NEW GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF THE WESTERN RALEIGH TERRANE IN THE NORTH CAROLINA EASTERN PIEDMONT

Correlations among crustal blocks juxtaposed by dextral strands of the Eastern Piedmont fault system (EPFS) during Pangea formation is an ongoing lithotectonic problem in the southern Appalachian hinterland due to a lack of discriminating geochemical and geochronological datasets. In the NC and VA eastern Piedmont, the Raleigh terrane (Rt) is an amphibolite-facies block that includes an eastern Early Devonian arc-basin complex (eRt) based on new mapping and geochronological datasets, and a well-mapped western portion (wRt) of unclear affinity and age due to a sparse geochronological dataset. The Lake Gordon strand of the EPFS has an unconstrained slip magnitude and separates these two portions of the Raleigh terrane. The wRt has been lithologically correlated with the ca. 633–528 Ma peri-Gondwanan Carolinia domain in NC, or the ca. 1100–385 Ma Grenville Goochland domain (terrane) in VA. It may also be linked with the eRt as a crustal block translated from the northern Appalachian Ganderian domain based on similar lithologies, high-grade metamorphism, and U-Pb zircon datasets.

The wRt includes the Raleigh Gneiss, a layered unit of amphibolite-facies mafic to felsic rocks, and the Falls Leucogneiss, a ca. 545 Ma leucogranite (Caslin et al., 2001) that intrudes the Raleigh Gneiss. New whole-rock geochemical analyses from these units are presented here. Major element diagrams highlight a crystal fractionation trend indicative of a calc-alkaline island-arc with a minor tholeiitic component. Chondrite-normalized REE diagrams show enriched light REEs, flat to moderately negative heavy REEs, and varying depletions and enrichments in Eu. N-MORB diagrams reinforce an island-arc origin due to depletions in Nb(Ta) and Ti and enriched LIL elements relative to HFS elements and REEs. On petrogenetic discrimination diagrams, most mafic and intermediate rocks plot in island-arc fields; few samples indicate N-MORB or BAB basalt. Felsic rocks plot as I-type granite having an island-arc signature for the Raleigh Gneiss and A-type, within-plate granite for the Falls Leucogneiss. The island-arc signature of the wRt may be related to the eRt as a single block of Neoproterozoic and Devonian rocks, or each may be distinct terranes across the Lake Gordon shear zone, perhaps displaced from Goochland or Ganderia rather than Carolinia.