ABSTRACT Ion microprobe U-Pb zircon rim ages from 39 samples from across the accreted terranes of the central Blue Ridge, eastward across the Inner Piedmont, delimit the timing and spatial extent of superposed metamorphism in the southern Appalachian orogen. Metamorphic zircon rims are 10–40 µm wide, mostly unzoned, and dark gray to black or bright white in cathodoluminescence, and truncate and/or embay interior oscillatory zoning. Black unzoned and rounded or ovoid-shaped metamorphic zircon morphologies also occur. Th/U values range from 0.01 to 1.4, with the majority of ratios less than 0.1. Results of 206 Pb/ 238 U ages, ±2% discordant, range from 481 to 305 Ma. Clustering within these data reveals that the Blue Ridge and Inner Piedmont terranes were affected by three tectonothermal events: (1) 462–448 Ma (Taconic); (2) 395–340 Ma (Acadian and Neoacadian); and (3) 335–322 Ma, related to the early phase of the Alleghanian orogeny. By combining zircon rim ages with metamorphic isograds and other published isotopic ages, we identify the thermal architecture of the southern Appalachian orogen: juxtaposed and superposed metamorphic domains have younger ages to the east related to the marginward addition of terranes, and these domains can serve as a proxy to delimit terrane accretion. Most 462–448 Ma ages occur in the western and central Blue Ridge and define a continuous progression from greenschist to granulite facies that identifies the intact Taconic core. The extent of 462–448 Ma metamorphism indicates that the central Blue Ridge and Tugaloo terranes were accreted to the western Blue Ridge during the Taconic orogeny. Zircon rim ages in the Inner Piedmont span almost 100 m.y., with peaks at 395–385, 376–340, and 335–322 Ma, and delimit the Acadian-Neoacadian and Alleghanian metamorphic core. The timing and distribution of metamorphism in the Inner Piedmont are consistent with the Devonian to Mississippian oblique collision of the Carolina superterrane, followed by an early phase of Alleghanian metamorphism at 335–322 Ma (temperature >500 °C). The eastern Blue Ridge contains evidence of three possible tectonothermal events: ~460 Ma, 376–340 Ma, and ~335 Ma. All of the crystalline terranes of the Blue Ridge–Piedmont megathrust sheet were affected by Alleghanian metamorphism and deformation.

The southern Appalachian crystalline core is composed of lithotectonic assemblages that are largely sedimentary in origin. Sixteen paragneiss samples from the Blue Ridge and Inner Piedmont of North Carolina and Georgia, and one sample of Middle Ordovician rocks from the Sevier-Blountian clastic wedge in the Tennessee Valley and Ridge were sampled for sensitive high-resolution ion microprobe (SHRIMP) U-Pb detrital zircon geochronology, whole-rock geochemistry, and zircon trace-element analyses. Detrital zircon ages range from Archean (~2.7 Ga) to Middle Paleozoic (~430 Ma), with a notable abundance of Mesoproterozoic zircons (1.3–0.9 Ga). Many samples also contain moderate populations of slightly older Mesoproterozoic zircons (1.5–1.3 Ga). Minor populations of Paleoproterozoic (2.3–1.5 Ga) and Neoproterozoic (754–717 and 629–614 Ma) ages occur in several samples; however, Paleozoic detrital zircons (478–435 Ma) are restricted to samples from the Cat Square terrane. Depositional periods of the metasedimentary terranes are bracketed by detrital zircon, metamorphic, and magmatic ages, and include: (1) Mesoproterozoic, (2) Neoproterozoic to early Paleozoic, and (3) middle Paleozoic. A xenolith from the ~1.15 Ga Wiley Gneiss suggests a post–~1.2 Ga period of sedimentation prior to the ~1.15 Ga Grenvillian magmatic event. Detrital zircon populations of Neoproterozoic to Middle Ordovician suggest a mixed Laurentian provenance with Amazonian and peri-Gondwanan sources deposited in divergent and convergent plate settings. Blue Ridge and Inner Piedmont detrital zircon ages, whole-rock geochemistry, lithologic assemblages, and field relationships are compatible with deposition of immature clastic material in a rift and passive-margin setting from the Neoproterozoic to early Paleozoic. Occurrence of 1.3–0.9 Ga, 1.5–1.3 Ga, and 754–717 Ma detrital zircon ages indicate a dominantly Laurentian provenance for the Cartoogechaye, Cowrock, Dahlonega gold belt, Smith River allochthon, and Tugaloo terranes. Minor Paleoproterozoic populations in these terranes suggest input from distal terranes of the Laurentian midcontinent or the Amazonian craton. Transition to a convergent plate margin in the Middle Ordovician resulted in collision of central Blue Ridge and Tugaloo terranes and recycling of material from these terranes into the Mineral Bluff Formation and Sevier Shale. Ordovician and 629–614 Ma detrital zircons from the Cat Square terrane document the first occurrence of peri-Gondwanan material, which was deposited in a convergent setting between the Laurentian margin and the accreting Carolina superterrane during the Late Silurian to Devonian.

