ION MICROPROBE ANALYSES OF ZIRCON RIMS FROM THE EASTERN BLUE RIDGE AND INNER PIEDMONT, NC-SC-GA: IMPLICATIONS FOR THE TIMING OF PALEOZOIC METAMORPHISM IN THE SOUTHERN APPALACHIANS

The timing of Paleozoic metamorphism in the southern Appalachian Piedmont Terrane remains controversial and enigmatic despite decades of research. Cathodoluminescence (CL) and back scattered electron (BSE) images of numerous polished zircons reveal thin rims (<10 microns) that contain valuable information concerning the tectonic evolution of this subdivision. Ion microprobe analyses of magmatic interiors of these samples have yielded ages that range from Mesoproterozoic (~1160 Ma) to Devonian (~370 Ma). Due to difficulty associated with the small rim size, we have been able to obtain only a small number of polished rim analyses. To overcome the difficulty associated with polished mounts, we mounted zircon grains in epoxy and analyzed the unpolished surface in order to date the outermost rims. Samples analyzed using this technique include the NC Toxaway Gneiss (magmatic age of ~1160 Ma, GA), NC Henderson Gneiss (~490 Ma), NC Whiteside pluton (~470 Ma), GA Rabun pluton (~370 Ma), NC Persimmon Creek Gneiss (~480 Ma), and the GA Tallulah Falls Formation (detrital cores ~1.1-1.2 Ga). Distinct rims ages were not detected in the Persimmon Creek Gneiss or Rabun pluton but the remaining samples yield rim ages of ~325-380 Ma. A probability plot of these data, combined with data obtained on polished grain mounts from the NC Dysartsville orthogneiss, NC Tallulah Falls Formation, and GA Toxaway Gneiss, yields a well-defined maximum at an age of 352 Ma, consistent with previous work from the eastern Blue Ridge and Inner Piedmont (e.g., Kish, 1990; Dennis and Wright, 1997; Mirante and Patino-Douce, 2000). The Henderson Gneiss, Dysartsville orthogneiss, and two Tallulah Falls sample localities are east of the Brevard fault zone in the Inner Piedmont and record the same metamorphic age as the samples in the eastern Blue Ridge, suggesting that juxtaposition of the Eastern Blue Ridge and Inner Piedmont occurred prior to rim growth. Zircon rim growth, which was probably a response to a thermal pulse and/or regional fluid fluxing, documents widespread late Acadian tectonism.