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