GEOCHRONOLOGY AND GEOCHEMISTRY OF MID PALEOZOIC GRANITIC MAGMATISM, CENTRAL AND EASTERN INNER PIEDMONT, NC & SC

New SHRIMP zircon U-Pb age data, major and trace element geochemistry, and Nd and Sr isotope data from six plutonic bodies in the central and eastern Inner Piedmont (IP) of North and South Carolina help constrain the development of the southern Appalachians throughout the mid-Paleozoic. The studied intrusions include: the Walker Top orthogneiss at Icy Knob, NC; the Pacolet Mills and Gray Court granites, SC; the Reedy River gneiss near Fork Shoals, SC (Wagener, 1977); an augen gneiss that crops out in the North Tyger River at Anderson’s Mill shoals, SC (Curl, 1997); and an augen gneiss from a quarry near Pelham, SC (Maybin, 1997).

Compositionally these rocks are peraluminous granites and granodiorites (SiO₂ 66-74 wt.%). They are characterized by low Sr (50-300 ppm), high Rb (100-300 ppm) and are relatively enriched in K₂O (~4.5 wt.%). They generally have slightly negative Eu anomalies and are enriched in HREE. Isotopically, at the time of formation, these intrusions were similar to metasedimentary lithologies of the IP [eNd (-2 to –6) and ⁸⁷Sr/⁸⁶Sr (0.756-0.712)] suggesting that they were derived from sources similar to the rocks they intrude.

Ages range from late-Silurian to Pennsylvanian (Walker Top, ~370 Ma; Pacolet Mills, ~310 Ma; Gray Court, ~355 Ma; Reedy River, ~320 Ma; Anderson’s Mill, ~425 Ma; Pelham, ~400 Ma). Curiously, these intrusions nearly or completely lack older inherited zircon cores that are characteristic of plutons of similar age in the eastern Blue Ridge to the west. Granitoid rocks in the range 425-310 Ma are currently unknown in the western IP (Vinson, 1999). Additionally, Taconic age intrusives are currently unknown from the central and eastern IP. These two areas of the IP therefore differ in their magmatic histories. These new ages, together with previous age data from the Toluca (~375 Ma) and Cherryville (~375 Ma) granites, show that magmatism occurred over a ~115 My time period during the mid Paleozoic and that no distinction between an Acadian or Alleghanian event can be made based on plutonism.