LAURENTIAN AND EXOTIC COMPONENTS OF THE SOUTHERN APPALACHIAN INNER PIEDMONT

The Brevard fault zone represents the western boundary of the Inner Piedmont and is an internal boundary of the Tugaloo terrane. Granitoid ages and chemistry indicate that the Brevard fault zone is also an important geochemical and thermal boundary. Sampled southern Appalachian crystalline core paragneisses from each side of the Brevard fault zone in the Blue Ridge and western Inner Piedmont have a Laurentian provenance. Relatively high initial ⁸⁷Sr/⁸⁶Sr values between 0.707 and 0.723, Early to Middle Proterozoic T_DM ages, high SiO₂ content, and LREE enrichment of these samples are remarkably similar to Blue Ridge Grenvillian basement data. Low chemical index of alteration values mostly between 41 and 66, and restricted Th/Sc and Zr/Sc values suggest both passive continental margin and continental arc depositional settings.

The eastern Inner Piedmont, separated from the western Inner Piedmont by the Brindle Creek fault and from the Carolina terrane by the Central Piedmont suture, is anomalous relative to the Laurentian and reworked Laurentian rocks of the Blue Ridge and western Inner Piedmont. The timing of deposition for two eastern Inner Piedmont samples is delimited by an Early Silurian detrital zircon component and the age of the oldest intrusive, ~380 Ma for the Toluca Granite. These constraints favor Late Silurian (possibly Early Devonian) deposition almost immediately followed by burial, tectonic thickening, and magma generation. In addition to Silurian and Ordovician detrital grains, the eastern Inner Piedmont samples also contained 504, 589, and 590 Ma detrital grains, ages not present in other samples except for a single 632 Ma analysis from the Dahlonega gold belt. These ages are best explained by proximal Panafrican crustal material at the time of deposition. Laurentian rocks of these ages are unlikely; magmatic rocks of this age, however, are present in the Carolina terrane. The eastern Inner Piedmont assemblage of mostly metapelite and middle Paleozoic granitoids could have developed as an accretionary prism in front of the accreting Carolina terrane. Eastern Inner Piedmont granitoid chemistry resembles that of the enclosing metasedimentary assemblage suggesting melting occurred in a thickened stack of thrust sheets beneath the overriding Carolina terrane during Neoacadian collision.