MID-PALEOZOIC MAGMATISM IN THE EASTERN BLUE RIDGE AND INNER PIEDMONT: CONSTRAINTS FROM GEOCHEMISTRY AND GEOCHRONOLOGY

Mid-Paleozoic plutons are common throughout the southern Appalachian eastern Blue Ridge and Inner Piedmont. Both terranes were magmatically active from the late Silurian to the Mississippian while magmatism in the Inner Piedmont continued until the latest Pennsylvanian.

Curiously, mid-Paleozic plutonic rocks are sparse in the western Inner Piedmont (presently limited to the ~330 Ma Zirconia pegmatite in southwestern North Carolina) between the Brevard zone and the Brindle Creek fault and age equivalent intermediate and mafic intrusive rocks are lacking in both terranes. Eastern Blue Ridge plutons are distinctly different from Inner Piedmont plutons, reflecting different sources and generation depths. Geochemical and isotopic data indicate significant input of juvenile mafic material at deep crustal levels for eastern Blue Ridge magmas although zircon saturation temperatures below 800°C and near ubiquitous zircon core inheritance suggest melting took place at low magmatic temperatures, probably in the presence of hydrous fluids. Isotopic ages for eastern Blue Ridge zircon cores indicate the dominant Zr source was Grenville aged basement or metasediments with Grenville detrital grains with subordinate sources as young as Ordovician and as old as Archean. Inner Piedmont plutons record higher zircon saturation temperatures (>800°C) and lack appreciable zircon core inheritance suggesting higher crustal temperatures during magma generation. Geochemical and isotopic data for mid-Paleozoic Inner Piedmont intrusions suggest magma generation at shallower crustal levels and a higher proportion of mature continental material than eastern Blue Ridge intrusions. Scarce inherited zircon cores in Inner Piedmont granitoids yielded Grenville and early Paleozoic U/Pb ages.

These relationships suggest that a considerable distance likely separated the two magmatic terranes during the mid-Paleozoic and that it is unlikely the magmas formed as a result of subduction tectonics. Recently described, Neoacadian orgenesis (360-340 Ma) coincides well with magmatic ages for some Inner Piedmont intrusions but the overall duration of magmatism in both the Inner Piedmont and eastern Blue Ridge suggests a prolonged period (80-100 Ma) of magmatic conditions instead of discrete orogenic pulses.