NEW PROBLEMS FROM NEW DATA - AN EXAMPLE FROM THE CAROLINA TERRANE IN NORTH CAROLINA AND BEARING ON INTERSTATE CORRELATION

The N.C. Geological Survey is in its 4th year of increased funding from the U.S. Geological Survey, National Cooperative Geologic Mapping Program under STATEMAP. Increased funding has led to new data that has generated new questions that cannot be answered within the contract period of STATEMAP projects, and the results may have cascading effects on previously delivered maps. We present new geochronology results from the Carolina terrane in NC and mention possible stratigraphic problems and interpretations that may require multiple years to fully develop.

New crystallization age data includes analyses of zircons by LA-ICP-MS at the Arizona LaserChron Center. Zircon ages reported here are ²⁰⁶Pb/²³⁸U weighted means, with errors reported at a 95% confidence interval. The weighted mean age of the Seagrove Felsite zircons (previously mapped as part of the Uwharrie Formation; n = 34) is 536.4 ± 6.1 Ma, with a mean standard of the weighted deviates (MSWD) of 4.5. The weighted mean age of the Tillery Felsite at Caraway Mountain zircons (n = 9) is 542.7 ± 5.3 Ma (MSWD = 0.5), with one inherited zircon age of 628.1 ± 22.7 Ma. The weighted mean age from zircons (n=41) from a granodiorite in the Greensboro Intrusive Suite is 591.2 ± 3.8 Ma (MSWD = 1.9), with one inherited zircon age of 628.1 ± 16.3 Ma. This granodiorite includes foliated xenoliths of Hyco Formation.

Preliminary ²⁰⁶Pb/²³⁸U detrital zircon ages (n=451) from a sample on the eastern limb of the Virgilina synclinorium within the Aaron Formation range from ca. 529 Ma to 3200 Ma. The youngest zircon grains have ages at ca. 529, 532 and 533 Ma.

These new crystallization ages call into question the age, extent and stratigraphic relationship of parts of the Uwharrie Formation. The new detrital age of the Aaron Formation brings into question the age, stratigraphic relationship and provenance of the Aaron Formation in the type area of the formation. Additionally, the accepted timing of the Virgilina deformation (ca. 578 – 554 Ma), which was based partly on the minimum age of the Aaron Formation of ca. 578 Ma, needs to be re-evaluated in-light of the new minimum age of the Aaron (ca. 529 Ma). The age of the granodiorite from the Greensboro Intrusive Suite may help bracket the minimum age of the Virgilina deformation in NC at ca. 591 Ma.