) exclusive of recognizable xenoliths is as follows: zoned plagioclase ( An65-45), 60 percent; hornblende, generally in clusters around a pyroxene nucleus, 15 percent; biotite, many with sieve textures, 10 percent; clinopyroxene, altering to hornblende, 5 percent; magnetite, 5 percent; quartz, 5 percent. |
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