167-9 Unravelling the Extent and Conditions of Multiple Paleozoic Metamorphic Events in the Southern Appalachian Blue Ridge in northern Georgia: Insights from REE Accessory Phase Petrochronology

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

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Abstract:

The southern Appalachian Blue Ridge records evidence for at least three Paleozoic tectonometamorphic events, including the Taconic (480-440 Ma), Neoacadian (380-340 Ma), and Alleghanian (335-280 Ma). Currently, the spatial extent, conditions, and even driving forces of primary metamorphism and metamorphic overprinting have remained contested. In many areas, including the northern Georgia Blue Ridge, this can be attributed to limited geo- thermochronologic constraints to date. Here, we use new monazite laser ablation split stream U-Pb dates to develop revised petrotectonic interpretations for the western (WBR) and central Blue Ridge (CBR), and the Brevard fault zone (BFZ). The WBR (Murphy belt) sample, which experienced peak kyanite-staurolite grade Taconic metamorphism is intensely mylonitized and contains euhedral garnet with up to four textural growth zones. Monazite in that sample dominantly occurs in the matrix and yields a weighted mean age of 327.0 ± 1.0 Ma. Two concordant grains yield significantly younger and older dates of 289 ± 8 Ma and 417 ± 14 Ma, respectively. One sample from the CBR (Dahlonega gold belt) contains kyanite that is slightly retrogressed and euhedral-subhedral garnets with at least two textural growth zones and curved inclusion trails. Monazite, some of which is included in garnet and kyanite, yields a weighted mean age of 318.5 ± 1.2 Ma.  One older date of 352 ± 10 Ma and multiple dates younger than 310 Ma were also recorded. Lastly, a BFZ mylonite sample collected near Atlanta contains subhedral-anhedral, heavily retrogressed garnets. Matrix monazites in this sample yield a weighted mean age of 323.2 ± 1.0 Ma, with two older concordant outliers at 343 ± 8 Ma and 351 ± 8 Ma. Combined, these data indicate that this part of the Georgia Blue Ridge records a complex metamorphic history of high-grade Neoacadian metamorphism overprinted by early  Alleghanian metamorphism at least to kyanite grade. Future work in this area will address the spatial extent of these events and their relationships with the pervasive Taconic event observed in the Blue Ridge of Tennessee and North Carolina. The results are relevant for elucidating (1) the exhumational/erosional history of the southern Appalachians; and (2) the assessment of provenance of monazite placer REE deposits in the coastal plain. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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