A new tectonic map of the southern and central Appalachians incorporates modern field and structural-stratigraphic, geochronologic (mostly sensitive high-resolution ion microprobe–reverse geometry, SHRIMP–RG, and Sm-Nd), geochemical, and geophysical data to identify crustal boundaries and blocks. Major tectonic units include the ∼735 Ma Laurentian failed rift, ∼565 Ma rifted margin sedimentary-volcanic assemblage deposited on Grenvillian and pre-Grenvillian crust, the Laurentian platform, and a series of distal Laurentian terranes (Cowrock, Cartoogechaye, Tugaloo-Chopawamsic-Potomac) accreted to Laurentia during the Taconian (Ordovician) or Neoacadian (Late Devonian–early Mississippian) orogenies. The Dahlonega gold belt consists of more proximal metasandstone and pelitic schist; it also contains Ordovician arc volcanic rocks, and a mixed detrital zircon suite of Laurentian and Gondwanan, or Penokean, affinity. The newly recognized Cat Square terrane contains Laurentian, Avalonian, and 430 Ma detrital zircons, and is considered a remnant ocean basin that closed during Acadian-Neoacadian accretion of the Carolina superterrane. The Pine Mountain terrane (southernmost exposed Appalachians) consists of Grenvillian basement and a cover sequence bearing Gondwanan or Penokean detrital zircons. The Carolina superterrane contains numerous peri-Gondwanan terranes that were deformed, metamorphosed, and amalgamated prior to 530 Ma, then accreted to Laurentia during the Neoacadian along the central Piedmont suture. The Raleigh-Goochland terrane contains blocks of Laurentian basement and cover that moved SW (dextrally) out of the collision zone to the north as the Theic ocean closed north to south during the early Alleghanian orogeny. This event also produced the Kiokee-Raleigh belt high-grade metamorphic core in the eastern Piedmont, and includes faults of the Pine Mountain window. The latter is framed by Alleghanian thrust and dextral faults formed at different crustal depths (times?). Subsurface components of the southern and central Appalachians are recognized in potential field and limited drill data. The Carolina superterrane extends beneath the Coastal Plain—possibly eastward to the East Coast magnetic anomaly. South of the Carolina superterrane and north of the Wiggins suture is the Brunswick (Charleston) terrane, another peri-Gondwanan terrane. The east–west Alleghanian Wiggins suture with the Suwannee terrane is recognizable to the south beneath Georgia and Alabama in potential field data, truncating all Appalachian structures and older crustal blocks west of the Appalachians. South of the suture, African basement and cover lie in the eastern Florida subsurface, while to the west are other Gondwanan or peri-Gondwanan components that may have originally connected with Yucatan.

