Age and zircon inheritance of eastern Blue Ridge plutons, southwestern North Carolina and northeastern Georgia,with implications for magma history and evolution of the southern Appalachian orogen

High-resolution ion microprobe analysis of zircon has provided ages for pre viously undated plutons of the high-grade eastern Blue Ridge of northeaster n Georgia and southwestern North Carolina. These data, together with backsc attered electron imaging, reveal the presence of nearly ubiquitous inherite d cores of highly variable age and magmatic rims that have experienced vari able Pb loss, thus making interpretation of conventional U-Pb analyses very difficult. Ion probe rim analyses indicate that the plutons were emplaced during both the mid-Ordovician (Taconian orogeny; Whiteside pluton, 465 Ma; Persimmon Creek Gneiss, 480 Ma) and mid-Devonian (Acadian orogeny; Rabun p luton, 375 Ma; Pink Beds pluton, 390 Ma;Looking Glass pluton, 380 Ma). A la rge trondhjemite dike north of Asheville is less confidently assigned an ap e of 415 Ma. Zircons from all intrusions have predominantly 1.0 to 1.25 Ga cores (Grenvillian). In addition, both Devonian and Ordovician plutons have smaller populations of Late Proterozoic-early Paleozoic (0.5-0.75 Ga), Mid dle Proterozoic (1.4 Ga), and Late Archean (2.6-2.9 Ga) cores. The ubiquitous, round cores and thick magmatic rims suggest significant res orption and then protracted growth within the melts. Zircon saturation temp eratures based on whole-rock (similar to melt) Zr concentrations are lower than expected for magma generation (710 degrees-760 degrees C). Zirconium c oncentrations may not reflect saturation at maximum temperature, if melting was very rapid (<similar to 10(5) yrs), or if zircon cores represent grain s that were shielded from melt inside host grains for much of the magma tic history The Late Proterozoic-early Paleozoic and Grenvillian inheritance is similar to documented ages of basement exposed in the eastern Blue Ridge. No expos ures of 1.4 Ga rocks are known in the southern Appalachians, but detrital a nd inherited zircons of this age have been reported, and 1.4 Ga granites ar e widespread in the craton to the northwest. However, the combination of ab sence of Early Proterozoic zircon and presence of Late Archean zircon is in consistent with the known distribution of basement rocks in southeastern No rth America. No Archean rocks or inherited zircons have been reported from the southern and central Appalachians; the nearest Archean exposures are 10 00 km north, across a dominantly Early Proterozoic terrane. This suggests e ither that the Laurentian configuration of Archean basement was very comple x or that the crust that underlay the plutons at the time of their emplacem ent included a far-travelled terrane (emplaced during Grenvillian or early Paleozoic orogeny?). Ages of magmatic and inherited zones of zircon from the plutons demonstrate that similar crust underlay the eastern Blue Ridge during both Taconian an d Acadian orogenies, that there was no single episode of voluminous magmati sm, and that metamorphism and deformation began before 470 Ma and continued after 370 Ma. These plutons do not constitute a significant convergence-re lated are, though it is possible that they represent a displaced part of an are that lies primarily to the east (in the Inner Piedmont?). Previous geochemical studies have demonstrated that the eastern Blue Ridge intrusions include both a very primitive component, either mafic magma or r elatively young mafic source rock, and a component derived from more mature felsic crust. The abundant inherited zircon cores verify contributions fro m similar crust to all the plutons, but they almost certainly magnify the r eal mass contribution from mature crust, especially in the more primitive r ocks. There is no discernible distinction in petrogenesis between Taconian plutons and Acadian plutons.