AN EXOTIC BLUE RIDGE BASEMENT: NEW ISOTOPIC EVIDENCE

New whole-rock Pb isotopic data combined with existing Pb, Sm-Nd, and U/Pb zircon data from the Blue Ridge and the Mid-Continent Granite Rhyolite terrane support recent suggestions that the southern and central Appalachian basement (SCAB) is exotic with respect to Laurentia. The conventional view that basement in the southern Appalachians represents juvenile Mesoproterozoic crust, the final stage of the southeastward growth of Laurentia prior to Rodinian collision, has been called into question based upon geochronological and isotopic data. Our expanded whole-rock Pb isotopic database for the Blue Ridge and the mid-continent Granite Rhyolite terrane, combined with existing Pb, Sm-Nd, and U/Pb zircon data, verify that SCAB is exotic to Laurentia. The 1.0-1.25 Ga Llano Uplift, 1.0-1.3 Ga Adirondacks, and 1.4-1.5 Ga Granite-Rhyolite terrane all have Sm-Nd depleted mantle model ages (TDM) close to their U/Pb zircon ages, and are thus apparently juvenile crustal material. In addition, all have low 207Pb/204Pb (for a given 206Pb/204Pb). Conversely, most 1.0-1.2 Ga SCAB massifs have TDM ages >1.5 Ga, indicating incorporation of pre-existing crustal material with TDM ages greater that those found in juvenile rocks of eastern North America. In addition, all SCAB massifs sampled thus far have a more elevated 207Pb/204Pb (for a given 206Pb/204Pb) than any nearby juvenile Laurentian crust. The combined Pb data suggest that two isotopically distinct reservoirs are required to produce the Pb isotopic array observed for Mesoproterozoic eastern North America. One reservoir, characterized by low 207Pb/204Pb, produced juvenile crust found in the Adirondacks, Llano, and mid-continent. A second reservoir, characterized by higher 207Pb/204Pb, produced the SCAB massifs. Thus, both Sm-Nd and Pb isotopic data, perhaps the most robust indicators of crustal origin in high-grade terranes, preclude derivation of SCAB from any nearby juvenile crustal material. The clearest evidence for exotic crust in the SCAB comes from the Roan Mountain area of eastern TN-western NC (northern Mars Hill terrrane). U/Pb SHRIMP ages up to 1.8 Ga for igneous rocks, abundant detrital zircons >2.0 Ga, TDM ages >2.0 Ga, and the most elevated 207Pb/204Pb observed in eastern North America demonstrate the anomalous age (Paleoproterozoic?) and isotopic characteristics of this crustal block with respect to the surrounding basement.