ORIGIN OF PEGMATITE-APLITE DIKES IN THE SPARTA GRANITE, GEORGIA

The Sparta Granite is a late orogenic, composite pluton in the Kiokee Belt of the southern Appalachian Piedmont. Field and petrographic investigations were conducted on granite and pegmatite-aplite dikes exposed in a quarry and bald 14 km southeast of Sparta. The granite to granodiorite (IUGS) pluton has allotriomorphic seriate texture with weak foliation defined by clusters of biotite, epidote and opaques. Quartz and microcline are the largest and most deformed grains in the granite. Narrow biotite-rich shear zones bearing bands and augen of microcline and/or smoky quartz strike N 9-15° W through the granite. The shear zones are cut by sets of aplite dikes (< 5 cm), which are cut by pegmatite-aplite dike (10-150 cm) swarms striking E-NE and NW. The pegmatite-aplite dikes have medium-coarse border zones of plagioclase, quartz and microcline with interior pegmatitic microcline perthite and quartz with accessory biotite, opaques and fluorite. Aplite cuts, fills fractures and envelopes the pegmatitic microcline and quartz within the dikes. Coarse to pegmatitic quartz, plagioclase and microcline display undulose extinction, bent twin lamellae and mortar texture. Aplite consists of undeformed albite, interstitial quartz, accessory garnet, opaques and epidote. Trachytoidal texture in aplite supports injection through fractured pegmatite. During pegmatite-aplite genesis the outer granite was rigid enough to generate linear fractures, but retained sufficient heat and residual melt to form dikes. Early crystallization of K feldpar, biotite and quartz in the granite enriched residual melt in Na. In this case aplite genesis may incorporate aspects of both pressure and compositional quenching as waning stress ruptured the pluton and sodic residual melt bled into the fractured dikes.