THE DADEVILLE COMPLEX OF EASTERN ALABAMA AND WESTERN GEORGIA: IMPLICATIONS FOR THE “MISSING” TACONIC ARC IN THE SOUTHERN APPALACHIANS

The Ordovician Taconic orogeny was one of the first events to be placed in a plate tectonic context, however significant differences in interpretation of this event exist between the northern and southern Appalachians. Principally, in the southern Appalachians (SA), a definitive accreted Taconic arc has not yet been identified. We suggest that the Dadeville complex (DC), located in the Inner Piedmont of E. Alabama and W. Georgia, is the best and most coherent Taconic arc candidate.

The Dadeville complex, unique among SA terranes, is composed of mafic/ultramafic intrusions, batholithic-scale granitic bodies and volcanic sequences with a tectonostratigraphic thickness of ≈17 km. It forms a broad klippe in the core of the shallowly NE-plunging Tallassee synform above terranes of the Early/Middle Ordovician Wedowee-Emuckfaw-Dahlonega (WED) back-arc basin. The Dadeville complex has been thrust over the WED back-arc basin and both must root outboard (SE) of the Mesoproterozoic Pine Mountain window. At the base of the Dadeville complex thrust sheet is the Ropes Creek Amphibolite (RCA) with a structural thickness of ≈ 6 km. The RCA was derived from metamorphism of tholeiitic basalts interlayered with minor felsic tuffs. Above the Ropes Creek (RCA), is the Agricola Schist (AS), a >1 km thick sequence of interbedded pelitic schist, metagreywacke, and amphibolite. The Ropes Creek (RCA) and Agricola Schist were intruded by several suites of felsic (e.g. Camp Hill batholith, 8 km thick) and mafic (e.g. the 1.5 km thick Doss Mountain suite with metagabbro and pyroxenite) plutonic rocks that comprise the majority of the Dadeville complex. All magmatic units, both intrusive and extrusive, exhibit arc-like geochemistry with large negative Nb-Ta anomalies and enriched LILE’s. Zircons from the Camp Hill batholith yield a Taconic U-Pb age of 460 ± 4 Ma. This is consistent with a Rb/Sr whole-rock age of 462 ± 4 Ma reported for the DC’s Franklin Gneiss (Seal and Kish, 1990). Also, detrital zircons from the Agricola Schist yield >50% Taconic ages with a lesser fraction of Grenville ages. The above observations indicate that the Dadeville complex is indeed a Taconic age arc, that it formed on Laurentian Mesoproterozoic crust outboard of a large back-arc basin (WED), and may have had reversed subduction polarity relative to the N. Appalachians.