THE TIMING OF METAMORPHISM IN THE SOUTHERN APPALACHIANS: FILLING IN THE GAPS USING GARNET SM-ND GEOCHRONOLOGY

The timing of metamorphism in the southernmost Appalachians is poorly known in comparison to the north. The lack of data underlines the need to directly date metamorphism in order to determine the extent and significance of orogenic events. The lack of ages hinders our ability to test tectonic models for the Appalachians. We present 3 garnet Sm-Nd ages from: the Standing Indian area, central Blue Ridge of North Carolina (12NC03); the Ducktown ore deposit, western Blue Ridge of Tennessee (14MP01); and the Mad Indian Group of the Ashland Supergroup, eastern Blue Ridge of Alabama (17Ashland01).

12NC03, from the Appalachian trail near Standing Indian Mountain, is a garnet-biotite-muscovite-K-feldspar migmatitic gneiss. Garnets have inclusion-rich cores and inclusion-free rims, which yield ages of 440.5±4 Ma (MSWD 0.44, 3 pts) and 411±12 Ma (MSWD 2.1, 3 pts), respectively. 14MP01, from the Bura Bura Mine in Ducktown, TN, is a clinoamphibole-garnet-chlorite schist. Garnet yields an age of 449±12 Ma (7 pts). Sample 17Ashland01, from the Mad Indian Group, is a garnet-sillimanite-kyanite-staurolite schist. The preliminary garnet age is 359±7.4 Ma (MSWD 2.0, 4 pts).

Timing of metamorphism in 14MP01 and 12NC03 are similar to published metamorphic zircon ages of c. 458 Ma from Winding Stair Gap, c. 77 and 7 km away from 14MP01 and 12NC03, respectively. Published garnet ages in the Wedowee and Emuckfaw groups, 20-50 km south of 17Ashland01, have garnet Sm-Nd ages of c. 331-319 Ma (Stowell et al., 2014). The difference in age between the Mad Indian and Wedowee-Emuckfaw groups could indicate either two thermal events or a >30 m.y. duration for metamorphism. Although these new garnet ages are widely spaced within the Blue Ridge, they draw attention to the following questions: What was the extent of Taconic metamorphism in Georgia or Alabama? What was the extent of Alleghanian or Acadian metamorphism in Georgia? The possibility of punctuated or long durations of metamorphism could have profound impacts on our understanding and definitions of these orogenies and additional garnet Sm-Nd ages and P-T data are required in the southern Appalachians to identify the extent and nature of these orogenies. Thermobarometry, isochemical phase diagram modeling, and additional garnet geochronology is in progress.