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The record of the Taconic orogeny in the southern Appalachian orogen lacks clear evidence for many attributes of this classic Ordovician orogeny in the Northern Appalachians, including subduction of Laurentian lithosphere, the presence of Taconic allochthons and associated metamorphism and deformation, obduction of arc terranes, and accretion of Gondwanan terranes. Consequently, identifying a Taconic arc fragment in the southern Appalachian orogen, constraining its origin (e.g., peri-Laurentian or peri-Gondwana or intra-oceanic), and determining the polarity of subduction it represents are critical factors needed to evaluate the character of Taconic geodynamics throughout the Appalachians and Caledonides, including the location of the Taconic suture(s). In the southern Appalachian orogen, the Dadeville Complex is an amphibolite- facies allochthon with prominent mafic and ultramafic units in the Inner Piedmont of Alabama and Georgia. It has been proposed to be an accreted, peri-Gondwanan arc fragment obducted onto the Laurentian (or peri-Laurentian) margin. In contrast, we interpret 450-440 Ma U-Pb ages of zircons from Dadeville Complex meta-igneous rocks, Paleozoic and Precambrian detrital zircons from Dadeville Complex metasedimentary rocks, initial εNd values of igneous protoliths (basalt-trondhjemite-granite), initial εHf values of igneous and detrital zircons, and normalized trace-element abundances of mafic rocks to indicate that the Dadeville Complex likely formed in an Ordovician suprasubduction environment on Laurentian crust. This conclusion is compatible with recent interpretations of west-directed subduction as the concluding event of the northern Appalachian Taconic orogeny, but it suggests that the southern Laurentian margin was open throughout the period of the Taconic orogeny.
The Taconic orogeny in the northern Appalachians is characterized by obduction of Ordovician arc terranes onto Laurentian crust, however exotic obducted arc terranes have not been identified in the southern Appalachians. The Dadeville complex of the Inner Piedmont comprises the most likely Taconic arc assemblage in the southern Appalachians because it contains mafic/ultramafic intrusions, batholith-scale granitic bodies, and volcanic sequences with a combined tectonostratigraphic thickness of ~15 km. It forms a broad klippe within a regional synform that was thrust from southeast of the Mesoproterozoic Pine Mountain basement massif over a proposed eastern Blue Ridge/Inner Piedmont Ordovician back-arc assemblage. The Ropes Creek Amphibolite, derived from ~ 9 km of tholeiitic basalt, metasedimentary rocks, and minor felsic tuff, forms the base of the Dadeville thrust sheet. It is overlain by the Agricola Schist, a >1 km thick sequence of pelitic schist, metagreywacke, and amphibolite. Both units were intruded by felsic and mafic-ultramafic plutonic rocks that comprise a majority of the complex. All magmatic units exhibit arc-like geochemistry with large negative Nb-Ta anomalies and enriched LILE's. Zircons from the felsic plutons yield Taconic U-Pb ages of 461 ± 4 Ma to 448 ± 2 Ma. Zircons from the Agricola Schist and Ropes Creek Amphibolite metasediments yield a significant fraction of Grenville and Granite-Rhyolite Province ages, but also grains as young as 440 Ma. Although it has been previously proposed that the Dadeville is an accreted, peri-Gondwanan arc fragment obducted onto the Laurentian (or peri-Laurentian) Inner Piedmont and eastern Blue Ridge/Brevard zone, we interpret 440-480 Ma U-Pb ages of zircons from Dadeville complex metaigneous rocks, Paleozoic and Precambrian detrital zircons from Dadeville complex metasedimentary rocks, initial ƩNd of igneous protoliths (basalt-trondhjemite-granite), initial ƩHf values of igneous and detrital zircons, and normalized trace element abundances of mafic rocks to indicate that the Dadeville complex formed in an Ordovician suprasubduction environment on Laurentian crust.
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.