Interpretation of the early Paleozoic tectonic history of the Appalachian Piedmont is commonly complicated by post-Taconian deformation that has disrupted and obscured early Paleozoic structure. Blocks that lie between zones where later deformation has been concentrated, however, may preserve evidence of Taconian high-grade metamorphism. In these cases, analysis of metamorphism allows an interpretation of the early Paleozoic tectonic history of the block and its surrounding region. A block of this type in the Pennsylvania–Delaware Piedmont includes three tectonic units: (1) the basement-cored massifs of southeastern Pennsylvania with Grenvillian gneiss unconformably overlain by the Setters Formation and Cockeysville Marble; (2) the Wilmington Complex arc terrane; and (3) the Wissahickon Group and associated metaigneous rocks. A metamorphic gradient and inferred temperature inversion within the Wissahickon Group and a metamorphic discontinuity at the Wissahickon Group–Cockeysville Marble contact are used to establish that the Wilmington Complex was first joined with the Wissahickon Group and that the two units were then emplaced together onto the rocks now exposed in the massifs. An observed correlation of amphibolite-facies mineral assemblages in the Cockeysville Marble with their distance from the Wissahickon Group indicates that peak metamorphism of the marble occurred after the metapelites were emplaced. As amphibolite-facies metamorphism of the basement-cored massifs has been previously dated as Taconian, it follows that the assembly of these tectonic units occurred in the early Paleozoic.

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