Sedimentary and metasedimentary rocks within the southern Appalachian Blue Ridge and Inner Piedmont contain a valuable record of Late Proterozoic Laurentian margin evolution following the breakup of Rodinia. Paleogeographic reconstructions and increasing amounts of geochronologic and isotopic data limit the derivation of these paragneisses to the Laurentian and/or west Gondwanan craton(s). Southern Appalachian crystalline core paragneiss samples have ε Nd values between –8.5 and –2.0 at the time of deposition and contain abundant 1.1–1.25 Ga zircon cores with Grenville 1.0–1.1 Ga metamorphic rims. Less abundant detrital zircons are pre-Grenvillian: Middle Proterozoic 1.25–1.6 Ga, Early Proterozoic 1.6–2.1 Ga, and Late Archean 2.7–2.9 Ga. Blue Ridge Grenvillian basement has almost identical ε Nd values and displays the same dominant magmatic core and metamorphic rim zircon ages. Based on our data, nonconformable basement-cover relationships, and crustal ages in eastern North America, we contend that the extensive sedimentary packages in the southern Appalachian Blue Ridge and western Inner Piedmont are derived from Laurentia. ε Nd values from Carolina terrane volcanic, plutonic, and volcaniclastic rocks are isotopically less evolved than southern Appalachian paragneisses and Blue Ridge Grenvillian basement, easily separating this composite terrane from the mostly Laurentian terranes to the west. Neoproterozoic and Ordovician, as well as Grenvillian and pre-Grenvillian, zircons in eastern Inner Piedmont paragneisses indicate that these samples were deposited much later and could have been derived entirely from a Panafrican source or possibly a mixture of Panafrican and recycled Laurentian margin assemblages.

A number of Grenvillian basement massifs occur in the southern Appalachian Blue Ridge. The largest are contained in the Blue Ridge anticlinorium, which extends northward from its widest point in western North Carolina to Maryland. The Tallulah Falls dome, Toxaway dome, and Trimont Ridge area contain small internal basement massifs in the eastern and central Blue Ridge of the Carolinas and northeastern Georgia. All are associated with Paleozoic antiformal culminations, but each contains different basement units and contrasting Paleozoic structure. The Tallulah Falls dome is a broad foliation antiform wherein basement rocks (coarse augen 1158 ± 19 Ma Wiley Gneiss [ion microprobe, 207 Pb/ 206 Pb], medium-grained 1156 ± 23 Ma [ 207 Pb/ 206 Pb] and 1126 ± 23 Ma [ 207 Pb/ 206 Pb] Sutton Creek Gneiss, and medium-grained to megacrystic 1129 ± 23 Ma Wolf Creek Gneiss [sensitive high resolution ion microprobe, SHRIMP, 207 Pb/ 206 Pb]) form a ring and spiral pattern on the west, south, and southeast sides of the dome. Basement rocks are preserved in the hinges of isoclinal anticlines whose axial surfaces dip off the flanks of the dome. The Wiley Gneiss was intruded by Sutton Creek Gneiss. The Toxaway dome consists predominantly of coarse, banded 1151 ± 17 Ma and coarse augen 1149 ± 32 Ma (SHRIMP 206 Pb/ 238 U) Toxaway Gneiss folded into a northwest-vergent, gently southwest- and northeast-plunging antiform that contains a boomerang structure of Tallulah Falls Formation metasedimentary rocks in the core near the southwest end. The coarse augen gneiss phase constitutes a larger proportion of the Toxaway Gneiss toward the northeast. Field evidence indicates that the augen phase intruded the banded Toxaway lithology; U/Pb isotopic ages of these lithologies, however, are statistically indistinguishable. The Trimont Ridge massif occurs in an east-west–trending antiform west of Franklin, North Carolina, and consists of felsic gneiss that yielded a 1103 ± 69 Ma SHRIMP 207 Pb/ 206 Pb age. An ε Nd -depleted mantle model age of 1.5–1.6 Ga permits derivation of all of these basement rocks (including most from the western Blue Ridge) from eastern granite-rhyolite province crust, except the Mars Hill terrane rocks, which yield 1.8–2.2-Ga model ages. The small Grenvillian internal massifs were probably rifted from Laurentia during the Neoproterozoic, and became islands in the Iapetus ocean that were later swept onto the eastern margin of Laurentia during Ordovician subduction and arc accretion. These massifs were additionally penetratively deformed and metamorphosed during the Taconian and Neoacadian orogenies.