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 explores igneous layering in the Morgantown Sheet, southeastern Pennsylvania, a Jurassic diabase intrusion that is part of the Central Atlantic Magmatic Province, formed during rifting of Pangea. The Pennsylvania Granite Quarry (Stop 1) is a dimension stone quarry in the southern side of the sheet, in which the cut walls display intermittent modal layering crosscut by channels of mafic diabase. Plagioclase-rich layers overlie pyroxene-rich layers in packages with slightly concave-up “wok” shapes ~ 0.3–0.4 m in dimension and ~ 0.35–0.5 m thick. Mafic diabase — both layers and crosscutting channels—contain 15–25 modal percent orthopyroxene phenocrysts and are interpreted as basaltic magma replenishments. Orientations of layering and channels suggest this part of the sheet was originally a horizontal sill ~ 400 m thick, at about six kilometers depth, and that the sheet was tilted 20° – 25° to the north after crystallization. The Dyer aggregate quarry (Stop 2) is in the northeast side of the sheet that dips ~ 80° southeast (Birdsboro dike). Here, rhythmic plagioclase-pyroxene layering also dipping ~ 80° is found in the interior and near the margin of the ~ 255-m-wide dike. Augite and plagioclase compositions are very similar in samples from different vertical heights in the sheet, suggesting localized rather than sheet-wide fractionation. We compare the Morgantown Sheet layering to similar features in the Palisades sill, New Jersey, and Basement sill, Antarctica, and discuss models for their formation.

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