DETRITAL-ZIRCON GEOCHRONOLOGY OF METASILICICLASTICS FROM THE SOUTHERNMOST APPALACHIAN OROGEN: IMPLICATIONS FOR THE CONFIGURATION OF THE SOUTHEASTERN LAURENTIAN MARGIN, ALABAMA AND GEORGIA

U-Pb ages of detrital zircons from metasiliciclastics is key for understanding the distribution of Laurentian margin sequences and identifying exotic terrane signatures in the southernmost Appalachians. The Jacksons Gap Group (JGG) of the Brevard zone and the easternmost Blue Ridge in the western limb of the regional Tallassee synform share similar lithologies (meta-arkosic schist and gneiss and quartzite intruded by Late Ordovician granites) with the Opelika Complex (OC) in the eastern limb. Detrital zircons (²⁰⁶Pb/²³⁸U ages for <1000 Ma and ²⁰⁷Pb/²⁰⁶Pb ages for >1000 Ma, discordance <10%) from the JGG and OC all show age concentrations of Grenville (cumulative 76%-87%) and Granite-Rhyolite (cumulative 5%-14%) sources. The stratigraphically lower Devil’s Backbone quartzite (n=112) of the JGG and Auburn Gneiss (n=98) of the OC each contains a youngest zircon of 933±40 Ma and 673±36 Ma (2σ), respectively. Their age spectra are similar to that of the Weisner Formation in the foreland, and, thus, are interpreted as the Laurentian rift-to-drift transition facies. The stratigraphically higher Tallassee quartzite (n=71) in the western limb and Saugahatchee quartzite (n=63) of the OC in the eastern limb each yields a youngest zircon of 454±6 Ma and 426±30 Ma, respectively. These detritus might be recycled from the Laurentian margin and deposited during or after the accretion of the Dadeville Complex, which contains meta-volcanic rocks of similar ages. The Alabama-Georgia Laurentian margin might have originated as an embayment with marginal/platformal strata to the west and parautochthonous basement/cover in the Pine Mountain window to the east, a shape roughly corresponding to the regional Tallassee synform. Laurentian margin strata toward the foreland are disrupted by Alleghanian faults. A sample of Cheaha Quartzite (n=90) of the Talladega slate belt, ~40 km northwest of the JGG, contains 84% Grenville and 6% Granite-Rhyolite zircons with a youngest grain of 860±44 Ma, showing an age spectrum similar to those of the Devil’s Backbone quartzite and Auburn Gneiss. Results also suggest that the Auburn Gneiss, displaced by the Towaliga normal fault, may be the sedimentary cover of the Grenville basement in the Pine Mountain window.