WHERE IS THE ARC? DISCONTINUITIES IN TACONIAN ARC MAGMATISM IN THE SOUTHERN APPALACHIANS

Recent ion probe zircon geochronology confirms the widely held interpretation that Taconian events were widespread in the Piedmont Terrane (PT; Eastern Blue Ridge + Inner Piedmont) of NC, SC, GA, and AL. Ages of mafic and felsic magmatism and high-grade metamorphism cluster between 460 and 470 Ma, possibly extending to 490 Ma. Almost all models for Taconian orogeny throughout the Appalachians call upon closure of an ocean basin east of autochthonous Laurentia by subduction. Isotopically primitive, MORB-like mafic complexes in NC and northern GA that were metamorphosed at high P appear to document formation and accretion of oceanic crust. However, evidence for a well-developed arc magmatic belt (akin to the Chopawamsic or Shelburne Falls/Bronson Hill Arcs) does not persist throughout the length of the PT, suggesting that either a subduction-arc margin was discontinuous or the resulting rock assemblage has been hidden or removed.

From the NC-GA border through AL, thrust sheets (Dahlonega Gold Belt, Hillabee Greenstone) contain 460-470 Ma low to medium-grade sequences of mafic and felsic volcanic rocks, immature metasediments, and exhalites that apparently represent an arc system. The compositionally diverse, dominantly intermediate, 465 Ma Persimmon Creek Gneiss (GA-NC) and Elkahatchee Quartz Diorite (AL) (~490 Ma, Russell et al., 1987) are likely the deep roots of this arc complex. North of the NC-GA border, no clear evidence for a subduction-related arc exists. Although felsic plutons of Ordovician age are common in the PT of NC, intermediate and mafic plutonic rocks are absent, and there is no evidence of volcanic rocks or volcanic input into sediments.

The absence of a lithologic record of an arc in NC indicates that, if it existed there, its remnants are either eroded away or, more likely, deeply buried. Alternatively, the ocean basin whose closure resulted in the southerly magmatic arc and accretion of ocean floor fragments may have been small and pinched out to the north (analogous to Gulf of California or Red Sea?). The postulated small basin thus would have closed rapidly and generated a short-lived, immature, discontinuous arc that never connected to the more northerly Appalachian arcs.