STATUS OF GEOLOGIC MAPPING AND LA-ICP-MS U-PB ZIRCON GEOCHRONOLOGY FROM THE RALEIGH TERRANE IN THE NORTH CAROLINA EASTERN PIEDMONT

The Raleigh terrane (Rt) is a third-order, high-grade infrastructural block in the North Carolina and Virginia Piedmont that is correlated with the second-order ca. 633–528 Ma peri-Gondwanan Carolinia or ca. 1100 and 385 Ma Laurentian Goochland lithotectonic domains. The dextral Nutbush Creek-Lake Gordon shear zones separate the Carolinia and Goochland domains from the western Rt. The Macon-Hollister fault zones separate the ca. 630–516 Ma Spring Hope and ca. 675–610 Ma Roanoke Rapids terranes from the eastern Rt. These low-grade suprastructural blocks are also linked with Carolinia.

Detailed mapping indicates that Rt meta-igneous and metasedimentary lithodemes record mylonitic and phyllonitic shear foliations, shallow mineral stretch lineation, and dextral indicators. Recumbent tight to isoclinal F₂ folds locally refold F₁ isoclinal folds in the higher grade western Rt; F₁ is not observed in the lower grade eastern Rt. Shallowly plunging, upright to overturned open to tight F₃ folds, including the macroscale Inez-Afton synform, reorient earlier structures. Regional fabric is compatible with folded west-side-north shear strain across the entire Rt block during late Paleozoic transpression in the southern Appalachian hinterland.

Zircon U-Pb geochronology on a quartz diorite gneiss yielded a unimodal 403.2 ± 3.2 Ma age, similar to a 410 ± 3.7 Ma age on an orthogneiss 5 km to the SW. These Rt calc-alkaline orthogneisses record Early Devonian magmatism. Three Rt detrital samples include: (1) ca. 2711–322 Ma ages on a garnet-rich schist (Z174), (2) ca. 2108–314 Ma ages on a porphyroclastic granitic gneiss migmatite (Z171), and (3) ca. 1955–356 Ma ages on a white mica schist (Z173). Z174 has ca. 2711–2020, 1900–850, 632–616, and 510–422 Ma ages, and a single 322 Ma grain. Z171 yielded five ca. 2108–842 Ma grains and has additional ages ca. 720–642, 580–415, and 350–314 Ma. Z173 has ca. 2000–1800, 1550–1100, 1000–798, and 733–356 Ma ages. Maximum depositional ages for these samples may be Early Devonian, which are not compatible with the Carolinia or Goochland domains. It is possible that ca. 410–340 Ma Acadian-Neoacadian orogenesis may have partially reset grains. CL-dark 340 Ma zircon with Th/U < 0.1 in Z171 are interpreted to be metamorphic grains. Pervasive Alleghenian granitic veins can account for ca. 330–314 Ma ages.