CRYPTIC CRUSTAL EVENTS ELUCIDATED THROUGH ZONE IMAGING AND ION MICROPROBE STUDIES OF ZIRCON, SOUTHERN APPALACHIAN BLUE-RIDGE, NORTH-CAROLINA GEORGIA

Compositional zoning reveals multistage growth histories and resorptio n events in zircon from a high-grade terrane in the eastern Blue Ridge of North Carolina and Georgia These zoning patterns,were used to guid e high-resolution ion microprobe dating that places important constrai nts on the evolution of the southern Appalachian crust. Zircons from g ranulite facies metapelite have unzoned rims that yield concordant U-P b ages of 495 +/- 14 Ma. We interpret this as the time of rim growth, which occurred during peak metamorphism early in the protracted orogen ic history of the region. Detrital cores, characterized by truncated e uhedral zoning, are of Grenville age (1.04-1.26 Ga). Zircons from the Whiteside and Rabun plutons have well-defined, rounded, inherited core s and euhedral, oscillatory-zoned magmatic rims. Rims of Rabun zircons record magmatic crystallization at 374 +/- 4 Ma, whereas Whiteside ri ms 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 ,vas no single, voluminous episode of plutonism in this area, that sim ilar material underpinned the region at least from 370 to 470 Ma, and that previously unrecognized Archean basement or Archean basement-deri ved 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 wit hout knowledge of zoning would have made interpreting the events recor ded in these zircons and the ages that they yield difficult or impossi ble.