Two high-grade metamorphic terranes of early Paleozoic age are exposed in southeastern Pennsylvania and northern Delaware. The Wilmington Complex in Delaware was metamorphosed at high temperatures to intermediate-pressure granulite facies during the early Paleozoic. In the West Chester prong, Pennsylvania, a granulite-facies metamorphism of Grenville age was overprinted by a high-pressure, moderate-temperature metamorphism during the Taconic orogeny. We propose a model to account for the different metamorphic conditions in these two terranes. In this model, the Piedmont of northern Delaware and southeastern Pennsylvania was the site of collision between a magmatic arc and the North American continent along an east-dipping subduction zone. The Wilmington Complex was the infrastructure of the arc, whereas the Grenville-age gneisses of the West Chester prong were basement of the North American continent. Between these two relatively rigid plates, there are nappes of remobilized basement and highly deformed forearc-basin and accretion-ary-prism sediments. The North American continent underthrust the hot infrastructure of the volcanic arc, resulting in the high-pressure (9-11 kb), moderate-temperature (650–700 °C), metamorphic overprint on Grenville assemblages of the West Chester prong. The Wilmington Complex was metamorphosed at moderate pressure (6–7 kb) and high temperature (∼800 °C) during the early Paleozoic, under the conditions of the high geothermal gradient that existed at the base of the volcanic arc. The intervening sediments and remobilized basement are all at amphibolite grade, but the grade is highest (above second sillimanite isograd) everywhere adjacent to the structurally overlying Wilmington Complex, suggesting that the isograds in the sediments are inverted beneath the Wilmington Complex. Uplift of this doubled thickness of crust was rapid enough that the gneisses of the West Chester prong were never heated above 700 °C.

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