DETERMINING THE MAGNITUDE AND EXTENT OF CRUSTAL THICKENING DURING ALLEGHANIAN OROGENESIS IN THE SOUTHERNMOST APPALACHIANS

Multiple orogenic events in the Appalachians result in complex metamorphic overprints and numerous intrusive events. In the southern Appalachians, sparse geochronological and thermobarometric data hamper evaluation of the Alleghanian orogenic architecture. We present new pressure-temperature paths and zircon hafnium isotope data, which coupled with garnet Sm-Nd and zircon U-Pb ages constrain the thickness of Alleghanian crust. These data indicate crustal thickening during Neoacadian to early Alleghanian orogeny in the eastern Blue Ridge of Alabama. Garnet Sm-Nd ages for amphibolite facies metamorphic rocks range from 331.0±4.0 to 319.8±2.6 Ma. Zircon U-Pb ages indicate that low- Sr/Y plutons are 376.6±1.5 to 363.8±2.9 Ma (Rockford Granite, & Bluff Springs intrusions) and high-Sr/Y plutons are 349.1±1.8 to 334.6±3.2 Ma (Almond, Wedowee, & Blakes Ferry plutons). Therefore, regional metamorphic garnet growth was ca. 4 m.y. after intrusion of the youngest high Sr/Y plutons. Garnet growth began at 5.5 to 6.0 kbar and 525 to 550°C, and ended at 7.0 to 7.5 kbar and 600 to 650°C in the Wedowee Group. Garnet in Emuckfaw Group rocks SE of the Wedowee began growth at similar conditions, but grew during a P increase to ca. 9.0 kbar. This indicates changing magma sources with older low-Sr/Y magmas sourced at depths < ~38 km, insufficient for garnet stability and younger high-Sr/Y magmas sourced at depths > ~38 km with garnet stable. Hf isotopes from zircon in all but the Wedowee Pluton (εHf_i=+3 to +4) range from - εHf_i to weakly + εHf_i, indicating mixing of mantle and deep-crustal sources. The garnet Sm-Nd ages and P estimates for metamorphic rocks require minimum crustal thicknesses of 21 to 33 km at 331 to 320 Ma. However, crustal thicknesses were likely 27 to 39 km including the underlying Alleghanian thrust sheet. Our data indicate hot tectonically-thickened crust during intrusion of the high Sr/Y plutons. Metamorphism and intrusion in the thickened crust were synchronous with deposition of the Bangor, Parkwood, and Pottsville formations in the foreland basin of Alabama between 350 and 320 Ma (Mississippian to Early Pennsylvanian). Therefore, crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.