Pressure-temperature conditions encountered by metamorphic rocks are often used to constrain tectonic environments of formation. Evidence of the prograde path during metamorphism is often obliterated by peak metamorphic conditions. However, pseudomorphs that form during prograde metamorphism may provide insights into heat and mass transport along this portion of the P-T-X-t path. Seventeen sillimanite zone samples, each containing 2-5 muscovite-rich pseudomorphs after staurolite, were investigated from pelitic schists of the Farmington Quadrangle, west-central Maine, USA. Pseudomorphs are easily recognized by an increase in average grain size (~cm) and change to randomly oriented muscovite-rich (~45-75%) modal mineralogy relative to the rock matrix (~mm). SEM cathodoluminescence images, X-ray maps, and electron microprobe analyses characterize the mineral distribution, modes, and compositions within the pseudomorphs and surrounding matrix. These data provide the basis for textural modeling studies, to test the influence of fluids in the system behavior of pseudomorph formation, and to test their use as indicators of metamorphic conditions along the prograde path.
Based on modal mineralogy, pseudomorphs are divided into four types: muscovite-rich (~70% muscovite), plagioclase muscovite (12-22% plagioclase with < 10% quartz, sillimanite); sillimanite plagioclase muscovite (20% plagioclase, 15-20% sillimanite with <8% quartz); and quartz muscovite (12-25% quartz with <8% plagioclase, sillimanite). A biotite-rich muscovite-poor mantle surrounds most pseudomorphs.
Irreversible thermodynamic modeling of pseudomorph development using analyzed mineral compositions and previously determined diffusion coefficients for metapelitic rocks suggests modes of ~75% muscovite, ~10% biotite, ~10% plagioclase, and ~5% quartz replace the staurolite in a system closed to fluids. These modes are substantially different from those observed. If the system is allowed to be open to H2O, K, Na, and Al, the modal variations of biotite, sillimanite, plagioclase, and quartz comprising the pseudomorphs can be reproduced with a different mechanism of formation for each pseudomorph type. In addition, changes in cathodoluminescence from core to rim in pseudomorph quartz grains suggest two growth stages characterized by trace element distributions. These data suggest that metapelitic pseudomorphs retain evidence for an infiltrating fluid phase, its composition, and therefore, conditions during prograde metamorphism, which provide additional information for the tectonic interpretation of west-central Maine.