The petrogenesis of the igneous and metamorphic rocks in the Wilmington Complex, Pennsylvania-Delaware Piedmont.

摘要

The granulite-facies Wilmington Complex in the Pennsylvania-Delaware Piedmont, is composed of felsic and mafic gneisses, rare aluminous gneisses, and numerous small igneous intrusions. The Bringhurst and Arden are the largest plutons. Most igneous rocks are gabbroids, but a pyroxene-bearing diorite-granodiorite suite intrudes older or contemporaneous gabbroids in the Arden pluton.;The conditions and timing of metamorphism are evaluated through field and petrographic observations, rock and mineral chemistry, and thermobarometry. Peak metamorphic conditions at ;Biotite-breakdown reactions in aluminous gneisses already above muscovite stability initiated the generation and segregation of leucocratic partial melts from a refractory residue containing hercynite, cordierite, garnet, orthopyroxene, corundum and calcic plagioclase. Reactions were overstepped and interface-controlled leading to the metastable preservation of reactant phases. Different mineral assemblages record different chemical potentials of fluid components and silica on the micro-scale, perhaps as a result of buffering during partial melting.;Coronas in olivine gabbroids probably formed during slow cooling at pressures in the spinel gabbro field. The Bringhurst and Arden gabbroids were derived from separate, but broadly co-genetic magmas. The calc-alkaline pyroxene-granodioritic suite of the Arden pluton formed by the crystallization of relatively dry granitic magma without significant fractionation, followed by differential migration of residual liquids relative to crystals.;The Wilmington Complex is a suspect terrane structurally overlying the Wissahickon. The tectonic setting most consistent with its igneous and metamorphic history is in the deep crustal infrastructure of a Cambro-Ordovician magmatic arc. Emplacement onto North America in the late Ordovician Taconic orogeny did not produce widespread deformation or recrystallization.