Pressure-temperature-reaction history of metapelitic rocks from the Maryland Piedmont on the basis of correlated garnet zoning and plagioclase-inclusion composition

Abstract

Regional structural interpretation, thermobarometry, and Gibbs-method calculations were combined to determine an approximate P- T path for kyanite-grade metapelitic rocks of the Baltimore Gneiss terrane. Garnet growth along the estimated P- T path was modeled using the computer program DiffGibbs. Models were evaluated by comparing calculated profiles of garnet components with measured profiles of a large garnet rich in plagioclase inclusions in a kyanite-bearing sample from the area. Modifications in the P- T path constrained by changes in garnet components along critical portions of the profile resulted in a very satisfactory match between measured and model-garnet profiles. In the most successful model, garnet growth began at 550°C and 6500 bars in the assemblage chlorite + biotite + garnet + muscovite + plagioclase + quartz with increasing temperature and with decreasing and then constant pressure. At 570 °C and 5724 bars, staurolite was added to the assemblage and some garnet was consumed. In the model, garnet growth resumed after chlorite was consumed along a path having a substantial increase in pressure. The region near the rim with low measured grossular content was reproduced by cessation of garnet growth as temperature increased at constant pressure to a maximum of 615°C at 6474 bars. After kyanite was added to the assemblage, grossular increased to the constant rim value only if temperature decreased and pressure increased. Cooling and decompression affected garnet zoning only very near the rim. Measured compositions of the rims of plagioclase inclusions in the garnet are consistent with plagioclase compositions predicted by the model. This study demonstrates that complex garnet zoning can be modeled successfully, even when the reaction history includes episodes of garnet consumption, over a path that is consistent with geologic evidence. Details of garnet zoning provide surprisingly tight constraints on the P-T path.