A TERRANE BOUNDARY IN THE CENTRAL GEORGIA (USA) INNER PIEDMONT CONFIRMED USING NEW U-PB SHRIMP AGES OF GRANITIC ROCKS

The southern Appalachian Inner Piedmont (IP) contains two lithotectonic terranes distinguished by sedimentary provenance and magmatic history, the Tugaloo (Tg, W) and Cat Square (CS, E) terranes, separated by the Brindle Creek fault (BCf). The Tugaloo terrane consists of Neoproterozoic to Ordovician(?) metasedimentary rocks with Laurentian provenance and a dominance of Ordo-silurian magmatism. CS terrane metasedimentary rocks reveal both Laurentian and Peri-Gondwanan sources, with mostly Devonian-Mississippian and Carboniferous plutonism. Numerous small Grenvillian basement massifs occur in the Tg terrane, while none have been found in the CS terrane. Detailed geologic mapping in the central GA IP along a prominent aeromagnetic lineament interpreted to represent the BCf revealed a distinct separation of granitoids on two sides of a high-temperature fault. New U-Pb SHRIMP dates contain a variety of crystallization ages and magmatic inheritance (rare for IP granitoids) of plutonic rocks from this area. The most voluminous granitoid NW of the fault is the Ordo-silurian (450-443 Ma) Lithonia gneiss, which also contains xenoliths of 1031 ± 23 Ma augen gneiss. SW of the BCf, the High Falls granodiorite is the most abundant granitoid, with three samples ranging from Silurian to Mississippian (424.1 ± 3.6, 382.7 ± 2.7, and 325.5 ± 1.3 Ma). A medium-fine grained granitoid was found on both sides of the BCf that crosscuts regional foliation. Samples from both sides of the fault contain abundant Carboniferous zircons (both whole grains and overgrowths) with inherited cores. Igneous zircons SE of the fault contain exclusively Peri–Gondwanan cores with no Laurentian inheritance, while to the NW, inheritance is solely Laurentian. Similar partitioning was first observed in analysis of detrital zircons in metasedimentary rocks from NC, and this partitioning of magmatic inheritance confirms the BCf and terrane boundary occurs at least this far SW in the IP.