REMNANTS OF A PAIRED LAURENTIAN TACONIC ARC/BACK-ARC SYSTEM IN THE SOUTHERNMOST APPALACHIANS?

Remnants of an Ordovician (Taconic) back-arc basin (Wedowee-Emuckfaw-Dahlonega Basin-WEDB) occur within the Appalachian Blue Ridge and Inner Piedmont (IP) in AL, GA, NC, and SC. The WEDB contains immature turbidites, bimodal metavolcanic rocks, and base metal deposits. Covering an area of >4,000 km², it is >10 km thick and contemporaneous with the Blount (Taconic) foreland basin. Geochemistry of the WEDB metavolcanic rocks indicates formation in a supra-subduction (back-arc) setting. U/Pb zircon ages of felsic metavolcanic rocks (~480–460 Ma) and granitoids that intrude the WEDB sequence (~463-450 Ma) provide age constraints on basin formation. U/Pb ages of detrital zircons in metasedimentary units are dominantly Mesoproterozoic (mostly ~1.2 to ~0.9 Ga, but up to ~1.5 Ga) and Paleozoic (~474-460 Ma), suggesting both Laurentian basement and contemporaneous arc sources. A potential, extra-basinal source of Paleozoic arc detritus is the Dadeville complex (DC), which now lies structurally above the WEDB sequence due to Alleghanian thrusting. The DC is nested in a large (>4,300 km²), synformal klippe along the west flank of the IP in AL and GA. The DC comprises a lower metavolcanic unit (>10 km thick) derived from mafic, intermediate, and minor felsic tuffs interlayered with tholeiitic basalts. A >1 km thick sequence of interbedded pelitic schist, metagreywacke, and minor amphibolite overlies the metavolcanic unit. Both sequences were intruded between ~450 and ~465 Ma by felsic (metatonalite and metagranite) and mafic-ultramafic (metanorite, metagabbro, and metapyroxenite) rocks that comprise >50% of the DC. Ages of detrital zircons from DC metasediments are similar to those of the WEDB with significant groupings at ~460 Ma and ~900 to1100 Ma; the oldest grains are ~1300 Ma. The similarities of the WEDB and DC detrital zircon age spectra, the range of lithologies in the DC, the major and trace element chemistry of WEDB and DC igneous rocks, the overlapping ages of the WEDB and DC igneous rocks, and their relative structural and spatial positions suggest the WEDB-DC formed on the Laurentian plate’s rifted margin, as a back-arc basin/fringing arc outboard of the continental hinge zone during the Taconic. Subsequently this association was thrust-telescoped to its current configuration during the Alleghanian.