ZIRCON U-PB GEOCHRONOLOGY OF THE EASTERN RALEIGH TERRANE IN THE NORTH CAROLINA EASTERN PIEDMONT

In the North Carolina eastern Piedmont, lithotectonic terranes are juxtaposed along strands of the dextral Eastern Piedmont fault system (EPFS). The Raleigh terrane (Rt) lies between and is crosscut by EPFS strands and is subdivided into western and eastern parts by the Lake Gordon shear zone and Penn–Perm granitoid plutons. The eastern Rt (eRt) has been correlated with the ca. 633–528 Ma Carolinia or the ca. 1100 and 385 Ma Goochland domains; however, conclusive data linking the eRt with either is lacking.

We present LA-ICP-MS zircon U-Pb geochronology for eleven eRt samples. Nine samples contain multimodal age distributions indicating sedimentary protoliths. Multi-dimensional scaling separates these into two groups and further separates the eRt into western and eastern parts. In the western eRt, the Vicksboro, Soul City and Union Mill gneisses, as well as a sample of garnet biotite schist contain scattered ages ca. 2800–2000 Ma and dominant ages ca. 1700–850 Ma and 650–400 Ma with modes 640, 580, 495, 475 and 405 Ma. Maximum Depositional Ages (MDAs; YGC 2σ) range from 460 Ma (Middle Ordovician) to 412 Ma (Early Devonian). Low Th/U grains (<0.1), CL-dark rims, and linear arrays to common-Pb suggest metamorphic ages ca. 355–335 Ma. In the eastern eRt, the Mill Branch schist (four samples) contains few grains ca. 2000–800 Ma and ages ca. 800–400 Ma dominate, with subordinate age modes ca. 540 and dominant age modes ca. 411 Ma. MDAs are 403 and 410 Ma (Early Devonian), and metamorphic ages are ca. 355–330 Ma. Three gneiss samples from the eastern eRt yield unimodal ages ca. 403 and 410 Ma (Early Devonian) indicating a magmatic or single-source sedimentary protoliths.

Our results show that the western eRt has greater proportions of Precambrian grains whereas 410 Ma ages and lesser ages ca. 2.0–1.0 Ga dominate the eastern eRt. We interpret this distinction as supporting the presence of a cryptic shear zone along which Penn–Perm granitoids intruded. Our results are inconsistent with linking eRt and Carolinia. Early Devonian ages and the spread of Precambrian ages are consistent with ages of the Goochland domain Maidens Gneiss (Owens et al., 2020), and units of Ganderian affinity in the northern Appalachian orogen. Both scenarios require east-side-south transport of the eRt from the north along the EPFS in late Paleozoic time.