GONDWANAN/PERI-GONDWANAN ORIGIN FOR THE UCHEE TERRANE, SOUTHERNMOST APPALACHIANS, ALABAMA AND GEORGIA: ZEROING IN ON SUTURES

The poorly known, suspect, Uchee terrane (UT) occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Grenville continental basement exposed in the Pine Mountain window (PMW) and overlying Gondwanan crust of the Suwannee terrane buried beneath sediments of the Gulf Coastal Plain. The UT was thought to be either a septum of Piedmont rocks that once was continuous across the erosionally breached PMW or part of the Carolina terrane. Existing isotopic dates are too sparse to determine which is correct so we initiated U-Pb (SHRIMP-RG) zircon studies to begin characterizing the provenance and ages of protolith deposition, metamorphism, and plutonism. U-Pb ages for zircons from the Phenix City Gneiss date igneous crystallization at ~620 Ma, inheritance ~1000 to ~1700 Ma, and a ~300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates clearly indicate Pan-African and/or Trans-Brasiliano elements, verifying an exotic peri-Gondwanan or Gondwanan origin. The UT likely correlates to other ~600 Ma terranes of the Carolina terrane (e.g., Savannah River, Goochland, and Virgilina) or, alternatively, represents basement to the Suwannee terrane. The ~300 Ma overgrowths on zircons are compatible with 295-288 Ma 40Ar/39Ar hornblende dates indicating deep tectonic burial of the UT contemporaneous with the Alleghanian orogeny recorded in the foreland. A new ~295 Ma U-Pb date on rutile from the PMW corroborates limited 40Ar/39Ar cooling dates supporting that it too had occupied middle-crustal levels during the Alleghanian. Temperature-time paths for the UT and PMT overlap after ~295 Ma indicating a minimum age for docking. U-Pb work on the UT is preliminary (four samples) but we have not detected any intermediate ‘Appalachian' dates (e.g., Acadian, Taconian, or Penobscotian). Together with initial fabric observations, early fabrics may be Neoproterozoic relics developed prior to formation of the eastern Laurentian, early Paleozoic passive margin, thus, wholly predating the Appalachian orogenic cycle.