PRELIMINARY U-PB GEOCHRONOLOGY OF PROTEROZOIC AND PALEOZOIC ROCKS BENEATH THE ATLANTIC COASTAL PLAIN FROM MARYLAND TO SOUTH CAROLINA

New U-Pb zircon geochronology of drill-core samples provides age constraints for extensions of the Appalachian orogen concealed beneath ≤2.2 km thick sediments of the Atlantic Coastal Plain (ACP). SHRIMP analyses of five samples from the Chesapeake zone in MD and VA indicate Neoproterozoic igneous ages. Two samples of tuffaceous metasiltstone from Crisfield, MD (geothermal core) yield a dominant young population of zircons at ~640 Ma, suggesting this is the age of the unit. Additionally, a felsite porphyry at Hampton, VA (57E7 core) yields a ²⁰⁶Pb/²³⁸U date of 654 ± 7 Ma (2σ). ²⁰⁶Pb/²³⁸U dates of 675 ± 9 Ma for granodiorite at Isle of Wight, VA (geothermal core) and 625 ± 6 Ma for gneissic granite of the Cambridge pluton (DO-Ce88 core) are interpreted as igneous crystallization ages. These dates suggest that large parts of the Chesapeake zone may be broadly correlative with peri-Gondwanan Neoproterozoic volcanic arc rocks of the Carolina and Roanoke Rapids terranes. Eastward of the Chesapeake zone, SHRIMP data for a few zircons from amphibolitic metagabbro of the Cape May zone near Berlin, MD (WO-Ce12 core) suggest an older Neoproterozoic (Tonian?) crystallization age and justify further study.

In SC, SHRIMP U-Pb zircon data from rocks beneath the ACP indicate a Neoproterozoic crystallization age of 615 ± 7 Ma for leucocratic granite (C10A core) of the Charleston terrane and confirm Pennsylvanian intrusion ages for widely distributed Alleghanian granitoids. The latter include 322 ± 4 Ma for megacrystic granodiorite intrusive into paragneiss of the Charleston terrane at Brittons Neck (MRN78 core), 311 ± 4 Ma for two-mica granitoid at Horrell Hill (RIC585 core), and 289 ± 5 Ma for the Springfield pluton (SAL-1 core). Zircon grains from samples of three rock units beneath the ACP in SC were analyzed by LA-ICP-MS. Sparse, generally euhedral zircon grains recovered from phyllite at Lake Darpo (DAR228 core) define a population between ~570-540 Ma; one 420 Ma grain shows resorption texture, a thin bright CL rim, and Th/U suggesting the grain is metamorphic. Two zircon grains recovered from metavolcanic hornblende gneiss (DRB6 core) at the Savannah River Site yield poorly constrained dates of ~560-540 Ma and ~350-325 Ma. Three zircon grains from metafelsite at Swansea (LEX844 core) yield poorly constrained ~715-700 Ma and ~212 Ma dates.