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Cryptic crustal events elucidated through zone imaging and ion microprobe studies of zircon, southern Appalachian Blue Ridge, North Carolina–Georgia

¹ Department of Geology, Vanderbilt University, Nashville, Tennessee 37235
² Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
³ W. M. Keck Foundation Center for Isotope Geochemistry, Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024

Compositional zoning reveals multistage growth histories and resorption events in zircon from a high-grade terrane in the eastern Blue Ridge of North Carolina and Georgia. These zoning patterns were used to guide high-resolution ion microprobe dating that places important constraints on the evolution of the southern Appalachian crust. Zircons from granulite facies metapelite have unzoned rims that yield concordant U-Pb ages of 495 ± 14 Ma. We interpret this as the time of rim growth, which occurred during peak metamorphism early in the protracted orogenic history of the region. Detrital cores, characterized by truncated euhedral zoning, are of Grenville age (1.04–1.26 Ga). Zircons from the Whiteside and Rabun plutons have well-defined, rounded, inherited cores and euhedral, oscillatory-zoned magmatic rims. Rims of Rabun zircons record magmatic crystallization at 374 ± 4 Ma, whereas Whiteside rims yield a 466 ± 10 Ma crystallization age. Cores from both plutons include 1.1–1.3 Ga and 2.6–2.7 Ga ages. These data indicate that there was no single, voluminous episode of plutonism in this area, that similar material underpinned the region at least from 370 to 470 Ma, and that previously unrecognized Archean basement or Archean basement–derived sedimentary rock was present in the southern Appalachians. Results of this study verify the value of combining zoning and ion microprobe studies: Using conventional U-Pb methods or ion microprobe dating without knowledge of zoning would have made interpreting the events recorded in these zircons and the ages that they yield difficult or impossible.

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