U-PB AGES FROM THE DADEVILLE COMPLEX, SOUTHERNMOST APPALACHIANS, EASTERN ALABAMA: AN ACCRETED TACONIC ARC

The Inner Piedmont comprises a suspect terrane of high-grade metasedimentary, metavolcanic, and metaplutonic rocks that extends throughout the southern Appalachians. In eastern Alabama, the precise age and tectonic affinity of these rocks is not well defined. Some units previously correlated to the Inner Piedmont in Alabama are in fact a part of the eastern Blue Ridge terrane (i.e., the Opelika Complex) so now only the Dadeville Complex is considered to have a true Inner Piedmont affinity. The Dadeville Complex is a package of interlayered amphibolite and felsic schist (Waresville Formation), layered to massive amphibolite and tonalitic gneiss (Ropes Creek Amphibolite), tonalitic gneiss (Waverly Gneiss), two different felsic intrusives suites (the Camp Hill Gneiss and the Rock Mills Granite Gneiss), and a migmatitic aluminous schist (Agricola Schist). The only previously reported age for Dadeville Complex rocks is a Rb-Sr whole-rock isochron age of 462 ± 4 Ma for the Franklin Gneiss in western Georgia (i.e., the Rock Mills Granite Gneiss in Alabama).

Samples were collected from the type localities of six constituent units comprising the Dadeville Complex and their zircons were extracted for LA-SF-ICPMS dating. Magmatic zircons yielded ²⁰⁶Pb/²³⁸U ages of 493.4 ± 6.0 Ma for the Waresville Formation, 476.6 ± 6.8 Ma for the Ropes Creek Amphibolite, 477.1 ± 4.8 Ma for the Waverly Gneiss, and 499.8 ± 6.9 Ma for the Camp Hill Gneiss. Although zircons from the Rock Mills Granite Gneiss were too strongly altered for analysis, the previously reported Rb-Sr date is compatible with our new geochronologic results documenting Early to Middle Ordovician development of the Dadeville Complex. Detrital zircons from the Agricola Schist yielded ²⁰⁶Pb/²³⁸U dates spanning from ~2.7 Ga to ~400 Ma, with peaks at ~883, ~755, and ~656 Ma; a prominent age population at ~480 Ma has Th/U values (<0.1) and is interpreted as metamorphic overgrowth. The detrital dates do not coincide with typical zircon producing events found along the southeastern (present-day) Laurentian margin; Grenville zircons are conspicuously sparse (5.3%) and only reflect the very latest stages of orogenesis. Results indicate that the Dadeville Complex is an exotic island arc emplaced onto the eastern Laurentian margin during the Taconic.