Thermal evolution of a convergent orogen: Pressure-temperature-deformation-time paths in the central Appalachian Piedmont of Pennsylvania and Delaware

The central Appalachian Piedmont in Pennsylvania and Delaware is underlain by rocks that preserve a history of early Paleozoic plate convergence that includes subduction related arc magmatism, arc-continent accretion, post-accretion magmatism and coincident low- to moderate-pressure high-temperature metamorphism, and regional metamorphism at moderate to deep levels resulting from crustal thickening during subsequent plate convergence. This study combines new data from field mapping, geothermobarometry, geochemistry, structural analysis and geochronology to constrain the baric, thermal, and structural history, a metamorphic pressure-temperature-deformation-time (P-T-D-t) path, of the highgrade metamorphic rocks of the Wissahickon Formation and Wilmington Complex in the central Appalachians. The absolute timing of metamorphism and deformation is constrained with new geochronologic techniques utilizing electron beam microanalysis of monazite, a rare earth element phosphate mineral. The trace element geochemical characteristics of metamorphosed mafic rocks presented in this dissertation provide strong evidence in support of interpretations that the Wilmington Complex originated as a magmatic arc. The presence of boninitic mafic dikes in both the Wissahickon Formation and Wilmington Complex near their contact and identification of a previously unrecognized period of early Ordovician metamorphism in the Wissahickon Formation, contemporaneous with arc magmatism in the Wilmington Complex, demonstrates that any tectonic juxtaposition of these units occurred early in the development of the arc. This study confirms and extends the previously described two-stage metamorphic history of the Wissahickon Formation and refines earlier mapping of isograds. Early low-pressure high temperature metamorphism in the Wissahickon Formation, associated with the main foliation forming deformation episode, is demonstrated to be latest Ordovician to Silurian (440–430 Ma), the same age as granulite facies metamorphism in the Wilmington Complex. Extensive folding, which may signal renewed crustal thickening, followed high-temperature low-pressure metamorphism in the earliest Devonian. Intermediate-pressure and temperature (7–9 kbar, 600°C) overprinting of the high-temperature low-pressure assemblages, intense deformation in shear zones along and parallel to the Rosemont fault, and Barrovian style metamorphism in the type section of the Wissahickon Formation in Philadelphia occurred during the Devonian (385–370 Ma).