DEHYDRATION PARTIAL MELTING AND DISEQUILIBRIUM IN THE GRANULITE-FACIES WILMINGTON COMPLEX, PENNSYLVANIA-DELAWARE PIEDMONT

Textural and compositional disequilibrium, on scales ranging from tens of micrometers to centimeters, preserves part of the prograde and retrograde metamorphic reaction history in garnet-bearing gneisses in the Wilmington Complex, a granulite-facies terrane located in southeastern Pennsylvania and northern Delaware. The Wilmington Complex is interpreted to be a deep-crustal remnant of a magmatic arc, tectonically emplaced onto the North American continent during the late-Ordovician Taconic orogeny (Wagner and Srogi. 1987). Results support previous conclusions (Wagner and Srogi, 1987) that peak metamorphic conditions of 800-degrees +/- 50-degrees-C at 700 +/- 100 MPa were associated with magmatic heating. We focus on garnet-bearing gneisses from one locality, in which dehydration melting reactions progressed to varying extents in samples collected different distances from an intrusive gabbroic stock. Reactions consumed biotite, sillimanite, quartz, and garnet and produced garnet, cordierite, hercynitic spinel, corundum, and orthopyroxene. Granitic melt produced by the reactions is preserved as leucocratic areas within less-extensively reacted rocks but apparently migrated out of more-reacted rocks closer to the intrusion. Reactions and re-equilibration during cooling from peak temperatures were localized and did not go to completion. Reactions postulated for the garnet-bearing gneisses could not have been initiated along any single P-T-time path. Apparently, reactions were overstepped and did not proceed at equilibrium. The original layered microfabric of the gneisses determined the nature and extent of reaction in layers of contrasting composition and mineralogy, resulting in gneisses, after reaction, with mimetic fabrics and enhanced chemical and mineralogical contrasts among assemblages in different layers or domains. Incongruent melting reactions produced restite assemblages with reduced values of mu(SiO2) and mu(H2O). Different product assemblages coexisting as domains within single thin sections indicate significant variations in mu(SiO2) on a scale of millimeters or less, suggesting a state of local equilibrium. Some textural features suggest garnet growth was influenced by slow diffusion rates of components such as alumina, but a model of diffusion-controlled growth cannot account adequately for the spatial distribution of minerals and the development of domains. There is also textural and chemical evidence for interface-controlled reactions. Reactant minerals, such as biotite and sillimanite, are preserved even in extensively-reacted rocks. The formation of some product minerals, such as spinel and sillimanite, was aided by epitaxial growth and nucleation on pre-existing trains of the same mineral. Variations in mineral compositions suggest partial and partitioning equilibrium among phases, rather than true chemical equilibrium (Loomis, 1976). The compositions of product minerals, such as cordierite and orthopyroxene, are nearly uniform in samples from the same locality; however, the compositions of reactant minerals, such as biotite, are significantly different among grains within single thin sections. Dehydration melting reactions in mineralogically heterogeneous rocks enhanced differences in chemical potentials and produced a pattern of domain assemblages which resembles a state of local equilibrium, but reactions were controlled by interface kinetics as well as diffusion.