ZIRCON GEOCHRONOLOGY OF THE RALEIGH TERRANE IN THE NORTH CAROLINA EASTERN PIEDMONT

In the southern Appalachian orogen, paleogeographic reconstructions are ongoing to establish the affinity of Laurentian versus Gondwanan terranes using mapping and zircon U-Pb analyses. A lingering debate focuses on the amphibolite facies Raleigh terrane (RT) in the North Carolina eastern Piedmont. Some lithotectonic maps link the RT to a peri-Gondwanan superterrane based on zircon ages for rocks falling in the range of the Neoproterozoic-Cambrian Carolinia island arc. Other maps depict the RT as a southern extension of the Laurentian-affinity Goochland terrane in the Virginia Piedmont that contains ca. 1.1 Ga Grenville basement rocks and ca. 385 Ma plutonic rocks.

We report LA-ICP-MS U-Pb zircon ages for three mapped localities of high-grade schist and gneiss in the northern Raleigh terrane. Samples Middleburg, Littleton, and Afton are named for the quadrangle in which they were collected. The Middleburg amphibolite yielded 60 ages from 2.9 Ga–336 Ma in three clusters ca. 2.0 –1.1 Ga, 1.6–1.3 Ga, and 1.2– 1.1 Ga. Disregarding micro-diking of apparent Pennsylvanian-Permian age, the maximum depositional age can be interpreted as ca. 1.1 Ga. An age range of 1.8 Ga – 410 Ma in two clusters ca. 650 – 540 Ma and 465 – 410 Ma and a maximum depositional age ca. 410 Ma, Early Devonian, characterize the Littleton schist. The Afton gneiss is interpreted as meta-igneous based on a unimodal age distribution of 35 zircon grains. The weighted mean age is 410.5 ± 3.7 Ma, Early Devonian.

Both samples Littleton and Afton have a youngest age mode at 415–410 Ma and an age spread from 450-350 Ma similar to meta-igneous rocks of the Early Devonian Concord and Salisbury plutonic suites. Ages ca. 2.0–1.0 Ga and 650–540 Ma in the Littleton sample are consistent with both primary sources and sedimentary recycling of zircon grains into a syn- to post-Early Devonian RT basin having a regional source area in Carolinia. In the Middleburg sample, the restriction of detrital zircon ages to 2.0–1.0 Ga and a Mesoproterozoic depositional age suggests a difference in provenance compared to our other RT samples and may be a fragment of Goochland terrane displaced along the Nutbush Creek-Lake Gordon fault system. Our results indicate that areas mapped as a single Raleigh terrane may in fact represent structural blocks with distinct Laurentian versus Gondwanan affinities.