SIGNIFICANCE OF DETRITAL ZIRCON AGES IN THE CENTRAL APPALACHIAN PIEDMONT OF SOUTHEASTERN PENNSYLVANIA AND NORTHERN DELAWARE

Detrital zircon age spectra obtained from units in the Central Appalachian Piedmont (southeast PA – northern DE) show significant differences between western and eastern units that are best explained by different depositional and tectonic settings, of different ages, and call into question accepted lithostratigraphic units and terrane boundaries. The western units are (from west to east): Harpers/Antietam, Octoraro, and Peters Creek Fms; and the West Grove Metamorphic Suite (formerly the “Glenarm Wissahickon”), which comprises the Doe Run Schist and Mount Cuba Gneiss. The eastern unit is the Wissahickon Fm, adjacent to and east of the Wilmington Complex arc.

All of the western samples are dominated by peaks between 960 and 1500 Ma. Excepting the Doe Run Schist, where the youngest zircon is 560 Ma, they also exhibit a small peak, (less than 2% of the grains in most samples) at approximately 530 Ma. A small Archean peak, ~2700 Ma, is present in all but the Mt. Cuba Gneiss. These ages suggest a Laurentian source. Following the analysis of Cawood et al. (2012), the paucity of detrital zircons within 150 Ma of the depositional age implies an extensional setting. This supports earlier lithostratigraphic analysis which considered these units as part of a late Neoproterozoic to Cambrian rift-to-drift transgressive sequence. While the source of the 530 Ma zircons is uncertain, metasedimentary rocks in these units are interlayered with rift- and MORB-like greenstone and amphibolite, demonstrating that extensional magmatism continued until at least 530 Ma.

Most zircon ages in samples from the eastern unit, the Wissahickon Fm, also fall between 1000 and 1500 Ma, but zircon as young as 470 Ma is present and several samples exhibit peaks between 750 and 900 Ma. The depositional age of the Wissahickon Fm, host to a 427 Ma pluton, is now constrained to be late Ordovician to early Silurian. Samples from the Wissahickon Fm were likely deposited in a basin proximal to the Wilmington Complex arc; the 750 – 900 Ma peaks indicate the influence of a non-Laurentian source. The zircon ages support a larger body of data which suggest that the name “Wissahickon Formation” be applied only to rock immediately adjacent to and east of the Wilmington Complex.