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 ret rograde metamorphic reaction history in garnet-bearing gneisses in the Wilmington Complex, a granulite-facies terrane located in southeaster n Pennsylvania and northern Delaware. The Wilmington Complex is interp reted to be a deep-crustal remnant of a magmatic arc, tectonically emp laced onto the North American continent during the late-Ordovician Tac onic orogeny (Wagner and Srogi. 1987). Results support previous conclu sions (Wagner and Srogi, 1987) that peak metamorphic conditions of 800 -degrees +/- 50-degrees-C at 700 +/- 100 MPa were associated with magm atic heating. We focus on garnet-bearing gneisses from one locality, i n which dehydration melting reactions progressed to varying extents in samples collected different distances from an intrusive gabbroic stoc k. Reactions consumed biotite, sillimanite, quartz, and garnet and pro duced garnet, cordierite, hercynitic spinel, corundum, and orthopyroxe ne. Granitic melt produced by the reactions is preserved as leucocrati c areas within less-extensively reacted rocks but apparently migrated out of more-reacted rocks closer to the intrusion. Reactions and re-eq uilibration during cooling from peak temperatures were localized and d id not go to completion. Reactions postulated for the garnet-bearing g neisses could not have been initiated along any single P-T-time path. Apparently, reactions were overstepped and did not proceed at equilibr ium. The original layered microfabric of the gneisses determined the n ature and extent of reaction in layers of contrasting composition and mineralogy, resulting in gneisses, after reaction, with mimetic fabric s 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). Diff erent product assemblages coexisting as domains within single thin sec tions indicate significant variations in mu(SiO2) on a scale of millim eters 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 gro wth cannot account adequately for the spatial distribution of minerals and the development of domains. There is also textural and chemical e vidence for interface-controlled reactions. Reactant minerals, such as biotite and sillimanite, are preserved even in extensively-reacted ro cks. The formation of some product minerals, such as spinel and sillim anite, was aided by epitaxial growth and nucleation on pre-existing tr ains of the same mineral. Variations in mineral compositions suggest p artial and partitioning equilibrium among phases, rather than true che mical 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 sin gle thin sections. Dehydration melting reactions in mineralogically he terogeneous rocks enhanced differences in chemical potentials and prod uced a pattern of domain assemblages which resembles a state of local equilibrium, but reactions were controlled by interface kinetics as we ll as diffusion.