In the southernmost Appalachians, the Hillabee Greenstone, an Ordovician volcanic arc fragment, lies directly atop the outermost Laurentian Devonian-earliest Mississippian(?) shelf sequence at the structural top of the greenschist facies Talladega belt, the frontal metamorphic allochthon along this orogenic segment. The Hillabee Greenstone was emplaced between latest Devonian and middle Mississippian time. It and the uppermost Laurentian section were later repeated together within a series of map-scale imbricate slices of a postmetamorphic, dextral, transpressional, Alleghanian thrust duplex system that placed the high-grade eastern Blue Ridge allochthon atop the Talladega belt. Geochemical and geochronologic (U-Pb zircon) studies indicate that the Hillabee Greenstone's interstratifi ed tholeiitic metabasalt and calc-alkaline metadacite/rhyolite formed within an extensional setting on continental crust ca. 460-470 Ma. Palinspastic reconstructions of the southern Appalachian Ordovician margin place the Hillabee Greenstone outboard of the present position of the Pine Mountain terrane and suggest links to Ordovician plutonism in the overlying eastern Blue Ridge, and possibly to widespread K-bentonite deposits within Ordovician platform units. The tectonic evolution of the Hillabee Greenstone exhibits many unusual and intriguing features, including: (1) premetamorphic emplacement along a basal cryptic thrust, which is remarkably concordant to both hanging wall and footwall sequences across its entire extent (>230 km), (2) forma-tion, transport, and emplacement of the arc fragment accompanied by minimal deformation of the Hillabee Greenstone and underlying outer-margin shelf rocks, (3) emplacement temporally coincident with the adjacent collision of the younger, tectonically independent Ouachita volcanic arc with southeastern Laurentia. These features highlight strong contrasts in the Ordovician-Taconian evolution of the southern and northern parts of the Appalachian orogen.

Fault-dismembered segments of a distinctive, extensive, highly allochthonous, and tectonically signifi cant Ordovician (ca. 480-460 Ma) basin, which contains suites of bimodal metavolcanic rocks, associated base metal deposits, and thick immature deepwater (turbiditic) metasediments, occur in parts of the southern Appalachian Talladega belt, eastern Blue Ridge, and Inner Piedmont of Alabama, Georgia, and North and South Carolina. The basin's predominantly metasedi mentary strata display geochemical and isotopic evidence of a mixed provenance, including an adjacent active volcanic arc and a provenance of mica (clay)-rich sedimentary and felsic plutonic rocks consistent with Laurentian (Grenvillian) upper-crustal continental rocks and their passive-margin cover sequences. Geochemical characteristics of the subordinate intercalated bimodal meta volcanic rocks indicate formation in a supra subduction environment, most likely a back-arc basin, whereas characteristics of metasedimentary units suggest deposition above Neoproterozoic rift and outer-margin lower Paleozoic slope and rise sediments within a marginal basin along Ordovician Laurentia's Iapetus margin. This tectonic setting indicates that southernmost Appalachian Ordovician orogenesis (Taconic orogeny) began as an extensional accretionary orogen along the outer margin of Laurentia, rather than in an exotic (non-Laurentian) arc collisional setting. B-type subduction polarity requires that the associated arc-trench system formed southeast of the palinspastic position of the backarc basin. This scenario can explain several unique features of the southern Appalachian Taconic orogen, including: the palinspastic geographic ordering of key tectonic elements (i.e., back-arc, arc, etc.), and a lack of (1) an obducted arc sensu stricto on the Laurentian margin, (2) widespread Ordovician regional metamorphism, and (3) Taconic klippen to supply detritus to the Taconic foreland basin.

Independent researchers working in the Talladega belt, Ashland-Wedowee-Emuckfaw belt, and Opelika Complex of Alabama, as well as the Dahlonega gold belt and western Inner Piedmont of Alabama, Georgia, and the Carolinas, have mapped stratigraphic sequences unique to each region. Although historically considered distinct terranes of disparate origin, a synthesis of data suggests that each includes lithologic units that formed in an Ordovician back-arc basin (Wedowee-Emuckfaw-Dahlonega basin-WEDB). Rocks in these terranes include varying proportions of metamorphosed mafi c and bimodal volcanic rock suites interlayered with deep-water metasedimentary rock sequences. Metavolcanic rocks yield ages that are Early-Middle Ordovician (480-460 Ma) and interlayered metasedimentary units are populated with both Grenville and Early-Middle Ordovician detrital zircons. Metamafi c rocks display geochemical trends ranging from mid-oceanic-ridge basalt to arc affi nity, similar to modern back-arc basalts. The collective data set limits formation of the WEDB to a suprasubduction system built on and adjacent to upper Neoproterozoiclower Paleozoic rocks of the passive Laurentian margin at the trailing edge of Iapetus, specifi cally in a continental margin back-arc setting. Overwhelmingly, the geologic history of the southern Appalachians, including rocks of the WEDB described here, indicates that the Ordovician Taconic orogeny in the southern Appalachians developed in an accretionary orogenic setting instead of the traditional collisional orogenic setting attributed to subduction of the Laurentian margin beneath an exotic or peri-Laurentian arc. Well-studied Cenozoic accretionary orogens provide excellent analogs for Taconic orogenesis, and an accretionary orogenic model for the southern Appalachian Taconic orogeny can account for aspects of Ordovician tectonics not easily explained through collisional orogenesis.

Ordovician and Silurian siliciclastic, volcanic, and plutonic rocks in the southern Appalachian eastern Blue Ridge and western Inner Piedmont, formerly interpreted as components of an obducted Taconic arc terrane, are recognized as parts of an Ordovician paired arc/back-arc system that formed on the seaward edge of the Laurentian plate. New crystallization ages from metamorphosed plutonic-volcanic components intruding and intercalated with metasedimentary back-arc basin lithologies of the Wedowee-Emuckfaw-Dahlonega back-arc basin confirm rapid deposition (hundreds of meters per million years) in an Ordovician−Silurian extensional basin that received input from continentally derived Mesoproterozoic crust and bimodal volcanic components. U-Pb (206Pb/238U) zircon ages from 12 samples of Zana Granite yield minimum crystallization ages between 459 Ma and 430 Ma, while zircon grains from six samples of Kowaliga Gneiss have minimum crystallization ages between 452 Ma and 435 Ma. Zircons from a probable metavolcanic unit within the Wedowee Group yield a 206Pb/238U age of 454 ± 3 Ma. Mesoproterozoic depleted mantle model ages and negative initial ƐHf values of zircons from the Ordovician−Silurian metaigneous rocks suggest a significant Mesoproterozoic component in their genesis. Formation of these silicic plutons was likely associated with the intrusion of mantle-derived mafic melts and decompression melting of Mesoproterozoic lower crust as part of a tectonically thickened crustal section with a contribution from sub-continental mantle lithosphere during back-arc extension. Silicic magma intruding thick successions of bimodal volcanic rocks and intercalated sedimentary rocks shortly after deposition is typical of back-arc extension and contraction phases above B-type subduction zones (i.e., Benioff oceanic subduction) in Lachlan-style orogens, which is consistent with the accretionary orogenic nature of the southern Appalachian Taconic orogen.

No abstract