NEW GARNET SM-ND AGE FOR GARNET ZONE METAMORPHISM IN THE WESTERN BLUE RIDGE, DUCKTOWN, TN
Pressure, temperature, and timing of metamorphism in the Ducktown area are poorly characterized. Monazite EPMA ages from the Ocoee Gorge, ca. 24 km west of Ducktown, indicate that metamorphism occurred ca. 400 Ma (Kohn and Malloy 2003). Zircon U-Pb from Winding Stair Gap, ca. 77 km east of Ducktown in the eastern Blue Ridge, indicate granulite facies metamorphism and partial melting ca. 458 Ma (Moecher et al., 2004). Whole rock 40Ar/39Ar cooling ages for the WBR, ca. 5-175 km north of Ducktown, range from 499-389 Ma (Connelly and Dallmeyer, 1993). These results are compatible with Ducktown metamorphism during the Taconic, Neoacadian or early Alleghanian orogenies.
The sample analyzed for this study is an clinoamphibole-garnet-chlorite schist. The protolith is likely a basalt based on major element chemical composition: 49.88% SiO2, 15.28% Al2O3, 5.05% MgO, 23.73% Fe2O3. Minerals present are actinolite-hornblende, garnet, chlorite, quartz, plagioclase, biotite, and arsenopyrite. The rock is foliated with 4-7 mm subhedral garnet containing predominantly quartz inclusions. Chlorite surrounding garnet suggests final mineral growth during decreasing temperatures.
Aliquots used for Sm-Nd geochronology are whole rock (WR), matrix (Mtx), amphibole (Amp), and garnet (core, rim, and bulk). 147Sm/144Nd isotope ratios are 0.0992-0.1149 for WR, Mtx, and Amp, and 0.6115-2.6306 for garnet separates. Sm-Nd data yield an age of 449±12 Ma (7 pts., 2s uncertainty) and an initial 147Sm/144Nd of 0.00017. Our new age indicates that garnet zone metamorphism is Taconic and not Alleghanian. Therefore, early Alleghanian metamorphism in the Blue Ridge of Alabama did not extend to the western Blue Ridge in southern Tennessee.