AUGMENTING GEOLOGIC MAPPING AND TERRANE CHARACTERIZATION WITH NEW GEOCHRONOLOGY FROM THE EASTERN PIEDMONT IN SOUTHEASTERN VIRGINIA AND NORTHEASTERN NORTH CAROLINA

New SHRIMP-RG and LA-ICP-MS U-Pb zircon ages from metaplutonic, metavolcanic, and metasedimentary rocks refine our understanding of mapped geologic units in the eastern Piedmont (eP) of southeastern Virginia and northeastern North Carolina. Neoproterozoic to late Paleozoic rocks are distributed in five peri-Gondwanan terranes in the eP: from west to east, the Raleigh terrane (Rt); Spring Hope terrane (SHt); Triplet terrane (Tt); Roanoke Rapids terrane (RRt); and Dinwiddie terrane (Dt). The Rt, Tt, and Dt were metamorphosed to amphibolite facies, and the SHt and RRt to greenschist facies.

A granodiorite from the RRt yields a SHRIMP-RG zircon crystallization age of 600 ± 5 Ma. This age confirms field observations that some granodiorite crosscuts diorite, gabbro, and hornblendite previously dated at ~610 Ma and is younger than other Ediacaran granodiorite plutons previously dated at ~625 Ma (Webbs Mill and Adsit) or ~615 Ma (Skippers). A SHRIMP-RG age of 299 ± 5 Ma of igneous zircons from a mylonitic granitoid places an upper bound on the timing of dextral transpression in the Nottoway River fault zone, which separates the Dt from the RRt. Inherited grains in this sample reflect ~400 Ma granitoid gneiss from the Dt, ~600 Ma rocks from the RRt, and Mesoproterozoic crust unrecognized in either terrane.

Detrital zircons from the RRt indicate that these rocks were sourced not only from proximal Cryogenian volcanic rocks, but also from Silurian-Devonian sources. Biotite paragneiss from the Tt and Rt also yield maximum depositional ages as young as the Early Devonian Period. All three terranes likely docked with Laurentia at more northerly latitudes during mid-Paleozoic orogenies.

One sample from the RRt is particularly intriguing. Petrography of this lower greenschist facies rock indicates that monocrystalline quartz crystals are embayed phenocrysts having a volcanic or volcaniclastic origin. The peak age mode of zircon grains in this sample is ~303 Ma but grains as old as 1550 Ma are present. If these data are reproducible, the unit is hypothesized to be either Alleghanian molasse (if the grains are detrital) or a hypabyssal quartz porphyry with a significant component of inherited zircon, in stark contrast to abundant catazonal to mesozonal Pennsylvanian-Permian granite plutons in the eP.