ORIGINS OF THE RALEIGH TERRANE: NEW LITHOLOGIC AND STRUCTURAL INSIGHTS TO AN OLD PROBLEM

The North Carolina eastern Piedmont exposes an orogen-scale crustal block called the Raleigh terrane. This lithotectonic element contains highly deformed amphibolite facies rocks. It has been linked to (1) Carolinia, a ca. 633-528 Ma island-arc that is peri-Gondwanan and exotic to Laurentia; or (2) Goochland, a ca. 1.1 Ga and ca. 385 Ma suite of magmatic rocks linked to Laurentia. In each scenario, the Nutbush Creek fault zone (NCFZ), a dextral strand of the Alleghanian orogeny Eastern Piedmont fault system (EPFS) separates the western Raleigh terrane from Carolinia and Goochland. The Macon fault zone (MFZ), another dextral strand of the EPFS, marks its eastern boundary.

NCGS STATEMAP and USGS EDMAP mapping between these high-strain zones define several Raleigh terrane lithodemes that include hornblende biotite gneiss, porphyroclastic K-feldspar or plagioclase biotite gneiss, and garnet-sillimanite white mica schist. They form an interlayered suite of meta-igneous and clastic metasedimentary rocks when map results are combined with preliminary U/Pb zircon analyses. A regionally penetrative dextral protomylonitic to ultramylonitic or phyllonitic shear foliation and mineral stretch lineation overprint these lithodemes across the Raleigh terrane. These fabric elements are associated with recumbent tight to isoclinal F₂ folds that refold isoclinal F₁ folds. Both fold generations record early phases of the EPFS. Shallowly plunging, upright open F₃ folds including the macroscale Inez-Afton synform reorient these fabric elements due to continued transpression between the NCFZ and MFZ.

U/Pb analyses from two lithodemes on opposite sides of the Raleigh terrane between the NCFZ and MFZ indicate the presence of detrital zircon. The western unit bears ages no younger than Mesoproterozoic and the eastern unit records ages as young as Early Devonian (Peach et al., 2017). Along-strike correlations of these units is problematic because Mesoproterozoic and Devonian detrital zircon ages are not reported in Carolinia, and Goochland rocks are reported to have igneous protoliths. The Raleigh terrane may be a far-traveled peri-Gondwanan or Laurentian crustal block dextrally translated along the EPFS, perhaps from the northern Appalachian orogen where Early Devonian rocks are reported in peri-Gondwanan terranes.