The Stone Mountain pluton, in Georgia, is composed of tourmaline-bearing biotite–muscovite granite. The pluton was intruded at midcrustal depths and is cut by late-stage dikes. Tourmaline occurs as large skeletal crystals in the granite in the same areas as late-stage aplite and pegmatite–aplite dikes. Skeletal tourmaline also occurs in the aplite zone of pegmatite–aplite dikes. Pegmatites and thin feldspar – quartz – tourmaline veins contain euhedral tourmaline. Isolated skeletal crystals of tourmaline in the granite and tourmaline-bearing dikes are surrounded by a fine-grained leucocratic, Fe-depleted halo of feldspar and quartz. Electron-microprobe analyses show that all of the tourmaline consists of schorl. Skeletal tourmaline shows an early brown to late blue pleochroism. Most euhedral tourmaline is uniformly brown, but a few grains from pegmatite have a blue core and a brown rim. Brown tourmaline is high in Ti (>0.1 apfu), whereas the blue tourmaline is lower in Ti (<0.05 apfu). There is little compositional variation among the different textures of the tourmaline. These textures indicate an initial crystallization at undercoolings of 40–100°C. A boron-enriched fluid reacted with the existing granite to form skeletal tourmaline with a Fe-depleted halo. In the pegmatite–aplite dikes, the B-enriched fluid (melt) segregated along the outer margins of the dikes and crystallized skeletal tourmaline from an undercooled melt. Removal of B from the melt raised the solidus temperature and caused the aplite that encloses the skeletal tourmaline to form. Crystallization of the remaining melt at lower undercoolings produced euhedral tourmaline of the pegmatite and veins. The development of skeletal crystals of tourmaline at Stone Mountain indicates that undercooling is an important process in some midcrustal granite systems.

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