AR-40 AR-39 THERMOCHRONOLOGY AND ALLEGHANIAN DEVELOPMENT OF THE SOUTHERNMOST APPALACHIAN PIEDMONT, ALABAMA AND SOUTHWEST GEORGIA/

Ar-40/Ar-39 age spectra of hornblende, muscovite, and microcline, and total fusion ages of biotite from metamorphic rocks of the Inner Piedm ont, Pine Mountain, and Uchee belts are reported. Mineral cooling ages from the eastern part of the Inner Piedmont are as follows: hornblend e, 320 Ma; muscovite, 296 Ma; biotite, 293 Ma; and microcline (diffusi onal release patterns) Tmax = 267 Ma, Tmin = 234 Ma. A 347 Ma hornblen de spectrum from the highest Inner Piedmont structural level sampled i s the oldest date determined and implies earlier passage of this level through the 500-degrees-C isotherm. Most release spectra from Pine Mo untain belt units are discordant with little or no apparent geologic m eaning. Modified saddle-shaped release patterns for hornblende indicat e extraneous argon with a maximum age of approximately 358 Ma. Muscovi te from the Pine Mountain belt cover sequence is 286 Ma (plateau age), and one from the underlying Grenville basement is 277 Ma (correlation age), indicating cooling below the 350-degrees-C isotherm. Plateau ag es on Uchee belt rocks are as follows: hornblende, from 297 to 288 Ma; muscovite, 285 Ma; biotite, 276 Ma; and microcline Tmax = 261 Ma, Tmi n 230 Ma. Muscovite fish from a Bartletts Ferry fault zone phyllonite have a plateau age of 283 Ma. The Ar-40/Ar-39 results combined with ot her geologic data indicate that (1) a large part of the southern and I nner Piedmonts of Alabama and southwest Georgia experienced a late Pal eozoic amphibolite-facies thermal and deformational event contemporane ous with the Alleghanian orogeny observed in the foreland; (2) the tec tonic development of this event, characterized by initial crustal thic kening followed by right-slip and normal-slip movements, is grossly si milar to that described for the amphibolite-facies Alleghanian belt in the eastern Piedmont of South Carolina and Georgia; and (3) extension al movements along the flanks of the Pine Mountain window occurred bet ween ca. 277 Ma and the Late Triassic-Early Jurassic and thus may refl ect latest Alleghanian extensional collapse or Mesozoic rifting.