THE WILMINGTON COMPLEX-WISSAHICKON FM-CHESTER PARK GNEISS: LINKING NORTHERN AND SOUTHERN PERI-GONDWANAN(?) ORDOVICIAN MAGMATIC ARCS

Recently obtained detrital zircon results reveal that the Chester Park Gneiss, which crops out along the Coastal Plain onlap in southeastern Pennsylvania, is a peri-Gondwanan terrane in the Central Appalachian Piedmont. Arc-related units with similar Gondwanan-derived detrital zircon populations have been identified to the south of the study area associated with the Chopawamsic and James Run arcs in Virginia (Hughes et al., 2014) and Maryland (Martin et al., 2015) and in the Shelburne Falls arc in New England. The Chester Park Gneiss zircon age spectrum is dominated by peaks at 550 Ma and 640 Ma, with smaller Neo-, Meso- and Paleoproterozoic peaks. Chester Park Gneiss is in contact with the Wissahickon Fm. and the 476-483 Ma Wilmington Complex magmatic arc. The Wissahickon Fm. contains zircon from both Laurentian and Gondwanan sources, with significant peaks at approximately 1050 and 1200 Ma and peaks of varying prominence between 510 – 530 Ma and 600 – 650 Ma. This study will utilize principal components analysis to compare detrital zircon results in the orogen.

The Wilmington Complex is sometimes regarded as the northeast continuation of the Chopawamsic - James Run arc. However, the Wilmington Complex and related rocks exhibit many similarities with rocks to the north. Magmatic zircon of the Wilmington Complex is the same age as the Shelburne Falls arc. The Wissahickon Fm., like the Hawley belt in New England, is host to boninitic amphibolites; other Wissahickon amphibolites are similar to back arc basin basalt-like dikes in the Hawley. Detrital zircon age spectra of the Wissahickon Fm. and Chester Park Gneiss are similar to those in the Hawley belt and Moretown terrane respectively. Both the Moretown and Chester Park Gneiss host tonalitic plutons (although the age of those in the latter are unknown). Timing of accretion of the Chopawamsic arc to Laurentia is constrained to the early Silurian, while the Wilmington Complex is outboard of the steeply-dipping Rosemont shear zone which was active well into the Devonian. While it remains possible that the Wilmington Complex is the northeastern extension of the southern arc, we suggest that these central Appalachian rocks may have originally been part of the Taconic arc in New England that were translated by strike-slip deformation to their present location.