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Paper No. 1-7
Presentation Time: 10:25 AM
GARNET GROWTH IN THE SOUTHERNMOST APPALACHIANS
The timing of metamorphism in the Southern Appalachians recorded in zircon, monazite, garnet, and mica ranges from Grenville to Alleghanian, and Interpreting these sparse and widely distributed ages remains difficult. Monazite U-Pb ages across the North Carolina Blue Ridge range from 464 to 441 Ma with no evidence for multiple growth/thermal events. South of the North Carolina - Georgia border, Grenville ages are restricted to the Pine Mountain terrane. In the North Carolina Blue Ridge, garnet Sm-Nd generally match monazite U-Pb ages; however, garnet in the Shooting Creek Schist from Standing Indian is complex: core = 441 & rim = 411 Ma. In the Blue Ridge of Georgia, Tennessee, and Alabama garnet Sm-Nd ages indicate a complex age pattern likely to result from structural juxtaposition of fault blocks with distinct thermal histories. In northernmost Georgia, garnet in biotite gneiss from Blood Mountain and Brasstown Bald yield Sm-Nd ages of 450±7 and 446±6 Ma, respectively. However, <100 km SW of Brasstown Bald, garnet in coarse kyanite-staurolite-hornblende-biotite gneiss near Canton, in the Dahlonega Gold Belt (Pumpkinvine Creek Fm.) yields a Sm-Nd age of 325±3 Ma. Garnet at this location has complex zoning, compatible with multistage growth or modification. Further south in Alabama, garnet Sm-Nd ages indicate ca. 355 Ma (2 ages) in the garnet-kyanite-biotite-muscovite schist of the Higgins Ferry and Mad Indian groups and 330 to 320 Ma (3 ages) in the predominantly biotite-muscovite-schist of the Wedowee Group. Garnet major element compositions indicate simple growth zoning with no evidence for polyphase metamorphism. These NE-trending belts are separated by the Goodwater-Enitachopco/Allatoona Fault.
Garnet ages indicate that the Dahlonega gold belt and Wedowee Group rocks comprise an Alleghanian belt of amphibolite facies rocks bounded by the Brevard zone to the south and the Allatoona Fault to the north. Recent mapping and structural analysis indicate that this fault tips out to the SW (Tull et al., 2023). In this case, the low- to mid-amphibolite facies Wedowee Group and upper amphibolite facies Higgins Ferry Group rocks form a continuous tectonostratigraphic sequence near the coastal plain onlap and garnet growth ages should likewise form a continual sequence in this region.