MINERALOGY OF PEGMATITE DIKES FROM THE KEYSTONE BLUE QUARRY ELBETON BATHOLITH, NORTHEAST GEORGIA AND IMPLICATIONS FOR THE ORIGIN OF THE PEGMATITC TEXTURE

The Keystone Blue Quarry in the Elberton granite of northeastern Georgia exposes numerous dikes of granite pegmatite in a fine-grained bluish-gray granite. Contacts between the thin (1-20 cm thick) dikes and the granite are generally sharp. Dikes and host granite generally have the same mineral assemblage, but the dikes are coarser grained. Grain size of the dikes (mms to cms) is larger than grains in the host granite (1-2 mm). Is the difference in grain size the result of crystallization from chemically fractionated magma or some kinetic process? This question was investigated by the students and faculty of the Earth Materials class at UGA during the fall of 2008.

The dikes were examined in the quarry and sampled for laboratory investigation. Students prepared thin sections and assisted in the collection of petrographic and electron microprobe data. The dikes contain about the same proportion of feldspars at quartz as the granite (about 30 % each of quartz, K-feldspar, and plagioclase). Quartz is anhedral and plagioclase is subhedral in dikes and granite. Perthitic K-feldspar is anhedral in the granite, but varies form anhedral to subhedral in the dikes. Feldspars are fresh in the granite, but alteration (calcite, white mica) of the plagioclase is common in the dikes. Biotite in the granite is unaltered, but alteration to chlorite is common in the dikes. Intergrowths of quartz in the feldspars are visible in both the granite and the dikes.

Feldspar compositions show a more limited range in the granite (plag: Ab75-Ab77, Kspar: Ab5-Ab10) than the dikes (plag: Ab76-Ab79, Kspar Ab1-Ab13). Biotite compositions are similar in granite (Mg# 39-42) and dikes (Mg# 40-42). Granite accessory minerals include ilmenite, zircon, apatite, sphene, and allanite. Accessory minerals are not common in the dikes, but the same assemblages is recorded in the granite. Euhedral grains of hematite are the most common accessory mineral in the dikes; zircon was noted in one dike and monazite in another. One dike contained euhedral crystals of hematite and an alteration assemblage (rutile, ilmenite, apatite) around sphene. The difference in grain size between the dikes and the granite is probably related to some kinetic process (nucleation, crystal growth rate) rather than crystallization of a fractionated granitic magma.