GARNET GEOCHRONOLOGY AND P-T-T PATHS FOR NEOACADIAN TO ALLEGHENIAN METAMORPHIC ROCKS IN THE EASTERN BLUE RIDGE, AL

Sm-Nd garnet ages, isochemical phase diagram sections (pseudosections), and thermobarometric estimates from the Wedowee Group of the Appalachian Eastern Blue Ridge (EBR) are being used to construct P-T-t paths for regional and contact metamorphism. The project will compare metamorphic ages and P-T-t paths from contact metamorphic rocks with those for regional metamorphic rocks to determine crustal thickness when the plutons were emplaced and the number of metamorphic events. These results will be used to evaluate the causes for metamorphism, e.g., relaxation of a depressed geothermal gradient and/or heating from plutonism.

Initial results indicated that garnet from the aureole of the Blake’s Ferry Pluton has complex zoning with two oscillatory cycles of high to low Ca concentrations varying from 0.163 to 0.073 mole fraction across the crystal. This seems compatible with garnet growth during both regional and contact metamorphism resulting in different ages for garnet rim and core. However, garnet core and rim ages are indistinguishable and the best age includes both: 331.0 ± 4.0 Ma (5 pt. isochron: 1 core, 2 rims, 1 whole rock, and 1 matrix = rock minus garnet). The new metamorphic age correlates with a previously reported U-Pb zircon age of 331± 3.8 Ma for the Blakes Ferry Pluton. However, U-Pb monazite ages for the Blakes Ferry Pluton are older at 345.9 ± 3.1 Ma. A MnNaCaKFMASH P–T pseudosection for garnet zone regional metamorphic rocks ca. 1 km from the pluton and garnet core compositions indicate that garnet growth began at 6.9 kbar and 525°C. The regional P-T-t path is simple with an increase in temperature to 600°C and little pressure change. The identical Sm-Nd garnet and U-Pb zircon ages suggest that garnet in the contact aureole grew soley due to input of heat during plutonism. The simple regional metamorphic P-T-t path suggests one regional thermal event. We infer that these 2 rocks had different heat sources based on pluton proximity. Future work will compare garnet ages and P-T-t paths in order to constrain crustal thickness, determine the ages for metamorphism, and evaluate heat souce(s).