NEOACADIAN CRUSTAL THICKENING, PLUTONISM AND METAMORPHISM IN THE EASTERN BLUE RIDGE, ALABAMA

Acadian to Neoacadian plutons in the eastern Blue Ridge indicate a temporal transition from low- to high-Sr/Y compositions compatible with progressive crustal thickening which stabilized garnet in the lower crustal source region by ca. 350 Ma. New P-T estimates and geochronologic data from pelitic rocks in the Wedowee and Emuckfaw groups provide a test of the crustal thickening hypothesis. Porphyroblastic garnet-bearing pelite was sampled from the Wedowee G. >3,000 m from the Wedowee Pluton near Wedowee, <100 m from the Blakes Ferry Pluton, and from the Emuckfaw G. in the Alexander City fault zone >5,000 m from known plutons. Garnet zoning is concentric in all samples and varies from simple monotonic variation in all elements to near-monotonic variation in Mn, Fe, & Mg with complex oscillations in Ca. MnNaCaKFeMgAlSiHTi P-T pseudosections and garnet zoning are compatible with a single generation of garnet growth with all samples reaching similar peak T of 600-650°C. Peak P ranges from 10±1 kbar in the Emuckfaw G. to the southwest to 7.5±1 kbar in the Wedowee G. to the northwest. Samples from the Emuckfaw G. and Blakes Ferry Pluton aureole indicate loading of 4 & 2 kbar, respectively during garnet growth; whereas, garnet from near the Wedowee Pluton grew at ca. 7 kbar with no clear evidence for loading. Garnet Sm-Nd isochron ages for Blakes Ferry Pluton contact aureole and near the Wedowee Pluton are 331.0±4.0 Ma (5 pt.) and 316.9±1.9 Ma (5 pt.), respectively. These new garnet ages from the Wedowee G. overlap with a 315±18 Ma Rb-Sr isochron for shear zone Au mineralization at Hog Mountain, ca. 7 km north of the Emuckfaw location.

Results are compatible with [1] garnet growth during a single regional metamorphic event which produced a metamorphic field gradient with growth occurring from 331 to 317 Ma, [2] garnet growth during both regional and contact metamorphism, or [3] all garnet growth in response to heating from local plutonism. The most likely explanation is that all or most garnet growth is regional. Regardless, loading identified in Emuckfaw G. and southwestern Wedowee G. rocks indicates significant crustal thickening prior to 331 Ma. Given the time required for conductive heating of the wedge, initial thickening could wholly or partly predate generation of 350-330 Ma high Sr/Y magmatism.