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

Abstract

High-resolution ion microprobe analysis of zircon has provided ages for previously undated plutons of the high-grade eastern Blue Ridge of northeastern Georgia and southwestern North Carolina. These data, together with backscattered electron imaging, reveal the presence of nearly ubiquitous inherited cores of highly variable age and magmatic rims that have experienced variable 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; Persim- mon Creek Gneiss, 480 Ma) and mid-Devonian (Acadian orogeny; Rabun pluton, 375 Ma; Pink Beds pluton, 390 Ma; Looking Glass pluton, 380 Ma). A large trondhjemite dike north of Asheville is less confidently assigned an age 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), Middle Proterozoic (1.4 Ga), and Late Archean (2.6-2.9 Ga) cores. The ubiquitous, round cores and thick magmatic rims suggest significant resorption and then protracted growth within the melts. Zircon saturation tem- peratures based on whole-rock (melt) Zr concentrations are lower than ex- pected for magma generation (710°-760°C). Zirconium concentrations may not reflect saturation at maximum temperature, if melting was very rapid (10 5 yrs), or if zircon cores represent grains that were shielded from melt inside host grains for much of the magmatic history. The Late Proterozoic-early Paleozoic and Grenvillian inheritance is similar to documented ages of basement exposed in the eastern Blue Ridge. No expo- sures of 1.4 Ga rocks are known in the southern Appalachians, but detrital and inherited zircons of this age have been reported, and 1.4 Ga granites are widespread in the craton to the northwest. However, the combination of absence of Early Proterozoic zircon and presence of Late Archean zircon is inconsistent with the known distribution of basement rocks in southeastern North America. No Archean rocks or inherited zircons have been reported from the southern and central Appalachians; the nearest Archean exposures are 1000 km north, across a dominantly Early Proterozoic terrane. This suggests either that the Laurentian configuration of Archean basement was very complex or that the crust that underlay the plutons at the time of their emplacement included a far-travelled terrane (emplaced during Grenvillian or early Paleozoic orogeny?). Ages of magmatic and inherited zones of zircon from the plutons demon- strate that similar crust underlay the eastern Blue Ridge during both Taconian and Acadian orogenies, that there was no single episode of voluminous magma- tism, and that metamorphism and deformation began before 470 Ma and contin- ued after 370 Ma. These plutons do not constitute a significant convergence- related arc, though it is possible that they represent a displaced part of an arc 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 relatively young mafic source rock, and a component derived from more mature