Skeletal tourmaline, undercooling, and crystallization history of the Stone Mountain granite
Longfellow, Kristen Michelle
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Stone Mountain is composed of a light colored, fine-grained two-mica granite to granodiorite. Large (3-15cm) skeletal tourmaline crystals occur singly or in clusters and are surrounded by a white halo of biotite-free granite. Skeletal tourmaline occurs in granite and in pegmatite-aplite dikes. Euhedral tourmaline (.1-3cm) occurs in cores of pegmatite-aplites. The skeletal tourmaline crystals display a morphology typically associated with crystallization from undercooled melts. Halos surrounding the skeletal tourmaline crystals are consistent with growing tourmaline that depleted local melt environment in Fe, suggesting a magmatic origin for the skeletal crystals. The appearance of skeletal crystals in the granite and early-formed parts of the pegmatite suggest a link between the crystallization history of these units. Microprobe compositions indicate little compositional variation in the different textures of the schorl tourmaline. Crystallization of the aplite began by removal of B, and the remaining melt crystallized to form a euhedral core at low undercooling.