THE CHESTER PARK GNEISS - A PERI-GONDWANAN TERRANE: NEW DETRITAL ZIRCON RESULTS FROM THE CENTRAL APPALACHIAN PIEDMONT

New detrital zircon results reveal that the Chester Park Gneiss, which crops out along the Coastal Plain onlap in southeastern Pennsylvania, is a previously unrecognized peri-Gondwanan terrane in the Central Appalachian Piedmont. The Chester Park Gneiss age spectrum is dominated by peaks at 550 Ma and 640 Ma, with an array of smaller Meso- and Paleoproterozoic peaks. Chester Park Gneiss is in contact with the Wissahickon Fm., a metasedimentary unit associated with the Wilmington Complex magmatic arc. Wissahickon Fm. age spectra contain 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. Other metasedimentary rock presently mapped as Wissahickon Fm., which lies to the west of the Wilmington Complex, contains mainly Mesoproterozoic zircon, but most samples yield a small number of early Cambrian and Ediacaran zircon, some of which may be derived from Gondwanan sources.

Detrital zircon populations, magmatic crystallization ages, and amphibolite geochemistry of the Piedmont units, above, are remarkably similar to early Ordovician magmatic arc and related rocks in the northern Appalachians. Published zircon results show that the Wilmington Complex (476 to 483 Ma) 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 amphibolite dikes in the Hawley. Zircon age spectra of the Wissahickon Fm. have similar peaks to the Hawley belt, including Gondwanan peaks and Grenville-aged peaks that are distinct from those in adjacent Laurentian rift-related metasediments (the West Grove Metamorphic Suite in the Central Appalachians, the Rowe belt in New England). Lithologically, the Chester Park Gneiss is similar to “pinstripe” granofels of the Moretown. Detrital zircon ages of the Chester Park Gneiss are very similar to the Moretown terrane. 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. As such, the Central Appalachian Piedmont may be the internal link between the northern and southern portions of the orogen.