UNDERCOOLING IN GRANITOID SYSTEMS: AN EXAMPLE FROM STONE MTN., GEORGIA

Crystallization of granitoid rocks normally takes place at very low undercoolings resulting in a granular texture. However, in some granitoid systems significant undercooling temperatures can be generated in association with the loss of a volatile component (e.g. the pegmatite-aplite association) or local changes in melt compositions associated with growing crystals (e.g. the graphic and myrmekitic textures). Crystals grown in undercooled systems are cellular or skeletal due to their rapid crystal grow rate in an over-saturated system. The purpose of this paper is to examine the evidence for undercooling in granitic systems using the Stone Mtn. granite. The Stone Mtn. granite is a homogeneous medium-to-fine-grained biotite white mica granite locally cut by granitoid dikes. Accessory minerals in the granite include coarse-grained skeletal crystals of black tourmaline surrounded by a halo of granite completely devoid of biotite and white mica. The earliest dikes are biotite white mica granite, similar to the host granite. Later tourmaline-bearing composite pegmatite-aplite dikes cut the early dikes. Tourmaline-quartz dikes (veins) cut all of the earlier dikes. The composite pegmatite-aplite dikes contain pegmatitic zones of feldspar, quartz, and euhedral tourmaline. Zones of graphic and aplitic texture with skeletal tourmaline formed later than the pegmatitic zones and reflect crystallization at some undercooling relative to the pegmatitic part of the system. The transition form pegmatitic to aplitic crystallization was abrupt in these dikes and was probably related to loss of a fluid phase as evidenced by tourmaline-quartz veins emanating from the the pegmatite-aplite dikes. The coarse-grained skeletal tourmaline crystals in the granite are an enigma. Some workers concluded they formed at subsolidus conditions from hydrothermal fluids by replacement of the granite. Other workers suggest a magmatic origin for the tourmaline and crystallization from a fluid-enriched magma. Whatever the origin, these tourmaline crystals also grew in an undercooled system.