Abstract Recent detrital zircon results in both the central Appalachians and New England demonstrate that middle Ordovician, ‘Taconic’ island arcs, long considered to be peri-Laurentian, are built upon or associated with rock of Gondwanan affinity. This trip will visit granulite-facies orthogneiss of the Wilmington Complex, a 475–480 Ma magmatic arc, and the adjacent Wissahickon Formation. The Wissahickon Formation is intruded by and interlayered with meta-igneous rocks with arc affinity and contains detrital zircon populations characteristic of both Gondwanan and Laurentian sources. The Chester Park Gneiss, now known to have detrital zircon age spectra which match the Gondwana-derived Moretown Terrane in New England, is also featured. The trip will examine contact relationships between arc and Laurentian rocks and a newly discovered location where metapelitic rock contains garnet with crystallographically oriented rutile inclusions, possibly indicative of ultrahigh-temperature or ultrahigh-pressure metamorphism. We will discuss similarities between rocks of the central and northern Appalachians and evaluate a new model wherein the central Appalachian rocks were originally part of the Taconic arc in New England and were translated by strike-slip deformation to their present position in the orogen.

Abstract This field trip examines two distinct lithotectonic groups of the Pennsylvania Piedmont, separated by the Martic Line, each consisting of massifs of Mesoproterozoic gneiss overlain unconformably by Paleozoic metasediments. To the north of the Martic Line, the Mesoproterozoic gneisses are lithologically similar to rocks of the Adirondack anorthosite-mangerite-charnockite-granite (AMCG) association, and also include amphibolite-facies gneisses of felsic to mafic bulk composition. The overlying Paleozoic quartzite and carbonate ± semipelite succession records only greenschist-facies metamorphism, with on-going debate as to the extent of Taconian, Acadian, and Alleghanian contribution to the low-grade metamorphism and pervasive deformation. South of the Martic Line, AMCG lithologies are absent from the Mesoproterozoic gneisses. Paleozoic rocks of the Wissahickon Formation record low-pressure, high-temperature (0.3–0.4 GPa, 600–700 °C; andalusite-sillimanite) Silurian metamorphism, and Devonian moderate-pressure, moderate-temperature (0.6–0.8 GPa, 500–600 °C; kyanitesillimanite) metamorphism. Additionally, the Wissahickon Formation east of the Rosemont Shear Zone records Ordovician magmatic activity and limited contact metamorphism associated with emplacement of the Wilmington Complex. The Wissahickon Formation (sensu lato) is informally subdivided into three units: Glenarm Wissahickon (overlying Baltimore Gneiss and Glenarm Group, between the Embreeville and Street Road Faults), Mount Cuba Wissahickon (south and east of the Street Road fault, including a strip immediately east of the Wilmington Complex), and Wissahickon Formation (sensu stricto) (east of the Wilmington Complex). Day 1 treats the rocks north of the Martic Line. Day 2 addresses rocks south of the Martic Line, and within and around the Wilmington Complex.