Paper No. 23
Presentation Time: 8:00 AM-12:00 PM
NEW ISOTOPIC DATES FROM THE UCHEE TERRANE, SOUTHERNMOST APPALACHIANS, ALABAMA AND GEORGIA: A GONDWANAN/PERI-GONDWANAN ORIGIN
New U-Pb (SHRIMP-RG) geochronology from the poorly known, suspect, Uchee terrane, the most internal terrane exposed in the southernmost Appalachians, strongly support a Gondwanan or peri-Gondwanan origin. The Uchee terrane lies immediately north of the proposed Suwannee suture, which is buried beneath Coastal Plain sediments, and is juxtaposed against the southern margin of Grenville continental basement of the Pine Mountain massif by some of the orogens widest mylonite zones (i.e., Goat Rock and Bartletts Ferry). Correlation of the upper-amphibolite-facies rocks of the Uchee is uncertain due to the lack of age information; proposals range from the more external, Laurentian eastern Blue Ridge and inner Piedmont to the peri-Gondwanan Carolina terrane. U-Pb dates for zircons from the Phenix City Gneiss, an orthogneiss complex that forms the bulk of the exposed Uchee terrane, indicate igneous crystallization at ~620 Ma with inheritance from ~1000 to ~1700 Ma, and an ~300 Ma (Alleghanian) overprint, the latter recorded by rims on zircons of all ages found in migmatitic samples. Zircons from the Moffits Mill Schist, a distinctly metasedimantary unit with uncertain relations to the Phenix City Gneiss, yield a bimodal population of dates at ca. 620 and 640 Ma. The ca. 300 Ma overgrowths on zircons from the migmatites are compatible with 295-288 Ma 40Ar/39Ar hornblende dates reported by Steltenpohl and Kunk (1993) and support their conclusion that the Uchee terrane was deeply, tectonically buried contemporaneous with the Alleghanian orogeny recorded in the foreland less than 100 km to the west. Limited 40Ar/39Ar cooling dates and our ca. 295 Ma U-Pb date on rutile from Pine Mountain rocks document its involvement in Alleghanian orogenesis as well. The 620 to 640 Ma dates from the Uchee rocks clearly indicate Pan-African and/or Trans-Brasiliano elements, verifying an exotic Gondwanan or peri-Gondwanan origin for the this terrane. We explore how the Uchee terrane may relate to other ~600 Ma rocks in the southern Appalachians (e.g., Savannah River, Goochland, Virgilina, Carolina, and Suwannee terranes) and what these data mean for Appalachian and Rodinian tectonics.