MONAZITE AGES FROM THE SOUTHERN APPALACHIAN INNER PIEDMONT - THE ATHENS GNEISS

The Athens Gneiss (AG) is a suite of upper amphibolite grade rocks within the Inner Piedmont (IP) of NE Georgia that consists primarily of metatexite and diatexite migmatites. The mineral assemblage of the migmatites is pl+kfs+bio+qtz+ms± grt. The peak metamorphic temperature recorded in the migmatite mineral assemblages is 790±50°C (Mirante and Patino-Douce, 2000).

Metamorphic monazites from migmatites in the AG are relatively large, up to 250 microns in length, euhedral, and surrounded by anhedral minerals. This suggests that the monazites formed at or near metamorphic peak conditions and that there is little or no monazite inherited from the protolith. Dating of these monazites using the U-Th-Total Pb method described by Montel et al. (1996) constrains the timing of peak metamorphism in the IP, at least in NE Georgia.

Monazites were analyzed for U, Th, and Pb using the electron microprobe at the University of Georgia. U, Th, and Pb were calibrated for 60 s at 15 KeV and 30 nA using U metal, ThO₂, and PbS standards respectively. Count times were 120 s for U and Th and 240 s for Pb. These long count times were used in order to improve counting statistics, especially for Pb, which is typically present in concentrations lower than 600 ppm.

Ages of peak metamorphism for metamorphic monazites from both metatexite and diatexite migmatites in the AG are 330±15 Ma, which corresponds to the later part of the Southern Appalachian Acadian orogeny. The peak metamorphic conditions of the AG and the formation of diatexite and metatexite migmatites by partial melting occurred at the same time. These results indicate that widespread, migmatite forming, partial melting in the IP core in NE Georgia is confined to the Acadian, as there are no monazite ages younger than Acadian in the AG.